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Linux/net/ipv4/tcp_output.c

  1 /*
  2  * INET         An implementation of the TCP/IP protocol suite for the LINUX
  3  *              operating system.  INET is implemented using the  BSD Socket
  4  *              interface as the means of communication with the user level.
  5  *
  6  *              Implementation of the Transmission Control Protocol(TCP).
  7  *
  8  * Authors:     Ross Biro
  9  *              Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
 10  *              Mark Evans, <evansmp@uhura.aston.ac.uk>
 11  *              Corey Minyard <wf-rch!minyard@relay.EU.net>
 12  *              Florian La Roche, <flla@stud.uni-sb.de>
 13  *              Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
 14  *              Linus Torvalds, <torvalds@cs.helsinki.fi>
 15  *              Alan Cox, <gw4pts@gw4pts.ampr.org>
 16  *              Matthew Dillon, <dillon@apollo.west.oic.com>
 17  *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 18  *              Jorge Cwik, <jorge@laser.satlink.net>
 19  */
 20 
 21 /*
 22  * Changes:     Pedro Roque     :       Retransmit queue handled by TCP.
 23  *                              :       Fragmentation on mtu decrease
 24  *                              :       Segment collapse on retransmit
 25  *                              :       AF independence
 26  *
 27  *              Linus Torvalds  :       send_delayed_ack
 28  *              David S. Miller :       Charge memory using the right skb
 29  *                                      during syn/ack processing.
 30  *              David S. Miller :       Output engine completely rewritten.
 31  *              Andrea Arcangeli:       SYNACK carry ts_recent in tsecr.
 32  *              Cacophonix Gaul :       draft-minshall-nagle-01
 33  *              J Hadi Salim    :       ECN support
 34  *
 35  */
 36 
 37 #define pr_fmt(fmt) "TCP: " fmt
 38 
 39 #include <net/tcp.h>
 40 
 41 #include <linux/compiler.h>
 42 #include <linux/gfp.h>
 43 #include <linux/module.h>
 44 
 45 /* People can turn this off for buggy TCP's found in printers etc. */
 46 int sysctl_tcp_retrans_collapse __read_mostly = 1;
 47 
 48 /* People can turn this on to work with those rare, broken TCPs that
 49  * interpret the window field as a signed quantity.
 50  */
 51 int sysctl_tcp_workaround_signed_windows __read_mostly = 0;
 52 
 53 /* Default TSQ limit of four TSO segments */
 54 int sysctl_tcp_limit_output_bytes __read_mostly = 262144;
 55 
 56 /* This limits the percentage of the congestion window which we
 57  * will allow a single TSO frame to consume.  Building TSO frames
 58  * which are too large can cause TCP streams to be bursty.
 59  */
 60 int sysctl_tcp_tso_win_divisor __read_mostly = 3;
 61 
 62 /* By default, RFC2861 behavior.  */
 63 int sysctl_tcp_slow_start_after_idle __read_mostly = 1;
 64 
 65 static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 66                            int push_one, gfp_t gfp);
 67 
 68 /* Account for new data that has been sent to the network. */
 69 static void tcp_event_new_data_sent(struct sock *sk, const struct sk_buff *skb)
 70 {
 71         struct inet_connection_sock *icsk = inet_csk(sk);
 72         struct tcp_sock *tp = tcp_sk(sk);
 73         unsigned int prior_packets = tp->packets_out;
 74 
 75         tcp_advance_send_head(sk, skb);
 76         tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
 77 
 78         tp->packets_out += tcp_skb_pcount(skb);
 79         if (!prior_packets || icsk->icsk_pending == ICSK_TIME_EARLY_RETRANS ||
 80             icsk->icsk_pending == ICSK_TIME_LOSS_PROBE) {
 81                 tcp_rearm_rto(sk);
 82         }
 83 
 84         NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT,
 85                       tcp_skb_pcount(skb));
 86 }
 87 
 88 /* SND.NXT, if window was not shrunk.
 89  * If window has been shrunk, what should we make? It is not clear at all.
 90  * Using SND.UNA we will fail to open window, SND.NXT is out of window. :-(
 91  * Anything in between SND.UNA...SND.UNA+SND.WND also can be already
 92  * invalid. OK, let's make this for now:
 93  */
 94 static inline __u32 tcp_acceptable_seq(const struct sock *sk)
 95 {
 96         const struct tcp_sock *tp = tcp_sk(sk);
 97 
 98         if (!before(tcp_wnd_end(tp), tp->snd_nxt))
 99                 return tp->snd_nxt;
100         else
101                 return tcp_wnd_end(tp);
102 }
103 
104 /* Calculate mss to advertise in SYN segment.
105  * RFC1122, RFC1063, draft-ietf-tcpimpl-pmtud-01 state that:
106  *
107  * 1. It is independent of path mtu.
108  * 2. Ideally, it is maximal possible segment size i.e. 65535-40.
109  * 3. For IPv4 it is reasonable to calculate it from maximal MTU of
110  *    attached devices, because some buggy hosts are confused by
111  *    large MSS.
112  * 4. We do not make 3, we advertise MSS, calculated from first
113  *    hop device mtu, but allow to raise it to ip_rt_min_advmss.
114  *    This may be overridden via information stored in routing table.
115  * 5. Value 65535 for MSS is valid in IPv6 and means "as large as possible,
116  *    probably even Jumbo".
117  */
118 static __u16 tcp_advertise_mss(struct sock *sk)
119 {
120         struct tcp_sock *tp = tcp_sk(sk);
121         const struct dst_entry *dst = __sk_dst_get(sk);
122         int mss = tp->advmss;
123 
124         if (dst) {
125                 unsigned int metric = dst_metric_advmss(dst);
126 
127                 if (metric < mss) {
128                         mss = metric;
129                         tp->advmss = mss;
130                 }
131         }
132 
133         return (__u16)mss;
134 }
135 
136 /* RFC2861. Reset CWND after idle period longer RTO to "restart window".
137  * This is the first part of cwnd validation mechanism.
138  */
139 void tcp_cwnd_restart(struct sock *sk, s32 delta)
140 {
141         struct tcp_sock *tp = tcp_sk(sk);
142         u32 restart_cwnd = tcp_init_cwnd(tp, __sk_dst_get(sk));
143         u32 cwnd = tp->snd_cwnd;
144 
145         tcp_ca_event(sk, CA_EVENT_CWND_RESTART);
146 
147         tp->snd_ssthresh = tcp_current_ssthresh(sk);
148         restart_cwnd = min(restart_cwnd, cwnd);
149 
150         while ((delta -= inet_csk(sk)->icsk_rto) > 0 && cwnd > restart_cwnd)
151                 cwnd >>= 1;
152         tp->snd_cwnd = max(cwnd, restart_cwnd);
153         tp->snd_cwnd_stamp = tcp_time_stamp;
154         tp->snd_cwnd_used = 0;
155 }
156 
157 /* Congestion state accounting after a packet has been sent. */
158 static void tcp_event_data_sent(struct tcp_sock *tp,
159                                 struct sock *sk)
160 {
161         struct inet_connection_sock *icsk = inet_csk(sk);
162         const u32 now = tcp_time_stamp;
163 
164         if (tcp_packets_in_flight(tp) == 0)
165                 tcp_ca_event(sk, CA_EVENT_TX_START);
166 
167         tp->lsndtime = now;
168 
169         /* If it is a reply for ato after last received
170          * packet, enter pingpong mode.
171          */
172         if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
173                 icsk->icsk_ack.pingpong = 1;
174 }
175 
176 /* Account for an ACK we sent. */
177 static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts)
178 {
179         tcp_dec_quickack_mode(sk, pkts);
180         inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
181 }
182 
183 
184 u32 tcp_default_init_rwnd(u32 mss)
185 {
186         /* Initial receive window should be twice of TCP_INIT_CWND to
187          * enable proper sending of new unsent data during fast recovery
188          * (RFC 3517, Section 4, NextSeg() rule (2)). Further place a
189          * limit when mss is larger than 1460.
190          */
191         u32 init_rwnd = TCP_INIT_CWND * 2;
192 
193         if (mss > 1460)
194                 init_rwnd = max((1460 * init_rwnd) / mss, 2U);
195         return init_rwnd;
196 }
197 
198 /* Determine a window scaling and initial window to offer.
199  * Based on the assumption that the given amount of space
200  * will be offered. Store the results in the tp structure.
201  * NOTE: for smooth operation initial space offering should
202  * be a multiple of mss if possible. We assume here that mss >= 1.
203  * This MUST be enforced by all callers.
204  */
205 void tcp_select_initial_window(int __space, __u32 mss,
206                                __u32 *rcv_wnd, __u32 *window_clamp,
207                                int wscale_ok, __u8 *rcv_wscale,
208                                __u32 init_rcv_wnd)
209 {
210         unsigned int space = (__space < 0 ? 0 : __space);
211 
212         /* If no clamp set the clamp to the max possible scaled window */
213         if (*window_clamp == 0)
214                 (*window_clamp) = (65535 << 14);
215         space = min(*window_clamp, space);
216 
217         /* Quantize space offering to a multiple of mss if possible. */
218         if (space > mss)
219                 space = (space / mss) * mss;
220 
221         /* NOTE: offering an initial window larger than 32767
222          * will break some buggy TCP stacks. If the admin tells us
223          * it is likely we could be speaking with such a buggy stack
224          * we will truncate our initial window offering to 32K-1
225          * unless the remote has sent us a window scaling option,
226          * which we interpret as a sign the remote TCP is not
227          * misinterpreting the window field as a signed quantity.
228          */
229         if (sysctl_tcp_workaround_signed_windows)
230                 (*rcv_wnd) = min(space, MAX_TCP_WINDOW);
231         else
232                 (*rcv_wnd) = space;
233 
234         (*rcv_wscale) = 0;
235         if (wscale_ok) {
236                 /* Set window scaling on max possible window
237                  * See RFC1323 for an explanation of the limit to 14
238                  */
239                 space = max_t(u32, space, sysctl_tcp_rmem[2]);
240                 space = max_t(u32, space, sysctl_rmem_max);
241                 space = min_t(u32, space, *window_clamp);
242                 while (space > 65535 && (*rcv_wscale) < 14) {
243                         space >>= 1;
244                         (*rcv_wscale)++;
245                 }
246         }
247 
248         if (mss > (1 << *rcv_wscale)) {
249                 if (!init_rcv_wnd) /* Use default unless specified otherwise */
250                         init_rcv_wnd = tcp_default_init_rwnd(mss);
251                 *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
252         }
253 
254         /* Set the clamp no higher than max representable value */
255         (*window_clamp) = min(65535U << (*rcv_wscale), *window_clamp);
256 }
257 EXPORT_SYMBOL(tcp_select_initial_window);
258 
259 /* Chose a new window to advertise, update state in tcp_sock for the
260  * socket, and return result with RFC1323 scaling applied.  The return
261  * value can be stuffed directly into th->window for an outgoing
262  * frame.
263  */
264 static u16 tcp_select_window(struct sock *sk)
265 {
266         struct tcp_sock *tp = tcp_sk(sk);
267         u32 old_win = tp->rcv_wnd;
268         u32 cur_win = tcp_receive_window(tp);
269         u32 new_win = __tcp_select_window(sk);
270 
271         /* Never shrink the offered window */
272         if (new_win < cur_win) {
273                 /* Danger Will Robinson!
274                  * Don't update rcv_wup/rcv_wnd here or else
275                  * we will not be able to advertise a zero
276                  * window in time.  --DaveM
277                  *
278                  * Relax Will Robinson.
279                  */
280                 if (new_win == 0)
281                         NET_INC_STATS(sock_net(sk),
282                                       LINUX_MIB_TCPWANTZEROWINDOWADV);
283                 new_win = ALIGN(cur_win, 1 << tp->rx_opt.rcv_wscale);
284         }
285         tp->rcv_wnd = new_win;
286         tp->rcv_wup = tp->rcv_nxt;
287 
288         /* Make sure we do not exceed the maximum possible
289          * scaled window.
290          */
291         if (!tp->rx_opt.rcv_wscale && sysctl_tcp_workaround_signed_windows)
292                 new_win = min(new_win, MAX_TCP_WINDOW);
293         else
294                 new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));
295 
296         /* RFC1323 scaling applied */
297         new_win >>= tp->rx_opt.rcv_wscale;
298 
299         /* If we advertise zero window, disable fast path. */
300         if (new_win == 0) {
301                 tp->pred_flags = 0;
302                 if (old_win)
303                         NET_INC_STATS(sock_net(sk),
304                                       LINUX_MIB_TCPTOZEROWINDOWADV);
305         } else if (old_win == 0) {
306                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPFROMZEROWINDOWADV);
307         }
308 
309         return new_win;
310 }
311 
312 /* Packet ECN state for a SYN-ACK */
313 static void tcp_ecn_send_synack(struct sock *sk, struct sk_buff *skb)
314 {
315         const struct tcp_sock *tp = tcp_sk(sk);
316 
317         TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_CWR;
318         if (!(tp->ecn_flags & TCP_ECN_OK))
319                 TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_ECE;
320         else if (tcp_ca_needs_ecn(sk))
321                 INET_ECN_xmit(sk);
322 }
323 
324 /* Packet ECN state for a SYN.  */
325 static void tcp_ecn_send_syn(struct sock *sk, struct sk_buff *skb)
326 {
327         struct tcp_sock *tp = tcp_sk(sk);
328         bool use_ecn = sock_net(sk)->ipv4.sysctl_tcp_ecn == 1 ||
329                        tcp_ca_needs_ecn(sk);
330 
331         if (!use_ecn) {
332                 const struct dst_entry *dst = __sk_dst_get(sk);
333 
334                 if (dst && dst_feature(dst, RTAX_FEATURE_ECN))
335                         use_ecn = true;
336         }
337 
338         tp->ecn_flags = 0;
339 
340         if (use_ecn) {
341                 TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ECE | TCPHDR_CWR;
342                 tp->ecn_flags = TCP_ECN_OK;
343                 if (tcp_ca_needs_ecn(sk))
344                         INET_ECN_xmit(sk);
345         }
346 }
347 
348 static void tcp_ecn_clear_syn(struct sock *sk, struct sk_buff *skb)
349 {
350         if (sock_net(sk)->ipv4.sysctl_tcp_ecn_fallback)
351                 /* tp->ecn_flags are cleared at a later point in time when
352                  * SYN ACK is ultimatively being received.
353                  */
354                 TCP_SKB_CB(skb)->tcp_flags &= ~(TCPHDR_ECE | TCPHDR_CWR);
355 }
356 
357 static void
358 tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th)
359 {
360         if (inet_rsk(req)->ecn_ok)
361                 th->ece = 1;
362 }
363 
364 /* Set up ECN state for a packet on a ESTABLISHED socket that is about to
365  * be sent.
366  */
367 static void tcp_ecn_send(struct sock *sk, struct sk_buff *skb,
368                          struct tcphdr *th, int tcp_header_len)
369 {
370         struct tcp_sock *tp = tcp_sk(sk);
371 
372         if (tp->ecn_flags & TCP_ECN_OK) {
373                 /* Not-retransmitted data segment: set ECT and inject CWR. */
374                 if (skb->len != tcp_header_len &&
375                     !before(TCP_SKB_CB(skb)->seq, tp->snd_nxt)) {
376                         INET_ECN_xmit(sk);
377                         if (tp->ecn_flags & TCP_ECN_QUEUE_CWR) {
378                                 tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
379                                 th->cwr = 1;
380                                 skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
381                         }
382                 } else if (!tcp_ca_needs_ecn(sk)) {
383                         /* ACK or retransmitted segment: clear ECT|CE */
384                         INET_ECN_dontxmit(sk);
385                 }
386                 if (tp->ecn_flags & TCP_ECN_DEMAND_CWR)
387                         th->ece = 1;
388         }
389 }
390 
391 /* Constructs common control bits of non-data skb. If SYN/FIN is present,
392  * auto increment end seqno.
393  */
394 static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
395 {
396         skb->ip_summed = CHECKSUM_PARTIAL;
397         skb->csum = 0;
398 
399         TCP_SKB_CB(skb)->tcp_flags = flags;
400         TCP_SKB_CB(skb)->sacked = 0;
401 
402         tcp_skb_pcount_set(skb, 1);
403 
404         TCP_SKB_CB(skb)->seq = seq;
405         if (flags & (TCPHDR_SYN | TCPHDR_FIN))
406                 seq++;
407         TCP_SKB_CB(skb)->end_seq = seq;
408 }
409 
410 static inline bool tcp_urg_mode(const struct tcp_sock *tp)
411 {
412         return tp->snd_una != tp->snd_up;
413 }
414 
415 #define OPTION_SACK_ADVERTISE   (1 << 0)
416 #define OPTION_TS               (1 << 1)
417 #define OPTION_MD5              (1 << 2)
418 #define OPTION_WSCALE           (1 << 3)
419 #define OPTION_FAST_OPEN_COOKIE (1 << 8)
420 
421 struct tcp_out_options {
422         u16 options;            /* bit field of OPTION_* */
423         u16 mss;                /* 0 to disable */
424         u8 ws;                  /* window scale, 0 to disable */
425         u8 num_sack_blocks;     /* number of SACK blocks to include */
426         u8 hash_size;           /* bytes in hash_location */
427         __u8 *hash_location;    /* temporary pointer, overloaded */
428         __u32 tsval, tsecr;     /* need to include OPTION_TS */
429         struct tcp_fastopen_cookie *fastopen_cookie;    /* Fast open cookie */
430 };
431 
432 /* Write previously computed TCP options to the packet.
433  *
434  * Beware: Something in the Internet is very sensitive to the ordering of
435  * TCP options, we learned this through the hard way, so be careful here.
436  * Luckily we can at least blame others for their non-compliance but from
437  * inter-operability perspective it seems that we're somewhat stuck with
438  * the ordering which we have been using if we want to keep working with
439  * those broken things (not that it currently hurts anybody as there isn't
440  * particular reason why the ordering would need to be changed).
441  *
442  * At least SACK_PERM as the first option is known to lead to a disaster
443  * (but it may well be that other scenarios fail similarly).
444  */
445 static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
446                               struct tcp_out_options *opts)
447 {
448         u16 options = opts->options;    /* mungable copy */
449 
450         if (unlikely(OPTION_MD5 & options)) {
451                 *ptr++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) |
452                                (TCPOPT_MD5SIG << 8) | TCPOLEN_MD5SIG);
453                 /* overload cookie hash location */
454                 opts->hash_location = (__u8 *)ptr;
455                 ptr += 4;
456         }
457 
458         if (unlikely(opts->mss)) {
459                 *ptr++ = htonl((TCPOPT_MSS << 24) |
460                                (TCPOLEN_MSS << 16) |
461                                opts->mss);
462         }
463 
464         if (likely(OPTION_TS & options)) {
465                 if (unlikely(OPTION_SACK_ADVERTISE & options)) {
466                         *ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
467                                        (TCPOLEN_SACK_PERM << 16) |
468                                        (TCPOPT_TIMESTAMP << 8) |
469                                        TCPOLEN_TIMESTAMP);
470                         options &= ~OPTION_SACK_ADVERTISE;
471                 } else {
472                         *ptr++ = htonl((TCPOPT_NOP << 24) |
473                                        (TCPOPT_NOP << 16) |
474                                        (TCPOPT_TIMESTAMP << 8) |
475                                        TCPOLEN_TIMESTAMP);
476                 }
477                 *ptr++ = htonl(opts->tsval);
478                 *ptr++ = htonl(opts->tsecr);
479         }
480 
481         if (unlikely(OPTION_SACK_ADVERTISE & options)) {
482                 *ptr++ = htonl((TCPOPT_NOP << 24) |
483                                (TCPOPT_NOP << 16) |
484                                (TCPOPT_SACK_PERM << 8) |
485                                TCPOLEN_SACK_PERM);
486         }
487 
488         if (unlikely(OPTION_WSCALE & options)) {
489                 *ptr++ = htonl((TCPOPT_NOP << 24) |
490                                (TCPOPT_WINDOW << 16) |
491                                (TCPOLEN_WINDOW << 8) |
492                                opts->ws);
493         }
494 
495         if (unlikely(opts->num_sack_blocks)) {
496                 struct tcp_sack_block *sp = tp->rx_opt.dsack ?
497                         tp->duplicate_sack : tp->selective_acks;
498                 int this_sack;
499 
500                 *ptr++ = htonl((TCPOPT_NOP  << 24) |
501                                (TCPOPT_NOP  << 16) |
502                                (TCPOPT_SACK <<  8) |
503                                (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
504                                                      TCPOLEN_SACK_PERBLOCK)));
505 
506                 for (this_sack = 0; this_sack < opts->num_sack_blocks;
507                      ++this_sack) {
508                         *ptr++ = htonl(sp[this_sack].start_seq);
509                         *ptr++ = htonl(sp[this_sack].end_seq);
510                 }
511 
512                 tp->rx_opt.dsack = 0;
513         }
514 
515         if (unlikely(OPTION_FAST_OPEN_COOKIE & options)) {
516                 struct tcp_fastopen_cookie *foc = opts->fastopen_cookie;
517                 u8 *p = (u8 *)ptr;
518                 u32 len; /* Fast Open option length */
519 
520                 if (foc->exp) {
521                         len = TCPOLEN_EXP_FASTOPEN_BASE + foc->len;
522                         *ptr = htonl((TCPOPT_EXP << 24) | (len << 16) |
523                                      TCPOPT_FASTOPEN_MAGIC);
524                         p += TCPOLEN_EXP_FASTOPEN_BASE;
525                 } else {
526                         len = TCPOLEN_FASTOPEN_BASE + foc->len;
527                         *p++ = TCPOPT_FASTOPEN;
528                         *p++ = len;
529                 }
530 
531                 memcpy(p, foc->val, foc->len);
532                 if ((len & 3) == 2) {
533                         p[foc->len] = TCPOPT_NOP;
534                         p[foc->len + 1] = TCPOPT_NOP;
535                 }
536                 ptr += (len + 3) >> 2;
537         }
538 }
539 
540 /* Compute TCP options for SYN packets. This is not the final
541  * network wire format yet.
542  */
543 static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
544                                 struct tcp_out_options *opts,
545                                 struct tcp_md5sig_key **md5)
546 {
547         struct tcp_sock *tp = tcp_sk(sk);
548         unsigned int remaining = MAX_TCP_OPTION_SPACE;
549         struct tcp_fastopen_request *fastopen = tp->fastopen_req;
550 
551 #ifdef CONFIG_TCP_MD5SIG
552         *md5 = tp->af_specific->md5_lookup(sk, sk);
553         if (*md5) {
554                 opts->options |= OPTION_MD5;
555                 remaining -= TCPOLEN_MD5SIG_ALIGNED;
556         }
557 #else
558         *md5 = NULL;
559 #endif
560 
561         /* We always get an MSS option.  The option bytes which will be seen in
562          * normal data packets should timestamps be used, must be in the MSS
563          * advertised.  But we subtract them from tp->mss_cache so that
564          * calculations in tcp_sendmsg are simpler etc.  So account for this
565          * fact here if necessary.  If we don't do this correctly, as a
566          * receiver we won't recognize data packets as being full sized when we
567          * should, and thus we won't abide by the delayed ACK rules correctly.
568          * SACKs don't matter, we never delay an ACK when we have any of those
569          * going out.  */
570         opts->mss = tcp_advertise_mss(sk);
571         remaining -= TCPOLEN_MSS_ALIGNED;
572 
573         if (likely(sysctl_tcp_timestamps && !*md5)) {
574                 opts->options |= OPTION_TS;
575                 opts->tsval = tcp_skb_timestamp(skb) + tp->tsoffset;
576                 opts->tsecr = tp->rx_opt.ts_recent;
577                 remaining -= TCPOLEN_TSTAMP_ALIGNED;
578         }
579         if (likely(sysctl_tcp_window_scaling)) {
580                 opts->ws = tp->rx_opt.rcv_wscale;
581                 opts->options |= OPTION_WSCALE;
582                 remaining -= TCPOLEN_WSCALE_ALIGNED;
583         }
584         if (likely(sysctl_tcp_sack)) {
585                 opts->options |= OPTION_SACK_ADVERTISE;
586                 if (unlikely(!(OPTION_TS & opts->options)))
587                         remaining -= TCPOLEN_SACKPERM_ALIGNED;
588         }
589 
590         if (fastopen && fastopen->cookie.len >= 0) {
591                 u32 need = fastopen->cookie.len;
592 
593                 need += fastopen->cookie.exp ? TCPOLEN_EXP_FASTOPEN_BASE :
594                                                TCPOLEN_FASTOPEN_BASE;
595                 need = (need + 3) & ~3U;  /* Align to 32 bits */
596                 if (remaining >= need) {
597                         opts->options |= OPTION_FAST_OPEN_COOKIE;
598                         opts->fastopen_cookie = &fastopen->cookie;
599                         remaining -= need;
600                         tp->syn_fastopen = 1;
601                         tp->syn_fastopen_exp = fastopen->cookie.exp ? 1 : 0;
602                 }
603         }
604 
605         return MAX_TCP_OPTION_SPACE - remaining;
606 }
607 
608 /* Set up TCP options for SYN-ACKs. */
609 static unsigned int tcp_synack_options(struct request_sock *req,
610                                        unsigned int mss, struct sk_buff *skb,
611                                        struct tcp_out_options *opts,
612                                        const struct tcp_md5sig_key *md5,
613                                        struct tcp_fastopen_cookie *foc)
614 {
615         struct inet_request_sock *ireq = inet_rsk(req);
616         unsigned int remaining = MAX_TCP_OPTION_SPACE;
617 
618 #ifdef CONFIG_TCP_MD5SIG
619         if (md5) {
620                 opts->options |= OPTION_MD5;
621                 remaining -= TCPOLEN_MD5SIG_ALIGNED;
622 
623                 /* We can't fit any SACK blocks in a packet with MD5 + TS
624                  * options. There was discussion about disabling SACK
625                  * rather than TS in order to fit in better with old,
626                  * buggy kernels, but that was deemed to be unnecessary.
627                  */
628                 ireq->tstamp_ok &= !ireq->sack_ok;
629         }
630 #endif
631 
632         /* We always send an MSS option. */
633         opts->mss = mss;
634         remaining -= TCPOLEN_MSS_ALIGNED;
635 
636         if (likely(ireq->wscale_ok)) {
637                 opts->ws = ireq->rcv_wscale;
638                 opts->options |= OPTION_WSCALE;
639                 remaining -= TCPOLEN_WSCALE_ALIGNED;
640         }
641         if (likely(ireq->tstamp_ok)) {
642                 opts->options |= OPTION_TS;
643                 opts->tsval = tcp_skb_timestamp(skb);
644                 opts->tsecr = req->ts_recent;
645                 remaining -= TCPOLEN_TSTAMP_ALIGNED;
646         }
647         if (likely(ireq->sack_ok)) {
648                 opts->options |= OPTION_SACK_ADVERTISE;
649                 if (unlikely(!ireq->tstamp_ok))
650                         remaining -= TCPOLEN_SACKPERM_ALIGNED;
651         }
652         if (foc != NULL && foc->len >= 0) {
653                 u32 need = foc->len;
654 
655                 need += foc->exp ? TCPOLEN_EXP_FASTOPEN_BASE :
656                                    TCPOLEN_FASTOPEN_BASE;
657                 need = (need + 3) & ~3U;  /* Align to 32 bits */
658                 if (remaining >= need) {
659                         opts->options |= OPTION_FAST_OPEN_COOKIE;
660                         opts->fastopen_cookie = foc;
661                         remaining -= need;
662                 }
663         }
664 
665         return MAX_TCP_OPTION_SPACE - remaining;
666 }
667 
668 /* Compute TCP options for ESTABLISHED sockets. This is not the
669  * final wire format yet.
670  */
671 static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb,
672                                         struct tcp_out_options *opts,
673                                         struct tcp_md5sig_key **md5)
674 {
675         struct tcp_sock *tp = tcp_sk(sk);
676         unsigned int size = 0;
677         unsigned int eff_sacks;
678 
679         opts->options = 0;
680 
681 #ifdef CONFIG_TCP_MD5SIG
682         *md5 = tp->af_specific->md5_lookup(sk, sk);
683         if (unlikely(*md5)) {
684                 opts->options |= OPTION_MD5;
685                 size += TCPOLEN_MD5SIG_ALIGNED;
686         }
687 #else
688         *md5 = NULL;
689 #endif
690 
691         if (likely(tp->rx_opt.tstamp_ok)) {
692                 opts->options |= OPTION_TS;
693                 opts->tsval = skb ? tcp_skb_timestamp(skb) + tp->tsoffset : 0;
694                 opts->tsecr = tp->rx_opt.ts_recent;
695                 size += TCPOLEN_TSTAMP_ALIGNED;
696         }
697 
698         eff_sacks = tp->rx_opt.num_sacks + tp->rx_opt.dsack;
699         if (unlikely(eff_sacks)) {
700                 const unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
701                 opts->num_sack_blocks =
702                         min_t(unsigned int, eff_sacks,
703                               (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
704                               TCPOLEN_SACK_PERBLOCK);
705                 size += TCPOLEN_SACK_BASE_ALIGNED +
706                         opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
707         }
708 
709         return size;
710 }
711 
712 
713 /* TCP SMALL QUEUES (TSQ)
714  *
715  * TSQ goal is to keep small amount of skbs per tcp flow in tx queues (qdisc+dev)
716  * to reduce RTT and bufferbloat.
717  * We do this using a special skb destructor (tcp_wfree).
718  *
719  * Its important tcp_wfree() can be replaced by sock_wfree() in the event skb
720  * needs to be reallocated in a driver.
721  * The invariant being skb->truesize subtracted from sk->sk_wmem_alloc
722  *
723  * Since transmit from skb destructor is forbidden, we use a tasklet
724  * to process all sockets that eventually need to send more skbs.
725  * We use one tasklet per cpu, with its own queue of sockets.
726  */
727 struct tsq_tasklet {
728         struct tasklet_struct   tasklet;
729         struct list_head        head; /* queue of tcp sockets */
730 };
731 static DEFINE_PER_CPU(struct tsq_tasklet, tsq_tasklet);
732 
733 static void tcp_tsq_handler(struct sock *sk)
734 {
735         if ((1 << sk->sk_state) &
736             (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 | TCPF_CLOSING |
737              TCPF_CLOSE_WAIT  | TCPF_LAST_ACK)) {
738                 struct tcp_sock *tp = tcp_sk(sk);
739 
740                 if (tp->lost_out > tp->retrans_out &&
741                     tp->snd_cwnd > tcp_packets_in_flight(tp))
742                         tcp_xmit_retransmit_queue(sk);
743 
744                 tcp_write_xmit(sk, tcp_current_mss(sk), tp->nonagle,
745                                0, GFP_ATOMIC);
746         }
747 }
748 /*
749  * One tasklet per cpu tries to send more skbs.
750  * We run in tasklet context but need to disable irqs when
751  * transferring tsq->head because tcp_wfree() might
752  * interrupt us (non NAPI drivers)
753  */
754 static void tcp_tasklet_func(unsigned long data)
755 {
756         struct tsq_tasklet *tsq = (struct tsq_tasklet *)data;
757         LIST_HEAD(list);
758         unsigned long flags;
759         struct list_head *q, *n;
760         struct tcp_sock *tp;
761         struct sock *sk;
762 
763         local_irq_save(flags);
764         list_splice_init(&tsq->head, &list);
765         local_irq_restore(flags);
766 
767         list_for_each_safe(q, n, &list) {
768                 tp = list_entry(q, struct tcp_sock, tsq_node);
769                 list_del(&tp->tsq_node);
770 
771                 sk = (struct sock *)tp;
772                 bh_lock_sock(sk);
773 
774                 if (!sock_owned_by_user(sk)) {
775                         tcp_tsq_handler(sk);
776                 } else {
777                         /* defer the work to tcp_release_cb() */
778                         set_bit(TCP_TSQ_DEFERRED, &tp->tsq_flags);
779                 }
780                 bh_unlock_sock(sk);
781 
782                 clear_bit(TSQ_QUEUED, &tp->tsq_flags);
783                 sk_free(sk);
784         }
785 }
786 
787 #define TCP_DEFERRED_ALL ((1UL << TCP_TSQ_DEFERRED) |           \
788                           (1UL << TCP_WRITE_TIMER_DEFERRED) |   \
789                           (1UL << TCP_DELACK_TIMER_DEFERRED) |  \
790                           (1UL << TCP_MTU_REDUCED_DEFERRED))
791 /**
792  * tcp_release_cb - tcp release_sock() callback
793  * @sk: socket
794  *
795  * called from release_sock() to perform protocol dependent
796  * actions before socket release.
797  */
798 void tcp_release_cb(struct sock *sk)
799 {
800         struct tcp_sock *tp = tcp_sk(sk);
801         unsigned long flags, nflags;
802 
803         /* perform an atomic operation only if at least one flag is set */
804         do {
805                 flags = tp->tsq_flags;
806                 if (!(flags & TCP_DEFERRED_ALL))
807                         return;
808                 nflags = flags & ~TCP_DEFERRED_ALL;
809         } while (cmpxchg(&tp->tsq_flags, flags, nflags) != flags);
810 
811         if (flags & (1UL << TCP_TSQ_DEFERRED))
812                 tcp_tsq_handler(sk);
813 
814         /* Here begins the tricky part :
815          * We are called from release_sock() with :
816          * 1) BH disabled
817          * 2) sk_lock.slock spinlock held
818          * 3) socket owned by us (sk->sk_lock.owned == 1)
819          *
820          * But following code is meant to be called from BH handlers,
821          * so we should keep BH disabled, but early release socket ownership
822          */
823         sock_release_ownership(sk);
824 
825         if (flags & (1UL << TCP_WRITE_TIMER_DEFERRED)) {
826                 tcp_write_timer_handler(sk);
827                 __sock_put(sk);
828         }
829         if (flags & (1UL << TCP_DELACK_TIMER_DEFERRED)) {
830                 tcp_delack_timer_handler(sk);
831                 __sock_put(sk);
832         }
833         if (flags & (1UL << TCP_MTU_REDUCED_DEFERRED)) {
834                 inet_csk(sk)->icsk_af_ops->mtu_reduced(sk);
835                 __sock_put(sk);
836         }
837 }
838 EXPORT_SYMBOL(tcp_release_cb);
839 
840 void __init tcp_tasklet_init(void)
841 {
842         int i;
843 
844         for_each_possible_cpu(i) {
845                 struct tsq_tasklet *tsq = &per_cpu(tsq_tasklet, i);
846 
847                 INIT_LIST_HEAD(&tsq->head);
848                 tasklet_init(&tsq->tasklet,
849                              tcp_tasklet_func,
850                              (unsigned long)tsq);
851         }
852 }
853 
854 /*
855  * Write buffer destructor automatically called from kfree_skb.
856  * We can't xmit new skbs from this context, as we might already
857  * hold qdisc lock.
858  */
859 void tcp_wfree(struct sk_buff *skb)
860 {
861         struct sock *sk = skb->sk;
862         struct tcp_sock *tp = tcp_sk(sk);
863         int wmem;
864 
865         /* Keep one reference on sk_wmem_alloc.
866          * Will be released by sk_free() from here or tcp_tasklet_func()
867          */
868         wmem = atomic_sub_return(skb->truesize - 1, &sk->sk_wmem_alloc);
869 
870         /* If this softirq is serviced by ksoftirqd, we are likely under stress.
871          * Wait until our queues (qdisc + devices) are drained.
872          * This gives :
873          * - less callbacks to tcp_write_xmit(), reducing stress (batches)
874          * - chance for incoming ACK (processed by another cpu maybe)
875          *   to migrate this flow (skb->ooo_okay will be eventually set)
876          */
877         if (wmem >= SKB_TRUESIZE(1) && this_cpu_ksoftirqd() == current)
878                 goto out;
879 
880         if (test_and_clear_bit(TSQ_THROTTLED, &tp->tsq_flags) &&
881             !test_and_set_bit(TSQ_QUEUED, &tp->tsq_flags)) {
882                 unsigned long flags;
883                 struct tsq_tasklet *tsq;
884 
885                 /* queue this socket to tasklet queue */
886                 local_irq_save(flags);
887                 tsq = this_cpu_ptr(&tsq_tasklet);
888                 list_add(&tp->tsq_node, &tsq->head);
889                 tasklet_schedule(&tsq->tasklet);
890                 local_irq_restore(flags);
891                 return;
892         }
893 out:
894         sk_free(sk);
895 }
896 
897 /* This routine actually transmits TCP packets queued in by
898  * tcp_do_sendmsg().  This is used by both the initial
899  * transmission and possible later retransmissions.
900  * All SKB's seen here are completely headerless.  It is our
901  * job to build the TCP header, and pass the packet down to
902  * IP so it can do the same plus pass the packet off to the
903  * device.
904  *
905  * We are working here with either a clone of the original
906  * SKB, or a fresh unique copy made by the retransmit engine.
907  */
908 static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
909                             gfp_t gfp_mask)
910 {
911         const struct inet_connection_sock *icsk = inet_csk(sk);
912         struct inet_sock *inet;
913         struct tcp_sock *tp;
914         struct tcp_skb_cb *tcb;
915         struct tcp_out_options opts;
916         unsigned int tcp_options_size, tcp_header_size;
917         struct tcp_md5sig_key *md5;
918         struct tcphdr *th;
919         int err;
920 
921         BUG_ON(!skb || !tcp_skb_pcount(skb));
922         tp = tcp_sk(sk);
923 
924         if (clone_it) {
925                 skb_mstamp_get(&skb->skb_mstamp);
926                 TCP_SKB_CB(skb)->tx.in_flight = TCP_SKB_CB(skb)->end_seq
927                         - tp->snd_una;
928                 tcp_rate_skb_sent(sk, skb);
929 
930                 if (unlikely(skb_cloned(skb)))
931                         skb = pskb_copy(skb, gfp_mask);
932                 else
933                         skb = skb_clone(skb, gfp_mask);
934                 if (unlikely(!skb))
935                         return -ENOBUFS;
936         }
937 
938         inet = inet_sk(sk);
939         tcb = TCP_SKB_CB(skb);
940         memset(&opts, 0, sizeof(opts));
941 
942         if (unlikely(tcb->tcp_flags & TCPHDR_SYN))
943                 tcp_options_size = tcp_syn_options(sk, skb, &opts, &md5);
944         else
945                 tcp_options_size = tcp_established_options(sk, skb, &opts,
946                                                            &md5);
947         tcp_header_size = tcp_options_size + sizeof(struct tcphdr);
948 
949         /* if no packet is in qdisc/device queue, then allow XPS to select
950          * another queue. We can be called from tcp_tsq_handler()
951          * which holds one reference to sk_wmem_alloc.
952          *
953          * TODO: Ideally, in-flight pure ACK packets should not matter here.
954          * One way to get this would be to set skb->truesize = 2 on them.
955          */
956         skb->ooo_okay = sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1);
957 
958         skb_push(skb, tcp_header_size);
959         skb_reset_transport_header(skb);
960 
961         skb_orphan(skb);
962         skb->sk = sk;
963         skb->destructor = skb_is_tcp_pure_ack(skb) ? __sock_wfree : tcp_wfree;
964         skb_set_hash_from_sk(skb, sk);
965         atomic_add(skb->truesize, &sk->sk_wmem_alloc);
966 
967         /* Build TCP header and checksum it. */
968         th = (struct tcphdr *)skb->data;
969         th->source              = inet->inet_sport;
970         th->dest                = inet->inet_dport;
971         th->seq                 = htonl(tcb->seq);
972         th->ack_seq             = htonl(tp->rcv_nxt);
973         *(((__be16 *)th) + 6)   = htons(((tcp_header_size >> 2) << 12) |
974                                         tcb->tcp_flags);
975 
976         th->check               = 0;
977         th->urg_ptr             = 0;
978 
979         /* The urg_mode check is necessary during a below snd_una win probe */
980         if (unlikely(tcp_urg_mode(tp) && before(tcb->seq, tp->snd_up))) {
981                 if (before(tp->snd_up, tcb->seq + 0x10000)) {
982                         th->urg_ptr = htons(tp->snd_up - tcb->seq);
983                         th->urg = 1;
984                 } else if (after(tcb->seq + 0xFFFF, tp->snd_nxt)) {
985                         th->urg_ptr = htons(0xFFFF);
986                         th->urg = 1;
987                 }
988         }
989 
990         tcp_options_write((__be32 *)(th + 1), tp, &opts);
991         skb_shinfo(skb)->gso_type = sk->sk_gso_type;
992         if (likely(!(tcb->tcp_flags & TCPHDR_SYN))) {
993                 th->window      = htons(tcp_select_window(sk));
994                 tcp_ecn_send(sk, skb, th, tcp_header_size);
995         } else {
996                 /* RFC1323: The window in SYN & SYN/ACK segments
997                  * is never scaled.
998                  */
999                 th->window      = htons(min(tp->rcv_wnd, 65535U));
1000         }
1001 #ifdef CONFIG_TCP_MD5SIG
1002         /* Calculate the MD5 hash, as we have all we need now */
1003         if (md5) {
1004                 sk_nocaps_add(sk, NETIF_F_GSO_MASK);
1005                 tp->af_specific->calc_md5_hash(opts.hash_location,
1006                                                md5, sk, skb);
1007         }
1008 #endif
1009 
1010         icsk->icsk_af_ops->send_check(sk, skb);
1011 
1012         if (likely(tcb->tcp_flags & TCPHDR_ACK))
1013                 tcp_event_ack_sent(sk, tcp_skb_pcount(skb));
1014 
1015         if (skb->len != tcp_header_size) {
1016                 tcp_event_data_sent(tp, sk);
1017                 tp->data_segs_out += tcp_skb_pcount(skb);
1018         }
1019 
1020         if (after(tcb->end_seq, tp->snd_nxt) || tcb->seq == tcb->end_seq)
1021                 TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS,
1022                               tcp_skb_pcount(skb));
1023 
1024         tp->segs_out += tcp_skb_pcount(skb);
1025         /* OK, its time to fill skb_shinfo(skb)->gso_{segs|size} */
1026         skb_shinfo(skb)->gso_segs = tcp_skb_pcount(skb);
1027         skb_shinfo(skb)->gso_size = tcp_skb_mss(skb);
1028 
1029         /* Our usage of tstamp should remain private */
1030         skb->tstamp.tv64 = 0;
1031 
1032         /* Cleanup our debris for IP stacks */
1033         memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
1034                                sizeof(struct inet6_skb_parm)));
1035 
1036         err = icsk->icsk_af_ops->queue_xmit(sk, skb, &inet->cork.fl);
1037 
1038         if (likely(err <= 0))
1039                 return err;
1040 
1041         tcp_enter_cwr(sk);
1042 
1043         return net_xmit_eval(err);
1044 }
1045 
1046 /* This routine just queues the buffer for sending.
1047  *
1048  * NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
1049  * otherwise socket can stall.
1050  */
1051 static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
1052 {
1053         struct tcp_sock *tp = tcp_sk(sk);
1054 
1055         /* Advance write_seq and place onto the write_queue. */
1056         tp->write_seq = TCP_SKB_CB(skb)->end_seq;
1057         __skb_header_release(skb);
1058         tcp_add_write_queue_tail(sk, skb);
1059         sk->sk_wmem_queued += skb->truesize;
1060         sk_mem_charge(sk, skb->truesize);
1061 }
1062 
1063 /* Initialize TSO segments for a packet. */
1064 static void tcp_set_skb_tso_segs(struct sk_buff *skb, unsigned int mss_now)
1065 {
1066         if (skb->len <= mss_now || skb->ip_summed == CHECKSUM_NONE) {
1067                 /* Avoid the costly divide in the normal
1068                  * non-TSO case.
1069                  */
1070                 tcp_skb_pcount_set(skb, 1);
1071                 TCP_SKB_CB(skb)->tcp_gso_size = 0;
1072         } else {
1073                 tcp_skb_pcount_set(skb, DIV_ROUND_UP(skb->len, mss_now));
1074                 TCP_SKB_CB(skb)->tcp_gso_size = mss_now;
1075         }
1076 }
1077 
1078 /* When a modification to fackets out becomes necessary, we need to check
1079  * skb is counted to fackets_out or not.
1080  */
1081 static void tcp_adjust_fackets_out(struct sock *sk, const struct sk_buff *skb,
1082                                    int decr)
1083 {
1084         struct tcp_sock *tp = tcp_sk(sk);
1085 
1086         if (!tp->sacked_out || tcp_is_reno(tp))
1087                 return;
1088 
1089         if (after(tcp_highest_sack_seq(tp), TCP_SKB_CB(skb)->seq))
1090                 tp->fackets_out -= decr;
1091 }
1092 
1093 /* Pcount in the middle of the write queue got changed, we need to do various
1094  * tweaks to fix counters
1095  */
1096 static void tcp_adjust_pcount(struct sock *sk, const struct sk_buff *skb, int decr)
1097 {
1098         struct tcp_sock *tp = tcp_sk(sk);
1099 
1100         tp->packets_out -= decr;
1101 
1102         if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
1103                 tp->sacked_out -= decr;
1104         if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
1105                 tp->retrans_out -= decr;
1106         if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
1107                 tp->lost_out -= decr;
1108 
1109         /* Reno case is special. Sigh... */
1110         if (tcp_is_reno(tp) && decr > 0)
1111                 tp->sacked_out -= min_t(u32, tp->sacked_out, decr);
1112 
1113         tcp_adjust_fackets_out(sk, skb, decr);
1114 
1115         if (tp->lost_skb_hint &&
1116             before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(tp->lost_skb_hint)->seq) &&
1117             (tcp_is_fack(tp) || (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)))
1118                 tp->lost_cnt_hint -= decr;
1119 
1120         tcp_verify_left_out(tp);
1121 }
1122 
1123 static bool tcp_has_tx_tstamp(const struct sk_buff *skb)
1124 {
1125         return TCP_SKB_CB(skb)->txstamp_ack ||
1126                 (skb_shinfo(skb)->tx_flags & SKBTX_ANY_TSTAMP);
1127 }
1128 
1129 static void tcp_fragment_tstamp(struct sk_buff *skb, struct sk_buff *skb2)
1130 {
1131         struct skb_shared_info *shinfo = skb_shinfo(skb);
1132 
1133         if (unlikely(tcp_has_tx_tstamp(skb)) &&
1134             !before(shinfo->tskey, TCP_SKB_CB(skb2)->seq)) {
1135                 struct skb_shared_info *shinfo2 = skb_shinfo(skb2);
1136                 u8 tsflags = shinfo->tx_flags & SKBTX_ANY_TSTAMP;
1137 
1138                 shinfo->tx_flags &= ~tsflags;
1139                 shinfo2->tx_flags |= tsflags;
1140                 swap(shinfo->tskey, shinfo2->tskey);
1141                 TCP_SKB_CB(skb2)->txstamp_ack = TCP_SKB_CB(skb)->txstamp_ack;
1142                 TCP_SKB_CB(skb)->txstamp_ack = 0;
1143         }
1144 }
1145 
1146 static void tcp_skb_fragment_eor(struct sk_buff *skb, struct sk_buff *skb2)
1147 {
1148         TCP_SKB_CB(skb2)->eor = TCP_SKB_CB(skb)->eor;
1149         TCP_SKB_CB(skb)->eor = 0;
1150 }
1151 
1152 /* Function to create two new TCP segments.  Shrinks the given segment
1153  * to the specified size and appends a new segment with the rest of the
1154  * packet to the list.  This won't be called frequently, I hope.
1155  * Remember, these are still headerless SKBs at this point.
1156  */
1157 int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len,
1158                  unsigned int mss_now, gfp_t gfp)
1159 {
1160         struct tcp_sock *tp = tcp_sk(sk);
1161         struct sk_buff *buff;
1162         int nsize, old_factor;
1163         int nlen;
1164         u8 flags;
1165 
1166         if (WARN_ON(len > skb->len))
1167                 return -EINVAL;
1168 
1169         nsize = skb_headlen(skb) - len;
1170         if (nsize < 0)
1171                 nsize = 0;
1172 
1173         if (skb_unclone(skb, gfp))
1174                 return -ENOMEM;
1175 
1176         /* Get a new skb... force flag on. */
1177         buff = sk_stream_alloc_skb(sk, nsize, gfp, true);
1178         if (!buff)
1179                 return -ENOMEM; /* We'll just try again later. */
1180 
1181         sk->sk_wmem_queued += buff->truesize;
1182         sk_mem_charge(sk, buff->truesize);
1183         nlen = skb->len - len - nsize;
1184         buff->truesize += nlen;
1185         skb->truesize -= nlen;
1186 
1187         /* Correct the sequence numbers. */
1188         TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
1189         TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
1190         TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;
1191 
1192         /* PSH and FIN should only be set in the second packet. */
1193         flags = TCP_SKB_CB(skb)->tcp_flags;
1194         TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
1195         TCP_SKB_CB(buff)->tcp_flags = flags;
1196         TCP_SKB_CB(buff)->sacked = TCP_SKB_CB(skb)->sacked;
1197         tcp_skb_fragment_eor(skb, buff);
1198 
1199         if (!skb_shinfo(skb)->nr_frags && skb->ip_summed != CHECKSUM_PARTIAL) {
1200                 /* Copy and checksum data tail into the new buffer. */
1201                 buff->csum = csum_partial_copy_nocheck(skb->data + len,
1202                                                        skb_put(buff, nsize),
1203                                                        nsize, 0);
1204 
1205                 skb_trim(skb, len);
1206 
1207                 skb->csum = csum_block_sub(skb->csum, buff->csum, len);
1208         } else {
1209                 skb->ip_summed = CHECKSUM_PARTIAL;
1210                 skb_split(skb, buff, len);
1211         }
1212 
1213         buff->ip_summed = skb->ip_summed;
1214 
1215         buff->tstamp = skb->tstamp;
1216         tcp_fragment_tstamp(skb, buff);
1217 
1218         old_factor = tcp_skb_pcount(skb);
1219 
1220         /* Fix up tso_factor for both original and new SKB.  */
1221         tcp_set_skb_tso_segs(skb, mss_now);
1222         tcp_set_skb_tso_segs(buff, mss_now);
1223 
1224         /* Update delivered info for the new segment */
1225         TCP_SKB_CB(buff)->tx = TCP_SKB_CB(skb)->tx;
1226 
1227         /* If this packet has been sent out already, we must
1228          * adjust the various packet counters.
1229          */
1230         if (!before(tp->snd_nxt, TCP_SKB_CB(buff)->end_seq)) {
1231                 int diff = old_factor - tcp_skb_pcount(skb) -
1232                         tcp_skb_pcount(buff);
1233 
1234                 if (diff)
1235                         tcp_adjust_pcount(sk, skb, diff);
1236         }
1237 
1238         /* Link BUFF into the send queue. */
1239         __skb_header_release(buff);
1240         tcp_insert_write_queue_after(skb, buff, sk);
1241 
1242         return 0;
1243 }
1244 
1245 /* This is similar to __pskb_pull_head() (it will go to core/skbuff.c
1246  * eventually). The difference is that pulled data not copied, but
1247  * immediately discarded.
1248  */
1249 static void __pskb_trim_head(struct sk_buff *skb, int len)
1250 {
1251         struct skb_shared_info *shinfo;
1252         int i, k, eat;
1253 
1254         eat = min_t(int, len, skb_headlen(skb));
1255         if (eat) {
1256                 __skb_pull(skb, eat);
1257                 len -= eat;
1258                 if (!len)
1259                         return;
1260         }
1261         eat = len;
1262         k = 0;
1263         shinfo = skb_shinfo(skb);
1264         for (i = 0; i < shinfo->nr_frags; i++) {
1265                 int size = skb_frag_size(&shinfo->frags[i]);
1266 
1267                 if (size <= eat) {
1268                         skb_frag_unref(skb, i);
1269                         eat -= size;
1270                 } else {
1271                         shinfo->frags[k] = shinfo->frags[i];
1272                         if (eat) {
1273                                 shinfo->frags[k].page_offset += eat;
1274                                 skb_frag_size_sub(&shinfo->frags[k], eat);
1275                                 eat = 0;
1276                         }
1277                         k++;
1278                 }
1279         }
1280         shinfo->nr_frags = k;
1281 
1282         skb_reset_tail_pointer(skb);
1283         skb->data_len -= len;
1284         skb->len = skb->data_len;
1285 }
1286 
1287 /* Remove acked data from a packet in the transmit queue. */
1288 int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
1289 {
1290         if (skb_unclone(skb, GFP_ATOMIC))
1291                 return -ENOMEM;
1292 
1293         __pskb_trim_head(skb, len);
1294 
1295         TCP_SKB_CB(skb)->seq += len;
1296         skb->ip_summed = CHECKSUM_PARTIAL;
1297 
1298         skb->truesize        -= len;
1299         sk->sk_wmem_queued   -= len;
1300         sk_mem_uncharge(sk, len);
1301         sock_set_flag(sk, SOCK_QUEUE_SHRUNK);
1302 
1303         /* Any change of skb->len requires recalculation of tso factor. */
1304         if (tcp_skb_pcount(skb) > 1)
1305                 tcp_set_skb_tso_segs(skb, tcp_skb_mss(skb));
1306 
1307         return 0;
1308 }
1309 
1310 /* Calculate MSS not accounting any TCP options.  */
1311 static inline int __tcp_mtu_to_mss(struct sock *sk, int pmtu)
1312 {
1313         const struct tcp_sock *tp = tcp_sk(sk);
1314         const struct inet_connection_sock *icsk = inet_csk(sk);
1315         int mss_now;
1316 
1317         /* Calculate base mss without TCP options:
1318            It is MMS_S - sizeof(tcphdr) of rfc1122
1319          */
1320         mss_now = pmtu - icsk->icsk_af_ops->net_header_len - sizeof(struct tcphdr);
1321 
1322         /* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */
1323         if (icsk->icsk_af_ops->net_frag_header_len) {
1324                 const struct dst_entry *dst = __sk_dst_get(sk);
1325 
1326                 if (dst && dst_allfrag(dst))
1327                         mss_now -= icsk->icsk_af_ops->net_frag_header_len;
1328         }
1329 
1330         /* Clamp it (mss_clamp does not include tcp options) */
1331         if (mss_now > tp->rx_opt.mss_clamp)
1332                 mss_now = tp->rx_opt.mss_clamp;
1333 
1334         /* Now subtract optional transport overhead */
1335         mss_now -= icsk->icsk_ext_hdr_len;
1336 
1337         /* Then reserve room for full set of TCP options and 8 bytes of data */
1338         if (mss_now < 48)
1339                 mss_now = 48;
1340         return mss_now;
1341 }
1342 
1343 /* Calculate MSS. Not accounting for SACKs here.  */
1344 int tcp_mtu_to_mss(struct sock *sk, int pmtu)
1345 {
1346         /* Subtract TCP options size, not including SACKs */
1347         return __tcp_mtu_to_mss(sk, pmtu) -
1348                (tcp_sk(sk)->tcp_header_len - sizeof(struct tcphdr));
1349 }
1350 
1351 /* Inverse of above */
1352 int tcp_mss_to_mtu(struct sock *sk, int mss)
1353 {
1354         const struct tcp_sock *tp = tcp_sk(sk);
1355         const struct inet_connection_sock *icsk = inet_csk(sk);
1356         int mtu;
1357 
1358         mtu = mss +
1359               tp->tcp_header_len +
1360               icsk->icsk_ext_hdr_len +
1361               icsk->icsk_af_ops->net_header_len;
1362 
1363         /* IPv6 adds a frag_hdr in case RTAX_FEATURE_ALLFRAG is set */
1364         if (icsk->icsk_af_ops->net_frag_header_len) {
1365                 const struct dst_entry *dst = __sk_dst_get(sk);
1366 
1367                 if (dst && dst_allfrag(dst))
1368                         mtu += icsk->icsk_af_ops->net_frag_header_len;
1369         }
1370         return mtu;
1371 }
1372 EXPORT_SYMBOL(tcp_mss_to_mtu);
1373 
1374 /* MTU probing init per socket */
1375 void tcp_mtup_init(struct sock *sk)
1376 {
1377         struct tcp_sock *tp = tcp_sk(sk);
1378         struct inet_connection_sock *icsk = inet_csk(sk);
1379         struct net *net = sock_net(sk);
1380 
1381         icsk->icsk_mtup.enabled = net->ipv4.sysctl_tcp_mtu_probing > 1;
1382         icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp + sizeof(struct tcphdr) +
1383                                icsk->icsk_af_ops->net_header_len;
1384         icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, net->ipv4.sysctl_tcp_base_mss);
1385         icsk->icsk_mtup.probe_size = 0;
1386         if (icsk->icsk_mtup.enabled)
1387                 icsk->icsk_mtup.probe_timestamp = tcp_time_stamp;
1388 }
1389 EXPORT_SYMBOL(tcp_mtup_init);
1390 
1391 /* This function synchronize snd mss to current pmtu/exthdr set.
1392 
1393    tp->rx_opt.user_mss is mss set by user by TCP_MAXSEG. It does NOT counts
1394    for TCP options, but includes only bare TCP header.
1395 
1396    tp->rx_opt.mss_clamp is mss negotiated at connection setup.
1397    It is minimum of user_mss and mss received with SYN.
1398    It also does not include TCP options.
1399 
1400    inet_csk(sk)->icsk_pmtu_cookie is last pmtu, seen by this function.
1401 
1402    tp->mss_cache is current effective sending mss, including
1403    all tcp options except for SACKs. It is evaluated,
1404    taking into account current pmtu, but never exceeds
1405    tp->rx_opt.mss_clamp.
1406 
1407    NOTE1. rfc1122 clearly states that advertised MSS
1408    DOES NOT include either tcp or ip options.
1409 
1410    NOTE2. inet_csk(sk)->icsk_pmtu_cookie and tp->mss_cache
1411    are READ ONLY outside this function.         --ANK (980731)
1412  */
1413 unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)
1414 {
1415         struct tcp_sock *tp = tcp_sk(sk);
1416         struct inet_connection_sock *icsk = inet_csk(sk);
1417         int mss_now;
1418 
1419         if (icsk->icsk_mtup.search_high > pmtu)
1420                 icsk->icsk_mtup.search_high = pmtu;
1421 
1422         mss_now = tcp_mtu_to_mss(sk, pmtu);
1423         mss_now = tcp_bound_to_half_wnd(tp, mss_now);
1424 
1425         /* And store cached results */
1426         icsk->icsk_pmtu_cookie = pmtu;
1427         if (icsk->icsk_mtup.enabled)
1428                 mss_now = min(mss_now, tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low));
1429         tp->mss_cache = mss_now;
1430 
1431         return mss_now;
1432 }
1433 EXPORT_SYMBOL(tcp_sync_mss);
1434 
1435 /* Compute the current effective MSS, taking SACKs and IP options,
1436  * and even PMTU discovery events into account.
1437  */
1438 unsigned int tcp_current_mss(struct sock *sk)
1439 {
1440         const struct tcp_sock *tp = tcp_sk(sk);
1441         const struct dst_entry *dst = __sk_dst_get(sk);
1442         u32 mss_now;
1443         unsigned int header_len;
1444         struct tcp_out_options opts;
1445         struct tcp_md5sig_key *md5;
1446 
1447         mss_now = tp->mss_cache;
1448 
1449         if (dst) {
1450                 u32 mtu = dst_mtu(dst);
1451                 if (mtu != inet_csk(sk)->icsk_pmtu_cookie)
1452                         mss_now = tcp_sync_mss(sk, mtu);
1453         }
1454 
1455         header_len = tcp_established_options(sk, NULL, &opts, &md5) +
1456                      sizeof(struct tcphdr);
1457         /* The mss_cache is sized based on tp->tcp_header_len, which assumes
1458          * some common options. If this is an odd packet (because we have SACK
1459          * blocks etc) then our calculated header_len will be different, and
1460          * we have to adjust mss_now correspondingly */
1461         if (header_len != tp->tcp_header_len) {
1462                 int delta = (int) header_len - tp->tcp_header_len;
1463                 mss_now -= delta;
1464         }
1465 
1466         return mss_now;
1467 }
1468 
1469 /* RFC2861, slow part. Adjust cwnd, after it was not full during one rto.
1470  * As additional protections, we do not touch cwnd in retransmission phases,
1471  * and if application hit its sndbuf limit recently.
1472  */
1473 static void tcp_cwnd_application_limited(struct sock *sk)
1474 {
1475         struct tcp_sock *tp = tcp_sk(sk);
1476 
1477         if (inet_csk(sk)->icsk_ca_state == TCP_CA_Open &&
1478             sk->sk_socket && !test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
1479                 /* Limited by application or receiver window. */
1480                 u32 init_win = tcp_init_cwnd(tp, __sk_dst_get(sk));
1481                 u32 win_used = max(tp->snd_cwnd_used, init_win);
1482                 if (win_used < tp->snd_cwnd) {
1483                         tp->snd_ssthresh = tcp_current_ssthresh(sk);
1484                         tp->snd_cwnd = (tp->snd_cwnd + win_used) >> 1;
1485                 }
1486                 tp->snd_cwnd_used = 0;
1487         }
1488         tp->snd_cwnd_stamp = tcp_time_stamp;
1489 }
1490 
1491 static void tcp_cwnd_validate(struct sock *sk, bool is_cwnd_limited)
1492 {
1493         struct tcp_sock *tp = tcp_sk(sk);
1494 
1495         /* Track the maximum number of outstanding packets in each
1496          * window, and remember whether we were cwnd-limited then.
1497          */
1498         if (!before(tp->snd_una, tp->max_packets_seq) ||
1499             tp->packets_out > tp->max_packets_out) {
1500                 tp->max_packets_out = tp->packets_out;
1501                 tp->max_packets_seq = tp->snd_nxt;
1502                 tp->is_cwnd_limited = is_cwnd_limited;
1503         }
1504 
1505         if (tcp_is_cwnd_limited(sk)) {
1506                 /* Network is feed fully. */
1507                 tp->snd_cwnd_used = 0;
1508                 tp->snd_cwnd_stamp = tcp_time_stamp;
1509         } else {
1510                 /* Network starves. */
1511                 if (tp->packets_out > tp->snd_cwnd_used)
1512                         tp->snd_cwnd_used = tp->packets_out;
1513 
1514                 if (sysctl_tcp_slow_start_after_idle &&
1515                     (s32)(tcp_time_stamp - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto)
1516                         tcp_cwnd_application_limited(sk);
1517         }
1518 }
1519 
1520 /* Minshall's variant of the Nagle send check. */
1521 static bool tcp_minshall_check(const struct tcp_sock *tp)
1522 {
1523         return after(tp->snd_sml, tp->snd_una) &&
1524                 !after(tp->snd_sml, tp->snd_nxt);
1525 }
1526 
1527 /* Update snd_sml if this skb is under mss
1528  * Note that a TSO packet might end with a sub-mss segment
1529  * The test is really :
1530  * if ((skb->len % mss) != 0)
1531  *        tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
1532  * But we can avoid doing the divide again given we already have
1533  *  skb_pcount = skb->len / mss_now
1534  */
1535 static void tcp_minshall_update(struct tcp_sock *tp, unsigned int mss_now,
1536                                 const struct sk_buff *skb)
1537 {
1538         if (skb->len < tcp_skb_pcount(skb) * mss_now)
1539                 tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
1540 }
1541 
1542 /* Return false, if packet can be sent now without violation Nagle's rules:
1543  * 1. It is full sized. (provided by caller in %partial bool)
1544  * 2. Or it contains FIN. (already checked by caller)
1545  * 3. Or TCP_CORK is not set, and TCP_NODELAY is set.
1546  * 4. Or TCP_CORK is not set, and all sent packets are ACKed.
1547  *    With Minshall's modification: all sent small packets are ACKed.
1548  */
1549 static bool tcp_nagle_check(bool partial, const struct tcp_sock *tp,
1550                             int nonagle)
1551 {
1552         return partial &&
1553                 ((nonagle & TCP_NAGLE_CORK) ||
1554                  (!nonagle && tp->packets_out && tcp_minshall_check(tp)));
1555 }
1556 
1557 /* Return how many segs we'd like on a TSO packet,
1558  * to send one TSO packet per ms
1559  */
1560 u32 tcp_tso_autosize(const struct sock *sk, unsigned int mss_now,
1561                      int min_tso_segs)
1562 {
1563         u32 bytes, segs;
1564 
1565         bytes = min(sk->sk_pacing_rate >> 10,
1566                     sk->sk_gso_max_size - 1 - MAX_TCP_HEADER);
1567 
1568         /* Goal is to send at least one packet per ms,
1569          * not one big TSO packet every 100 ms.
1570          * This preserves ACK clocking and is consistent
1571          * with tcp_tso_should_defer() heuristic.
1572          */
1573         segs = max_t(u32, bytes / mss_now, min_tso_segs);
1574 
1575         return min_t(u32, segs, sk->sk_gso_max_segs);
1576 }
1577 EXPORT_SYMBOL(tcp_tso_autosize);
1578 
1579 /* Return the number of segments we want in the skb we are transmitting.
1580  * See if congestion control module wants to decide; otherwise, autosize.
1581  */
1582 static u32 tcp_tso_segs(struct sock *sk, unsigned int mss_now)
1583 {
1584         const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
1585         u32 tso_segs = ca_ops->tso_segs_goal ? ca_ops->tso_segs_goal(sk) : 0;
1586 
1587         return tso_segs ? :
1588                 tcp_tso_autosize(sk, mss_now, sysctl_tcp_min_tso_segs);
1589 }
1590 
1591 /* Returns the portion of skb which can be sent right away */
1592 static unsigned int tcp_mss_split_point(const struct sock *sk,
1593                                         const struct sk_buff *skb,
1594                                         unsigned int mss_now,
1595                                         unsigned int max_segs,
1596                                         int nonagle)
1597 {
1598         const struct tcp_sock *tp = tcp_sk(sk);
1599         u32 partial, needed, window, max_len;
1600 
1601         window = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
1602         max_len = mss_now * max_segs;
1603 
1604         if (likely(max_len <= window && skb != tcp_write_queue_tail(sk)))
1605                 return max_len;
1606 
1607         needed = min(skb->len, window);
1608 
1609         if (max_len <= needed)
1610                 return max_len;
1611 
1612         partial = needed % mss_now;
1613         /* If last segment is not a full MSS, check if Nagle rules allow us
1614          * to include this last segment in this skb.
1615          * Otherwise, we'll split the skb at last MSS boundary
1616          */
1617         if (tcp_nagle_check(partial != 0, tp, nonagle))
1618                 return needed - partial;
1619 
1620         return needed;
1621 }
1622 
1623 /* Can at least one segment of SKB be sent right now, according to the
1624  * congestion window rules?  If so, return how many segments are allowed.
1625  */
1626 static inline unsigned int tcp_cwnd_test(const struct tcp_sock *tp,
1627                                          const struct sk_buff *skb)
1628 {
1629         u32 in_flight, cwnd, halfcwnd;
1630 
1631         /* Don't be strict about the congestion window for the final FIN.  */
1632         if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) &&
1633             tcp_skb_pcount(skb) == 1)
1634                 return 1;
1635 
1636         in_flight = tcp_packets_in_flight(tp);
1637         cwnd = tp->snd_cwnd;
1638         if (in_flight >= cwnd)
1639                 return 0;
1640 
1641         /* For better scheduling, ensure we have at least
1642          * 2 GSO packets in flight.
1643          */
1644         halfcwnd = max(cwnd >> 1, 1U);
1645         return min(halfcwnd, cwnd - in_flight);
1646 }
1647 
1648 /* Initialize TSO state of a skb.
1649  * This must be invoked the first time we consider transmitting
1650  * SKB onto the wire.
1651  */
1652 static int tcp_init_tso_segs(struct sk_buff *skb, unsigned int mss_now)
1653 {
1654         int tso_segs = tcp_skb_pcount(skb);
1655 
1656         if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now)) {
1657                 tcp_set_skb_tso_segs(skb, mss_now);
1658                 tso_segs = tcp_skb_pcount(skb);
1659         }
1660         return tso_segs;
1661 }
1662 
1663 
1664 /* Return true if the Nagle test allows this packet to be
1665  * sent now.
1666  */
1667 static inline bool tcp_nagle_test(const struct tcp_sock *tp, const struct sk_buff *skb,
1668                                   unsigned int cur_mss, int nonagle)
1669 {
1670         /* Nagle rule does not apply to frames, which sit in the middle of the
1671          * write_queue (they have no chances to get new data).
1672          *
1673          * This is implemented in the callers, where they modify the 'nonagle'
1674          * argument based upon the location of SKB in the send queue.
1675          */
1676         if (nonagle & TCP_NAGLE_PUSH)
1677                 return true;
1678 
1679         /* Don't use the nagle rule for urgent data (or for the final FIN). */
1680         if (tcp_urg_mode(tp) || (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN))
1681                 return true;
1682 
1683         if (!tcp_nagle_check(skb->len < cur_mss, tp, nonagle))
1684                 return true;
1685 
1686         return false;
1687 }
1688 
1689 /* Does at least the first segment of SKB fit into the send window? */
1690 static bool tcp_snd_wnd_test(const struct tcp_sock *tp,
1691                              const struct sk_buff *skb,
1692                              unsigned int cur_mss)
1693 {
1694         u32 end_seq = TCP_SKB_CB(skb)->end_seq;
1695 
1696         if (skb->len > cur_mss)
1697                 end_seq = TCP_SKB_CB(skb)->seq + cur_mss;
1698 
1699         return !after(end_seq, tcp_wnd_end(tp));
1700 }
1701 
1702 /* This checks if the data bearing packet SKB (usually tcp_send_head(sk))
1703  * should be put on the wire right now.  If so, it returns the number of
1704  * packets allowed by the congestion window.
1705  */
1706 static unsigned int tcp_snd_test(const struct sock *sk, struct sk_buff *skb,
1707                                  unsigned int cur_mss, int nonagle)
1708 {
1709         const struct tcp_sock *tp = tcp_sk(sk);
1710         unsigned int cwnd_quota;
1711 
1712         tcp_init_tso_segs(skb, cur_mss);
1713 
1714         if (!tcp_nagle_test(tp, skb, cur_mss, nonagle))
1715                 return 0;
1716 
1717         cwnd_quota = tcp_cwnd_test(tp, skb);
1718         if (cwnd_quota && !tcp_snd_wnd_test(tp, skb, cur_mss))
1719                 cwnd_quota = 0;
1720 
1721         return cwnd_quota;
1722 }
1723 
1724 /* Test if sending is allowed right now. */
1725 bool tcp_may_send_now(struct sock *sk)
1726 {
1727         const struct tcp_sock *tp = tcp_sk(sk);
1728         struct sk_buff *skb = tcp_send_head(sk);
1729 
1730         return skb &&
1731                 tcp_snd_test(sk, skb, tcp_current_mss(sk),
1732                              (tcp_skb_is_last(sk, skb) ?
1733                               tp->nonagle : TCP_NAGLE_PUSH));
1734 }
1735 
1736 /* Trim TSO SKB to LEN bytes, put the remaining data into a new packet
1737  * which is put after SKB on the list.  It is very much like
1738  * tcp_fragment() except that it may make several kinds of assumptions
1739  * in order to speed up the splitting operation.  In particular, we
1740  * know that all the data is in scatter-gather pages, and that the
1741  * packet has never been sent out before (and thus is not cloned).
1742  */
1743 static int tso_fragment(struct sock *sk, struct sk_buff *skb, unsigned int len,
1744                         unsigned int mss_now, gfp_t gfp)
1745 {
1746         struct sk_buff *buff;
1747         int nlen = skb->len - len;
1748         u8 flags;
1749 
1750         /* All of a TSO frame must be composed of paged data.  */
1751         if (skb->len != skb->data_len)
1752                 return tcp_fragment(sk, skb, len, mss_now, gfp);
1753 
1754         buff = sk_stream_alloc_skb(sk, 0, gfp, true);
1755         if (unlikely(!buff))
1756                 return -ENOMEM;
1757 
1758         sk->sk_wmem_queued += buff->truesize;
1759         sk_mem_charge(sk, buff->truesize);
1760         buff->truesize += nlen;
1761         skb->truesize -= nlen;
1762 
1763         /* Correct the sequence numbers. */
1764         TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
1765         TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
1766         TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;
1767 
1768         /* PSH and FIN should only be set in the second packet. */
1769         flags = TCP_SKB_CB(skb)->tcp_flags;
1770         TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
1771         TCP_SKB_CB(buff)->tcp_flags = flags;
1772 
1773         /* This packet was never sent out yet, so no SACK bits. */
1774         TCP_SKB_CB(buff)->sacked = 0;
1775 
1776         tcp_skb_fragment_eor(skb, buff);
1777 
1778         buff->ip_summed = skb->ip_summed = CHECKSUM_PARTIAL;
1779         skb_split(skb, buff, len);
1780         tcp_fragment_tstamp(skb, buff);
1781 
1782         /* Fix up tso_factor for both original and new SKB.  */
1783         tcp_set_skb_tso_segs(skb, mss_now);
1784         tcp_set_skb_tso_segs(buff, mss_now);
1785 
1786         /* Link BUFF into the send queue. */
1787         __skb_header_release(buff);
1788         tcp_insert_write_queue_after(skb, buff, sk);
1789 
1790         return 0;
1791 }
1792 
1793 /* Try to defer sending, if possible, in order to minimize the amount
1794  * of TSO splitting we do.  View it as a kind of TSO Nagle test.
1795  *
1796  * This algorithm is from John Heffner.
1797  */
1798 static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb,
1799                                  bool *is_cwnd_limited, u32 max_segs)
1800 {
1801         const struct inet_connection_sock *icsk = inet_csk(sk);
1802         u32 age, send_win, cong_win, limit, in_flight;
1803         struct tcp_sock *tp = tcp_sk(sk);
1804         struct skb_mstamp now;
1805         struct sk_buff *head;
1806         int win_divisor;
1807 
1808         if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
1809                 goto send_now;
1810 
1811         if (icsk->icsk_ca_state >= TCP_CA_Recovery)
1812                 goto send_now;
1813 
1814         /* Avoid bursty behavior by allowing defer
1815          * only if the last write was recent.
1816          */
1817         if ((s32)(tcp_time_stamp - tp->lsndtime) > 0)
1818                 goto send_now;
1819 
1820         in_flight = tcp_packets_in_flight(tp);
1821 
1822         BUG_ON(tcp_skb_pcount(skb) <= 1 || (tp->snd_cwnd <= in_flight));
1823 
1824         send_win = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
1825 
1826         /* From in_flight test above, we know that cwnd > in_flight.  */
1827         cong_win = (tp->snd_cwnd - in_flight) * tp->mss_cache;
1828 
1829         limit = min(send_win, cong_win);
1830 
1831         /* If a full-sized TSO skb can be sent, do it. */
1832         if (limit >= max_segs * tp->mss_cache)
1833                 goto send_now;
1834 
1835         /* Middle in queue won't get any more data, full sendable already? */
1836         if ((skb != tcp_write_queue_tail(sk)) && (limit >= skb->len))
1837                 goto send_now;
1838 
1839         win_divisor = ACCESS_ONCE(sysctl_tcp_tso_win_divisor);
1840         if (win_divisor) {
1841                 u32 chunk = min(tp->snd_wnd, tp->snd_cwnd * tp->mss_cache);
1842 
1843                 /* If at least some fraction of a window is available,
1844                  * just use it.
1845                  */
1846                 chunk /= win_divisor;
1847                 if (limit >= chunk)
1848                         goto send_now;
1849         } else {
1850                 /* Different approach, try not to defer past a single
1851                  * ACK.  Receiver should ACK every other full sized
1852                  * frame, so if we have space for more than 3 frames
1853                  * then send now.
1854                  */
1855                 if (limit > tcp_max_tso_deferred_mss(tp) * tp->mss_cache)
1856                         goto send_now;
1857         }
1858 
1859         head = tcp_write_queue_head(sk);
1860         skb_mstamp_get(&now);
1861         age = skb_mstamp_us_delta(&now, &head->skb_mstamp);
1862         /* If next ACK is likely to come too late (half srtt), do not defer */
1863         if (age < (tp->srtt_us >> 4))
1864                 goto send_now;
1865 
1866         /* Ok, it looks like it is advisable to defer. */
1867 
1868         if (cong_win < send_win && cong_win <= skb->len)
1869                 *is_cwnd_limited = true;
1870 
1871         return true;
1872 
1873 send_now:
1874         return false;
1875 }
1876 
1877 static inline void tcp_mtu_check_reprobe(struct sock *sk)
1878 {
1879         struct inet_connection_sock *icsk = inet_csk(sk);
1880         struct tcp_sock *tp = tcp_sk(sk);
1881         struct net *net = sock_net(sk);
1882         u32 interval;
1883         s32 delta;
1884 
1885         interval = net->ipv4.sysctl_tcp_probe_interval;
1886         delta = tcp_time_stamp - icsk->icsk_mtup.probe_timestamp;
1887         if (unlikely(delta >= interval * HZ)) {
1888                 int mss = tcp_current_mss(sk);
1889 
1890                 /* Update current search range */
1891                 icsk->icsk_mtup.probe_size = 0;
1892                 icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp +
1893                         sizeof(struct tcphdr) +
1894                         icsk->icsk_af_ops->net_header_len;
1895                 icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
1896 
1897                 /* Update probe time stamp */
1898                 icsk->icsk_mtup.probe_timestamp = tcp_time_stamp;
1899         }
1900 }
1901 
1902 /* Create a new MTU probe if we are ready.
1903  * MTU probe is regularly attempting to increase the path MTU by
1904  * deliberately sending larger packets.  This discovers routing
1905  * changes resulting in larger path MTUs.
1906  *
1907  * Returns 0 if we should wait to probe (no cwnd available),
1908  *         1 if a probe was sent,
1909  *         -1 otherwise
1910  */
1911 static int tcp_mtu_probe(struct sock *sk)
1912 {
1913         struct tcp_sock *tp = tcp_sk(sk);
1914         struct inet_connection_sock *icsk = inet_csk(sk);
1915         struct sk_buff *skb, *nskb, *next;
1916         struct net *net = sock_net(sk);
1917         int len;
1918         int probe_size;
1919         int size_needed;
1920         int copy;
1921         int mss_now;
1922         int interval;
1923 
1924         /* Not currently probing/verifying,
1925          * not in recovery,
1926          * have enough cwnd, and
1927          * not SACKing (the variable headers throw things off) */
1928         if (!icsk->icsk_mtup.enabled ||
1929             icsk->icsk_mtup.probe_size ||
1930             inet_csk(sk)->icsk_ca_state != TCP_CA_Open ||
1931             tp->snd_cwnd < 11 ||
1932             tp->rx_opt.num_sacks || tp->rx_opt.dsack)
1933                 return -1;
1934 
1935         /* Use binary search for probe_size between tcp_mss_base,
1936          * and current mss_clamp. if (search_high - search_low)
1937          * smaller than a threshold, backoff from probing.
1938          */
1939         mss_now = tcp_current_mss(sk);
1940         probe_size = tcp_mtu_to_mss(sk, (icsk->icsk_mtup.search_high +
1941                                     icsk->icsk_mtup.search_low) >> 1);
1942         size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
1943         interval = icsk->icsk_mtup.search_high - icsk->icsk_mtup.search_low;
1944         /* When misfortune happens, we are reprobing actively,
1945          * and then reprobe timer has expired. We stick with current
1946          * probing process by not resetting search range to its orignal.
1947          */
1948         if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high) ||
1949                 interval < net->ipv4.sysctl_tcp_probe_threshold) {
1950                 /* Check whether enough time has elaplased for
1951                  * another round of probing.
1952                  */
1953                 tcp_mtu_check_reprobe(sk);
1954                 return -1;
1955         }
1956 
1957         /* Have enough data in the send queue to probe? */
1958         if (tp->write_seq - tp->snd_nxt < size_needed)
1959                 return -1;
1960 
1961         if (tp->snd_wnd < size_needed)
1962                 return -1;
1963         if (after(tp->snd_nxt + size_needed, tcp_wnd_end(tp)))
1964                 return 0;
1965 
1966         /* Do we need to wait to drain cwnd? With none in flight, don't stall */
1967         if (tcp_packets_in_flight(tp) + 2 > tp->snd_cwnd) {
1968                 if (!tcp_packets_in_flight(tp))
1969                         return -1;
1970                 else
1971                         return 0;
1972         }
1973 
1974         /* We're allowed to probe.  Build it now. */
1975         nskb = sk_stream_alloc_skb(sk, probe_size, GFP_ATOMIC, false);
1976         if (!nskb)
1977                 return -1;
1978         sk->sk_wmem_queued += nskb->truesize;
1979         sk_mem_charge(sk, nskb->truesize);
1980 
1981         skb = tcp_send_head(sk);
1982 
1983         TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(skb)->seq;
1984         TCP_SKB_CB(nskb)->end_seq = TCP_SKB_CB(skb)->seq + probe_size;
1985         TCP_SKB_CB(nskb)->tcp_flags = TCPHDR_ACK;
1986         TCP_SKB_CB(nskb)->sacked = 0;
1987         nskb->csum = 0;
1988         nskb->ip_summed = skb->ip_summed;
1989 
1990         tcp_insert_write_queue_before(nskb, skb, sk);
1991 
1992         len = 0;
1993         tcp_for_write_queue_from_safe(skb, next, sk) {
1994                 copy = min_t(int, skb->len, probe_size - len);
1995                 if (nskb->ip_summed) {
1996                         skb_copy_bits(skb, 0, skb_put(nskb, copy), copy);
1997                 } else {
1998                         __wsum csum = skb_copy_and_csum_bits(skb, 0,
1999                                                              skb_put(nskb, copy),
2000                                                              copy, 0);
2001                         nskb->csum = csum_block_add(nskb->csum, csum, len);
2002                 }
2003 
2004                 if (skb->len <= copy) {
2005                         /* We've eaten all the data from this skb.
2006                          * Throw it away. */
2007                         TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
2008                         tcp_unlink_write_queue(skb, sk);
2009                         sk_wmem_free_skb(sk, skb);
2010                 } else {
2011                         TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags &
2012                                                    ~(TCPHDR_FIN|TCPHDR_PSH);
2013                         if (!skb_shinfo(skb)->nr_frags) {
2014                                 skb_pull(skb, copy);
2015                                 if (skb->ip_summed != CHECKSUM_PARTIAL)
2016                                         skb->csum = csum_partial(skb->data,
2017                                                                  skb->len, 0);
2018                         } else {
2019                                 __pskb_trim_head(skb, copy);
2020                                 tcp_set_skb_tso_segs(skb, mss_now);
2021                         }
2022                         TCP_SKB_CB(skb)->seq += copy;
2023                 }
2024 
2025                 len += copy;
2026 
2027                 if (len >= probe_size)
2028                         break;
2029         }
2030         tcp_init_tso_segs(nskb, nskb->len);
2031 
2032         /* We're ready to send.  If this fails, the probe will
2033          * be resegmented into mss-sized pieces by tcp_write_xmit().
2034          */
2035         if (!tcp_transmit_skb(sk, nskb, 1, GFP_ATOMIC)) {
2036                 /* Decrement cwnd here because we are sending
2037                  * effectively two packets. */
2038                 tp->snd_cwnd--;
2039                 tcp_event_new_data_sent(sk, nskb);
2040 
2041                 icsk->icsk_mtup.probe_size = tcp_mss_to_mtu(sk, nskb->len);
2042                 tp->mtu_probe.probe_seq_start = TCP_SKB_CB(nskb)->seq;
2043                 tp->mtu_probe.probe_seq_end = TCP_SKB_CB(nskb)->end_seq;
2044 
2045                 return 1;
2046         }
2047 
2048         return -1;
2049 }
2050 
2051 /* TCP Small Queues :
2052  * Control number of packets in qdisc/devices to two packets / or ~1 ms.
2053  * (These limits are doubled for retransmits)
2054  * This allows for :
2055  *  - better RTT estimation and ACK scheduling
2056  *  - faster recovery
2057  *  - high rates
2058  * Alas, some drivers / subsystems require a fair amount
2059  * of queued bytes to ensure line rate.
2060  * One example is wifi aggregation (802.11 AMPDU)
2061  */
2062 static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
2063                                   unsigned int factor)
2064 {
2065         unsigned int limit;
2066 
2067         limit = max(2 * skb->truesize, sk->sk_pacing_rate >> 10);
2068         limit = min_t(u32, limit, sysctl_tcp_limit_output_bytes);
2069         limit <<= factor;
2070 
2071         if (atomic_read(&sk->sk_wmem_alloc) > limit) {
2072                 set_bit(TSQ_THROTTLED, &tcp_sk(sk)->tsq_flags);
2073                 /* It is possible TX completion already happened
2074                  * before we set TSQ_THROTTLED, so we must
2075                  * test again the condition.
2076                  */
2077                 smp_mb__after_atomic();
2078                 if (atomic_read(&sk->sk_wmem_alloc) > limit)
2079                         return true;
2080         }
2081         return false;
2082 }
2083 
2084 /* This routine writes packets to the network.  It advances the
2085  * send_head.  This happens as incoming acks open up the remote
2086  * window for us.
2087  *
2088  * LARGESEND note: !tcp_urg_mode is overkill, only frames between
2089  * snd_up-64k-mss .. snd_up cannot be large. However, taking into
2090  * account rare use of URG, this is not a big flaw.
2091  *
2092  * Send at most one packet when push_one > 0. Temporarily ignore
2093  * cwnd limit to force at most one packet out when push_one == 2.
2094 
2095  * Returns true, if no segments are in flight and we have queued segments,
2096  * but cannot send anything now because of SWS or another problem.
2097  */
2098 static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
2099                            int push_one, gfp_t gfp)
2100 {
2101         struct tcp_sock *tp = tcp_sk(sk);
2102         struct sk_buff *skb;
2103         unsigned int tso_segs, sent_pkts;
2104         int cwnd_quota;
2105         int result;
2106         bool is_cwnd_limited = false;
2107         u32 max_segs;
2108 
2109         sent_pkts = 0;
2110 
2111         if (!push_one) {
2112                 /* Do MTU probing. */
2113                 result = tcp_mtu_probe(sk);
2114                 if (!result) {
2115                         return false;
2116                 } else if (result > 0) {
2117                         sent_pkts = 1;
2118                 }
2119         }
2120 
2121         max_segs = tcp_tso_segs(sk, mss_now);
2122         while ((skb = tcp_send_head(sk))) {
2123                 unsigned int limit;
2124 
2125                 tso_segs = tcp_init_tso_segs(skb, mss_now);
2126                 BUG_ON(!tso_segs);
2127 
2128                 if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE) {
2129                         /* "skb_mstamp" is used as a start point for the retransmit timer */
2130                         skb_mstamp_get(&skb->skb_mstamp);
2131                         goto repair; /* Skip network transmission */
2132                 }
2133 
2134                 cwnd_quota = tcp_cwnd_test(tp, skb);
2135                 if (!cwnd_quota) {
2136                         if (push_one == 2)
2137                                 /* Force out a loss probe pkt. */
2138                                 cwnd_quota = 1;
2139                         else
2140                                 break;
2141                 }
2142 
2143                 if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now)))
2144                         break;
2145 
2146                 if (tso_segs == 1) {
2147                         if (unlikely(!tcp_nagle_test(tp, skb, mss_now,
2148                                                      (tcp_skb_is_last(sk, skb) ?
2149                                                       nonagle : TCP_NAGLE_PUSH))))
2150                                 break;
2151                 } else {
2152                         if (!push_one &&
2153                             tcp_tso_should_defer(sk, skb, &is_cwnd_limited,
2154                                                  max_segs))
2155                                 break;
2156                 }
2157 
2158                 limit = mss_now;
2159                 if (tso_segs > 1 && !tcp_urg_mode(tp))
2160                         limit = tcp_mss_split_point(sk, skb, mss_now,
2161                                                     min_t(unsigned int,
2162                                                           cwnd_quota,
2163                                                           max_segs),
2164                                                     nonagle);
2165 
2166                 if (skb->len > limit &&
2167                     unlikely(tso_fragment(sk, skb, limit, mss_now, gfp)))
2168                         break;
2169 
2170                 if (tcp_small_queue_check(sk, skb, 0))
2171                         break;
2172 
2173                 if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
2174                         break;
2175 
2176 repair:
2177                 /* Advance the send_head.  This one is sent out.
2178                  * This call will increment packets_out.
2179                  */
2180                 tcp_event_new_data_sent(sk, skb);
2181 
2182                 tcp_minshall_update(tp, mss_now, skb);
2183                 sent_pkts += tcp_skb_pcount(skb);
2184 
2185                 if (push_one)
2186                         break;
2187         }
2188 
2189         if (likely(sent_pkts)) {
2190                 if (tcp_in_cwnd_reduction(sk))
2191                         tp->prr_out += sent_pkts;
2192 
2193                 /* Send one loss probe per tail loss episode. */
2194                 if (push_one != 2)
2195                         tcp_schedule_loss_probe(sk);
2196                 is_cwnd_limited |= (tcp_packets_in_flight(tp) >= tp->snd_cwnd);
2197                 tcp_cwnd_validate(sk, is_cwnd_limited);
2198                 return false;
2199         }
2200         return !tp->packets_out && tcp_send_head(sk);
2201 }
2202 
2203 bool tcp_schedule_loss_probe(struct sock *sk)
2204 {
2205         struct inet_connection_sock *icsk = inet_csk(sk);
2206         struct tcp_sock *tp = tcp_sk(sk);
2207         u32 timeout, tlp_time_stamp, rto_time_stamp;
2208         u32 rtt = usecs_to_jiffies(tp->srtt_us >> 3);
2209 
2210         if (WARN_ON(icsk->icsk_pending == ICSK_TIME_EARLY_RETRANS))
2211                 return false;
2212         /* No consecutive loss probes. */
2213         if (WARN_ON(icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)) {
2214                 tcp_rearm_rto(sk);
2215                 return false;
2216         }
2217         /* Don't do any loss probe on a Fast Open connection before 3WHS
2218          * finishes.
2219          */
2220         if (tp->fastopen_rsk)
2221                 return false;
2222 
2223         /* TLP is only scheduled when next timer event is RTO. */
2224         if (icsk->icsk_pending != ICSK_TIME_RETRANS)
2225                 return false;
2226 
2227         /* Schedule a loss probe in 2*RTT for SACK capable connections
2228          * in Open state, that are either limited by cwnd or application.
2229          */
2230         if (sysctl_tcp_early_retrans < 3 || !tp->packets_out ||
2231             !tcp_is_sack(tp) || inet_csk(sk)->icsk_ca_state != TCP_CA_Open)
2232                 return false;
2233 
2234         if ((tp->snd_cwnd > tcp_packets_in_flight(tp)) &&
2235              tcp_send_head(sk))
2236                 return false;
2237 
2238         /* Probe timeout is at least 1.5*rtt + TCP_DELACK_MAX to account
2239          * for delayed ack when there's one outstanding packet. If no RTT
2240          * sample is available then probe after TCP_TIMEOUT_INIT.
2241          */
2242         timeout = rtt << 1 ? : TCP_TIMEOUT_INIT;
2243         if (tp->packets_out == 1)
2244                 timeout = max_t(u32, timeout,
2245                                 (rtt + (rtt >> 1) + TCP_DELACK_MAX));
2246         timeout = max_t(u32, timeout, msecs_to_jiffies(10));
2247 
2248         /* If RTO is shorter, just schedule TLP in its place. */
2249         tlp_time_stamp = tcp_time_stamp + timeout;
2250         rto_time_stamp = (u32)inet_csk(sk)->icsk_timeout;
2251         if ((s32)(tlp_time_stamp - rto_time_stamp) > 0) {
2252                 s32 delta = rto_time_stamp - tcp_time_stamp;
2253                 if (delta > 0)
2254                         timeout = delta;
2255         }
2256 
2257         inet_csk_reset_xmit_timer(sk, ICSK_TIME_LOSS_PROBE, timeout,
2258                                   TCP_RTO_MAX);
2259         return true;
2260 }
2261 
2262 /* Thanks to skb fast clones, we can detect if a prior transmit of
2263  * a packet is still in a qdisc or driver queue.
2264  * In this case, there is very little point doing a retransmit !
2265  */
2266 static bool skb_still_in_host_queue(const struct sock *sk,
2267                                     const struct sk_buff *skb)
2268 {
2269         if (unlikely(skb_fclone_busy(sk, skb))) {
2270                 NET_INC_STATS(sock_net(sk),
2271                               LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
2272                 return true;
2273         }
2274         return false;
2275 }
2276 
2277 /* When probe timeout (PTO) fires, try send a new segment if possible, else
2278  * retransmit the last segment.
2279  */
2280 void tcp_send_loss_probe(struct sock *sk)
2281 {
2282         struct tcp_sock *tp = tcp_sk(sk);
2283         struct sk_buff *skb;
2284         int pcount;
2285         int mss = tcp_current_mss(sk);
2286 
2287         skb = tcp_send_head(sk);
2288         if (skb) {
2289                 if (tcp_snd_wnd_test(tp, skb, mss)) {
2290                         pcount = tp->packets_out;
2291                         tcp_write_xmit(sk, mss, TCP_NAGLE_OFF, 2, GFP_ATOMIC);
2292                         if (tp->packets_out > pcount)
2293                                 goto probe_sent;
2294                         goto rearm_timer;
2295                 }
2296                 skb = tcp_write_queue_prev(sk, skb);
2297         } else {
2298                 skb = tcp_write_queue_tail(sk);
2299         }
2300 
2301         /* At most one outstanding TLP retransmission. */
2302         if (tp->tlp_high_seq)
2303                 goto rearm_timer;
2304 
2305         /* Retransmit last segment. */
2306         if (WARN_ON(!skb))
2307                 goto rearm_timer;
2308 
2309         if (skb_still_in_host_queue(sk, skb))
2310                 goto rearm_timer;
2311 
2312         pcount = tcp_skb_pcount(skb);
2313         if (WARN_ON(!pcount))
2314                 goto rearm_timer;
2315 
2316         if ((pcount > 1) && (skb->len > (pcount - 1) * mss)) {
2317                 if (unlikely(tcp_fragment(sk, skb, (pcount - 1) * mss, mss,
2318                                           GFP_ATOMIC)))
2319                         goto rearm_timer;
2320                 skb = tcp_write_queue_next(sk, skb);
2321         }
2322 
2323         if (WARN_ON(!skb || !tcp_skb_pcount(skb)))
2324                 goto rearm_timer;
2325 
2326         if (__tcp_retransmit_skb(sk, skb, 1))
2327                 goto rearm_timer;
2328 
2329         /* Record snd_nxt for loss detection. */
2330         tp->tlp_high_seq = tp->snd_nxt;
2331 
2332 probe_sent:
2333         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSSPROBES);
2334         /* Reset s.t. tcp_rearm_rto will restart timer from now */
2335         inet_csk(sk)->icsk_pending = 0;
2336 rearm_timer:
2337         tcp_rearm_rto(sk);
2338 }
2339 
2340 /* Push out any pending frames which were held back due to
2341  * TCP_CORK or attempt at coalescing tiny packets.
2342  * The socket must be locked by the caller.
2343  */
2344 void __tcp_push_pending_frames(struct sock *sk, unsigned int cur_mss,
2345                                int nonagle)
2346 {
2347         /* If we are closed, the bytes will have to remain here.
2348          * In time closedown will finish, we empty the write queue and
2349          * all will be happy.
2350          */
2351         if (unlikely(sk->sk_state == TCP_CLOSE))
2352                 return;
2353 
2354         if (tcp_write_xmit(sk, cur_mss, nonagle, 0,
2355                            sk_gfp_mask(sk, GFP_ATOMIC)))
2356                 tcp_check_probe_timer(sk);
2357 }
2358 
2359 /* Send _single_ skb sitting at the send head. This function requires
2360  * true push pending frames to setup probe timer etc.
2361  */
2362 void tcp_push_one(struct sock *sk, unsigned int mss_now)
2363 {
2364         struct sk_buff *skb = tcp_send_head(sk);
2365 
2366         BUG_ON(!skb || skb->len < mss_now);
2367 
2368         tcp_write_xmit(sk, mss_now, TCP_NAGLE_PUSH, 1, sk->sk_allocation);
2369 }
2370 
2371 /* This function returns the amount that we can raise the
2372  * usable window based on the following constraints
2373  *
2374  * 1. The window can never be shrunk once it is offered (RFC 793)
2375  * 2. We limit memory per socket
2376  *
2377  * RFC 1122:
2378  * "the suggested [SWS] avoidance algorithm for the receiver is to keep
2379  *  RECV.NEXT + RCV.WIN fixed until:
2380  *  RCV.BUFF - RCV.USER - RCV.WINDOW >= min(1/2 RCV.BUFF, MSS)"
2381  *
2382  * i.e. don't raise the right edge of the window until you can raise
2383  * it at least MSS bytes.
2384  *
2385  * Unfortunately, the recommended algorithm breaks header prediction,
2386  * since header prediction assumes th->window stays fixed.
2387  *
2388  * Strictly speaking, keeping th->window fixed violates the receiver
2389  * side SWS prevention criteria. The problem is that under this rule
2390  * a stream of single byte packets will cause the right side of the
2391  * window to always advance by a single byte.
2392  *
2393  * Of course, if the sender implements sender side SWS prevention
2394  * then this will not be a problem.
2395  *
2396  * BSD seems to make the following compromise:
2397  *
2398  *      If the free space is less than the 1/4 of the maximum
2399  *      space available and the free space is less than 1/2 mss,
2400  *      then set the window to 0.
2401  *      [ Actually, bsd uses MSS and 1/4 of maximal _window_ ]
2402  *      Otherwise, just prevent the window from shrinking
2403  *      and from being larger than the largest representable value.
2404  *
2405  * This prevents incremental opening of the window in the regime
2406  * where TCP is limited by the speed of the reader side taking
2407  * data out of the TCP receive queue. It does nothing about
2408  * those cases where the window is constrained on the sender side
2409  * because the pipeline is full.
2410  *
2411  * BSD also seems to "accidentally" limit itself to windows that are a
2412  * multiple of MSS, at least until the free space gets quite small.
2413  * This would appear to be a side effect of the mbuf implementation.
2414  * Combining these two algorithms results in the observed behavior
2415  * of having a fixed window size at almost all times.
2416  *
2417  * Below we obtain similar behavior by forcing the offered window to
2418  * a multiple of the mss when it is feasible to do so.
2419  *
2420  * Note, we don't "adjust" for TIMESTAMP or SACK option bytes.
2421  * Regular options like TIMESTAMP are taken into account.
2422  */
2423 u32 __tcp_select_window(struct sock *sk)
2424 {
2425         struct inet_connection_sock *icsk = inet_csk(sk);
2426         struct tcp_sock *tp = tcp_sk(sk);
2427         /* MSS for the peer's data.  Previous versions used mss_clamp
2428          * here.  I don't know if the value based on our guesses
2429          * of peer's MSS is better for the performance.  It's more correct
2430          * but may be worse for the performance because of rcv_mss
2431          * fluctuations.  --SAW  1998/11/1
2432          */
2433         int mss = icsk->icsk_ack.rcv_mss;
2434         int free_space = tcp_space(sk);
2435         int allowed_space = tcp_full_space(sk);
2436         int full_space = min_t(int, tp->window_clamp, allowed_space);
2437         int window;
2438 
2439         if (mss > full_space)
2440                 mss = full_space;
2441 
2442         if (free_space < (full_space >> 1)) {
2443                 icsk->icsk_ack.quick = 0;
2444 
2445                 if (tcp_under_memory_pressure(sk))
2446                         tp->rcv_ssthresh = min(tp->rcv_ssthresh,
2447                                                4U * tp->advmss);
2448 
2449                 /* free_space might become our new window, make sure we don't
2450                  * increase it due to wscale.
2451                  */
2452                 free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);
2453 
2454                 /* if free space is less than mss estimate, or is below 1/16th
2455                  * of the maximum allowed, try to move to zero-window, else
2456                  * tcp_clamp_window() will grow rcv buf up to tcp_rmem[2], and
2457                  * new incoming data is dropped due to memory limits.
2458                  * With large window, mss test triggers way too late in order
2459                  * to announce zero window in time before rmem limit kicks in.
2460                  */
2461                 if (free_space < (allowed_space >> 4) || free_space < mss)
2462                         return 0;
2463         }
2464 
2465         if (free_space > tp->rcv_ssthresh)
2466                 free_space = tp->rcv_ssthresh;
2467 
2468         /* Don't do rounding if we are using window scaling, since the
2469          * scaled window will not line up with the MSS boundary anyway.
2470          */
2471         window = tp->rcv_wnd;
2472         if (tp->rx_opt.rcv_wscale) {
2473                 window = free_space;
2474 
2475                 /* Advertise enough space so that it won't get scaled away.
2476                  * Import case: prevent zero window announcement if
2477                  * 1<<rcv_wscale > mss.
2478                  */
2479                 if (((window >> tp->rx_opt.rcv_wscale) << tp->rx_opt.rcv_wscale) != window)
2480                         window = (((window >> tp->rx_opt.rcv_wscale) + 1)
2481                                   << tp->rx_opt.rcv_wscale);
2482         } else {
2483                 /* Get the largest window that is a nice multiple of mss.
2484                  * Window clamp already applied above.
2485                  * If our current window offering is within 1 mss of the
2486                  * free space we just keep it. This prevents the divide
2487                  * and multiply from happening most of the time.
2488                  * We also don't do any window rounding when the free space
2489                  * is too small.
2490                  */
2491                 if (window <= free_space - mss || window > free_space)
2492                         window = (free_space / mss) * mss;
2493                 else if (mss == full_space &&
2494                          free_space > window + (full_space >> 1))
2495                         window = free_space;
2496         }
2497 
2498         return window;
2499 }
2500 
2501 void tcp_skb_collapse_tstamp(struct sk_buff *skb,
2502                              const struct sk_buff *next_skb)
2503 {
2504         if (unlikely(tcp_has_tx_tstamp(next_skb))) {
2505                 const struct skb_shared_info *next_shinfo =
2506                         skb_shinfo(next_skb);
2507                 struct skb_shared_info *shinfo = skb_shinfo(skb);
2508 
2509                 shinfo->tx_flags |= next_shinfo->tx_flags & SKBTX_ANY_TSTAMP;
2510                 shinfo->tskey = next_shinfo->tskey;
2511                 TCP_SKB_CB(skb)->txstamp_ack |=
2512                         TCP_SKB_CB(next_skb)->txstamp_ack;
2513         }
2514 }
2515 
2516 /* Collapses two adjacent SKB's during retransmission. */
2517 static void tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
2518 {
2519         struct tcp_sock *tp = tcp_sk(sk);
2520         struct sk_buff *next_skb = tcp_write_queue_next(sk, skb);
2521         int skb_size, next_skb_size;
2522 
2523         skb_size = skb->len;
2524         next_skb_size = next_skb->len;
2525 
2526         BUG_ON(tcp_skb_pcount(skb) != 1 || tcp_skb_pcount(next_skb) != 1);
2527 
2528         tcp_highest_sack_combine(sk, next_skb, skb);
2529 
2530         tcp_unlink_write_queue(next_skb, sk);
2531 
2532         skb_copy_from_linear_data(next_skb, skb_put(skb, next_skb_size),
2533                                   next_skb_size);
2534 
2535         if (next_skb->ip_summed == CHECKSUM_PARTIAL)
2536                 skb->ip_summed = CHECKSUM_PARTIAL;
2537 
2538         if (skb->ip_summed != CHECKSUM_PARTIAL)
2539                 skb->csum = csum_block_add(skb->csum, next_skb->csum, skb_size);
2540 
2541         /* Update sequence range on original skb. */
2542         TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;
2543 
2544         /* Merge over control information. This moves PSH/FIN etc. over */
2545         TCP_SKB_CB(skb)->tcp_flags |= TCP_SKB_CB(next_skb)->tcp_flags;
2546 
2547         /* All done, get rid of second SKB and account for it so
2548          * packet counting does not break.
2549          */
2550         TCP_SKB_CB(skb)->sacked |= TCP_SKB_CB(next_skb)->sacked & TCPCB_EVER_RETRANS;
2551         TCP_SKB_CB(skb)->eor = TCP_SKB_CB(next_skb)->eor;
2552 
2553         /* changed transmit queue under us so clear hints */
2554         tcp_clear_retrans_hints_partial(tp);
2555         if (next_skb == tp->retransmit_skb_hint)
2556                 tp->retransmit_skb_hint = skb;
2557 
2558         tcp_adjust_pcount(sk, next_skb, tcp_skb_pcount(next_skb));
2559 
2560         tcp_skb_collapse_tstamp(skb, next_skb);
2561 
2562         sk_wmem_free_skb(sk, next_skb);
2563 }
2564 
2565 /* Check if coalescing SKBs is legal. */
2566 static bool tcp_can_collapse(const struct sock *sk, const struct sk_buff *skb)
2567 {
2568         if (tcp_skb_pcount(skb) > 1)
2569                 return false;
2570         /* TODO: SACK collapsing could be used to remove this condition */
2571         if (skb_shinfo(skb)->nr_frags != 0)
2572                 return false;
2573         if (skb_cloned(skb))
2574                 return false;
2575         if (skb == tcp_send_head(sk))
2576                 return false;
2577         /* Some heurestics for collapsing over SACK'd could be invented */
2578         if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
2579                 return false;
2580 
2581         return true;
2582 }
2583 
2584 /* Collapse packets in the retransmit queue to make to create
2585  * less packets on the wire. This is only done on retransmission.
2586  */
2587 static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
2588                                      int space)
2589 {
2590         struct tcp_sock *tp = tcp_sk(sk);
2591         struct sk_buff *skb = to, *tmp;
2592         bool first = true;
2593 
2594         if (!sysctl_tcp_retrans_collapse)
2595                 return;
2596         if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
2597                 return;
2598 
2599         tcp_for_write_queue_from_safe(skb, tmp, sk) {
2600                 if (!tcp_can_collapse(sk, skb))
2601                         break;
2602 
2603                 if (!tcp_skb_can_collapse_to(to))
2604                         break;
2605 
2606                 space -= skb->len;
2607 
2608                 if (first) {
2609                         first = false;
2610                         continue;
2611                 }
2612 
2613                 if (space < 0)
2614                         break;
2615                 /* Punt if not enough space exists in the first SKB for
2616                  * the data in the second
2617                  */
2618                 if (skb->len > skb_availroom(to))
2619                         break;
2620 
2621                 if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
2622                         break;
2623 
2624                 tcp_collapse_retrans(sk, to);
2625         }
2626 }
2627 
2628 /* This retransmits one SKB.  Policy decisions and retransmit queue
2629  * state updates are done by the caller.  Returns non-zero if an
2630  * error occurred which prevented the send.
2631  */
2632 int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
2633 {
2634         struct inet_connection_sock *icsk = inet_csk(sk);
2635         struct tcp_sock *tp = tcp_sk(sk);
2636         unsigned int cur_mss;
2637         int diff, len, err;
2638 
2639 
2640         /* Inconclusive MTU probe */
2641         if (icsk->icsk_mtup.probe_size)
2642                 icsk->icsk_mtup.probe_size = 0;
2643 
2644         /* Do not sent more than we queued. 1/4 is reserved for possible
2645          * copying overhead: fragmentation, tunneling, mangling etc.
2646          */
2647         if (atomic_read(&sk->sk_wmem_alloc) >
2648             min_t(u32, sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2),
2649                   sk->sk_sndbuf))
2650                 return -EAGAIN;
2651 
2652         if (skb_still_in_host_queue(sk, skb))
2653                 return -EBUSY;
2654 
2655         if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
2656                 if (before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
2657                         BUG();
2658                 if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
2659                         return -ENOMEM;
2660         }
2661 
2662         if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
2663                 return -EHOSTUNREACH; /* Routing failure or similar. */
2664 
2665         cur_mss = tcp_current_mss(sk);
2666 
2667         /* If receiver has shrunk his window, and skb is out of
2668          * new window, do not retransmit it. The exception is the
2669          * case, when window is shrunk to zero. In this case
2670          * our retransmit serves as a zero window probe.
2671          */
2672         if (!before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp)) &&
2673             TCP_SKB_CB(skb)->seq != tp->snd_una)
2674                 return -EAGAIN;
2675 
2676         len = cur_mss * segs;
2677         if (skb->len > len) {
2678                 if (tcp_fragment(sk, skb, len, cur_mss, GFP_ATOMIC))
2679                         return -ENOMEM; /* We'll try again later. */
2680         } else {
2681                 if (skb_unclone(skb, GFP_ATOMIC))
2682                         return -ENOMEM;
2683 
2684                 diff = tcp_skb_pcount(skb);
2685                 tcp_set_skb_tso_segs(skb, cur_mss);
2686                 diff -= tcp_skb_pcount(skb);
2687                 if (diff)
2688                         tcp_adjust_pcount(sk, skb, diff);
2689                 if (skb->len < cur_mss)
2690                         tcp_retrans_try_collapse(sk, skb, cur_mss);
2691         }
2692 
2693         /* RFC3168, section 6.1.1.1. ECN fallback */
2694         if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
2695                 tcp_ecn_clear_syn(sk, skb);
2696 
2697         /* make sure skb->data is aligned on arches that require it
2698          * and check if ack-trimming & collapsing extended the headroom
2699          * beyond what csum_start can cover.
2700          */
2701         if (unlikely((NET_IP_ALIGN && ((unsigned long)skb->data & 3)) ||
2702                      skb_headroom(skb) >= 0xFFFF)) {
2703                 struct sk_buff *nskb;
2704 
2705                 skb_mstamp_get(&skb->skb_mstamp);
2706                 nskb = __pskb_copy(skb, MAX_TCP_HEADER, GFP_ATOMIC);
2707                 err = nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :
2708                              -ENOBUFS;
2709         } else {
2710                 err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
2711         }
2712 
2713         if (likely(!err)) {
2714                 segs = tcp_skb_pcount(skb);
2715 
2716                 TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;
2717                 /* Update global TCP statistics. */
2718                 TCP_ADD_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS, segs);
2719                 if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
2720                         __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
2721                 tp->total_retrans += segs;
2722         }
2723         return err;
2724 }
2725 
2726 int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
2727 {
2728         struct tcp_sock *tp = tcp_sk(sk);
2729         int err = __tcp_retransmit_skb(sk, skb, segs);
2730 
2731         if (err == 0) {
2732 #if FASTRETRANS_DEBUG > 0
2733                 if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
2734                         net_dbg_ratelimited("retrans_out leaked\n");
2735                 }
2736 #endif
2737                 TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
2738                 tp->retrans_out += tcp_skb_pcount(skb);
2739 
2740                 /* Save stamp of the first retransmit. */
2741                 if (!tp->retrans_stamp)
2742                         tp->retrans_stamp = tcp_skb_timestamp(skb);
2743 
2744         } else if (err != -EBUSY) {
2745                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL);
2746         }
2747 
2748         if (tp->undo_retrans < 0)
2749                 tp->undo_retrans = 0;
2750         tp->undo_retrans += tcp_skb_pcount(skb);
2751         return err;
2752 }
2753 
2754 /* Check if we forward retransmits are possible in the current
2755  * window/congestion state.
2756  */
2757 static bool tcp_can_forward_retransmit(struct sock *sk)
2758 {
2759         const struct inet_connection_sock *icsk = inet_csk(sk);
2760         const struct tcp_sock *tp = tcp_sk(sk);
2761 
2762         /* Forward retransmissions are possible only during Recovery. */
2763         if (icsk->icsk_ca_state != TCP_CA_Recovery)
2764                 return false;
2765 
2766         /* No forward retransmissions in Reno are possible. */
2767         if (tcp_is_reno(tp))
2768                 return false;
2769 
2770         /* Yeah, we have to make difficult choice between forward transmission
2771          * and retransmission... Both ways have their merits...
2772          *
2773          * For now we do not retransmit anything, while we have some new
2774          * segments to send. In the other cases, follow rule 3 for
2775          * NextSeg() specified in RFC3517.
2776          */
2777 
2778         if (tcp_may_send_now(sk))
2779                 return false;
2780 
2781         return true;
2782 }
2783 
2784 /* This gets called after a retransmit timeout, and the initially
2785  * retransmitted data is acknowledged.  It tries to continue
2786  * resending the rest of the retransmit queue, until either
2787  * we've sent it all or the congestion window limit is reached.
2788  * If doing SACK, the first ACK which comes back for a timeout
2789  * based retransmit packet might feed us FACK information again.
2790  * If so, we use it to avoid unnecessarily retransmissions.
2791  */
2792 void tcp_xmit_retransmit_queue(struct sock *sk)
2793 {
2794         const struct inet_connection_sock *icsk = inet_csk(sk);
2795         struct tcp_sock *tp = tcp_sk(sk);
2796         struct sk_buff *skb;
2797         struct sk_buff *hole = NULL;
2798         u32 max_segs, last_lost;
2799         int mib_idx;
2800         int fwd_rexmitting = 0;
2801 
2802         if (!tp->packets_out)
2803                 return;
2804 
2805         if (!tp->lost_out)
2806                 tp->retransmit_high = tp->snd_una;
2807 
2808         if (tp->retransmit_skb_hint) {
2809                 skb = tp->retransmit_skb_hint;
2810                 last_lost = TCP_SKB_CB(skb)->end_seq;
2811                 if (after(last_lost, tp->retransmit_high))
2812                         last_lost = tp->retransmit_high;
2813         } else {
2814                 skb = tcp_write_queue_head(sk);
2815                 last_lost = tp->snd_una;
2816         }
2817 
2818         max_segs = tcp_tso_segs(sk, tcp_current_mss(sk));
2819         tcp_for_write_queue_from(skb, sk) {
2820                 __u8 sacked;
2821                 int segs;
2822 
2823                 if (skb == tcp_send_head(sk))
2824                         break;
2825                 /* we could do better than to assign each time */
2826                 if (!hole)
2827                         tp->retransmit_skb_hint = skb;
2828 
2829                 segs = tp->snd_cwnd - tcp_packets_in_flight(tp);
2830                 if (segs <= 0)
2831                         return;
2832                 sacked = TCP_SKB_CB(skb)->sacked;
2833                 /* In case tcp_shift_skb_data() have aggregated large skbs,
2834                  * we need to make sure not sending too bigs TSO packets
2835                  */
2836                 segs = min_t(int, segs, max_segs);
2837 
2838                 if (fwd_rexmitting) {
2839 begin_fwd:
2840                         if (!before(TCP_SKB_CB(skb)->seq, tcp_highest_sack_seq(tp)))
2841                                 break;
2842                         mib_idx = LINUX_MIB_TCPFORWARDRETRANS;
2843 
2844                 } else if (!before(TCP_SKB_CB(skb)->seq, tp->retransmit_high)) {
2845                         tp->retransmit_high = last_lost;
2846                         if (!tcp_can_forward_retransmit(sk))
2847                                 break;
2848                         /* Backtrack if necessary to non-L'ed skb */
2849                         if (hole) {
2850                                 skb = hole;
2851                                 hole = NULL;
2852                         }
2853                         fwd_rexmitting = 1;
2854                         goto begin_fwd;
2855 
2856                 } else if (!(sacked & TCPCB_LOST)) {
2857                         if (!hole && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
2858                                 hole = skb;
2859                         continue;
2860 
2861                 } else {
2862                         last_lost = TCP_SKB_CB(skb)->end_seq;
2863                         if (icsk->icsk_ca_state != TCP_CA_Loss)
2864                                 mib_idx = LINUX_MIB_TCPFASTRETRANS;
2865                         else
2866                                 mib_idx = LINUX_MIB_TCPSLOWSTARTRETRANS;
2867                 }
2868 
2869                 if (sacked & (TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))
2870                         continue;
2871 
2872                 if (tcp_small_queue_check(sk, skb, 1))
2873                         return;
2874 
2875                 if (tcp_retransmit_skb(sk, skb, segs))
2876                         return;
2877 
2878                 NET_ADD_STATS(sock_net(sk), mib_idx, tcp_skb_pcount(skb));
2879 
2880                 if (tcp_in_cwnd_reduction(sk))
2881                         tp->prr_out += tcp_skb_pcount(skb);
2882 
2883                 if (skb == tcp_write_queue_head(sk))
2884                         inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
2885                                                   inet_csk(sk)->icsk_rto,
2886                                                   TCP_RTO_MAX);
2887         }
2888 }
2889 
2890 /* We allow to exceed memory limits for FIN packets to expedite
2891  * connection tear down and (memory) recovery.
2892  * Otherwise tcp_send_fin() could be tempted to either delay FIN
2893  * or even be forced to close flow without any FIN.
2894  * In general, we want to allow one skb per socket to avoid hangs
2895  * with edge trigger epoll()
2896  */
2897 void sk_forced_mem_schedule(struct sock *sk, int size)
2898 {
2899         int amt;
2900 
2901         if (size <= sk->sk_forward_alloc)
2902                 return;
2903         amt = sk_mem_pages(size);
2904         sk->sk_forward_alloc += amt * SK_MEM_QUANTUM;
2905         sk_memory_allocated_add(sk, amt);
2906 
2907         if (mem_cgroup_sockets_enabled && sk->sk_memcg)
2908                 mem_cgroup_charge_skmem(sk->sk_memcg, amt);
2909 }
2910 
2911 /* Send a FIN. The caller locks the socket for us.
2912  * We should try to send a FIN packet really hard, but eventually give up.
2913  */
2914 void tcp_send_fin(struct sock *sk)
2915 {
2916         struct sk_buff *skb, *tskb = tcp_write_queue_tail(sk);
2917         struct tcp_sock *tp = tcp_sk(sk);
2918 
2919         /* Optimization, tack on the FIN if we have one skb in write queue and
2920          * this skb was not yet sent, or we are under memory pressure.
2921          * Note: in the latter case, FIN packet will be sent after a timeout,
2922          * as TCP stack thinks it has already been transmitted.
2923          */
2924         if (tskb && (tcp_send_head(sk) || tcp_under_memory_pressure(sk))) {
2925 coalesce:
2926                 TCP_SKB_CB(tskb)->tcp_flags |= TCPHDR_FIN;
2927                 TCP_SKB_CB(tskb)->end_seq++;
2928                 tp->write_seq++;
2929                 if (!tcp_send_head(sk)) {
2930                         /* This means tskb was already sent.
2931                          * Pretend we included the FIN on previous transmit.
2932                          * We need to set tp->snd_nxt to the value it would have
2933                          * if FIN had been sent. This is because retransmit path
2934                          * does not change tp->snd_nxt.
2935                          */
2936                         tp->snd_nxt++;
2937                         return;
2938                 }
2939         } else {
2940                 skb = alloc_skb_fclone(MAX_TCP_HEADER, sk->sk_allocation);
2941                 if (unlikely(!skb)) {
2942                         if (tskb)
2943                                 goto coalesce;
2944                         return;
2945                 }
2946                 skb_reserve(skb, MAX_TCP_HEADER);
2947                 sk_forced_mem_schedule(sk, skb->truesize);
2948                 /* FIN eats a sequence byte, write_seq advanced by tcp_queue_skb(). */
2949                 tcp_init_nondata_skb(skb, tp->write_seq,
2950                                      TCPHDR_ACK | TCPHDR_FIN);
2951                 tcp_queue_skb(sk, skb);
2952         }
2953         __tcp_push_pending_frames(sk, tcp_current_mss(sk), TCP_NAGLE_OFF);
2954 }
2955 
2956 /* We get here when a process closes a file descriptor (either due to
2957  * an explicit close() or as a byproduct of exit()'ing) and there
2958  * was unread data in the receive queue.  This behavior is recommended
2959  * by RFC 2525, section 2.17.  -DaveM
2960  */
2961 void tcp_send_active_reset(struct sock *sk, gfp_t priority)
2962 {
2963         struct sk_buff *skb;
2964 
2965         /* NOTE: No TCP options attached and we never retransmit this. */
2966         skb = alloc_skb(MAX_TCP_HEADER, priority);
2967         if (!skb) {
2968                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
2969                 return;
2970         }
2971 
2972         /* Reserve space for headers and prepare control bits. */
2973         skb_reserve(skb, MAX_TCP_HEADER);
2974         tcp_init_nondata_skb(skb, tcp_acceptable_seq(sk),
2975                              TCPHDR_ACK | TCPHDR_RST);
2976         skb_mstamp_get(&skb->skb_mstamp);
2977         /* Send it off. */
2978         if (tcp_transmit_skb(sk, skb, 0, priority))
2979                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
2980 
2981         TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTRSTS);
2982 }
2983 
2984 /* Send a crossed SYN-ACK during socket establishment.
2985  * WARNING: This routine must only be called when we have already sent
2986  * a SYN packet that crossed the incoming SYN that caused this routine
2987  * to get called. If this assumption fails then the initial rcv_wnd
2988  * and rcv_wscale values will not be correct.
2989  */
2990 int tcp_send_synack(struct sock *sk)
2991 {
2992         struct sk_buff *skb;
2993 
2994         skb = tcp_write_queue_head(sk);
2995         if (!skb || !(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
2996                 pr_debug("%s: wrong queue state\n", __func__);
2997                 return -EFAULT;
2998         }
2999         if (!(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_ACK)) {
3000                 if (skb_cloned(skb)) {
3001                         struct sk_buff *nskb = skb_copy(skb, GFP_ATOMIC);
3002                         if (!nskb)
3003                                 return -ENOMEM;
3004                         tcp_unlink_write_queue(skb, sk);
3005                         __skb_header_release(nskb);
3006                         __tcp_add_write_queue_head(sk, nskb);
3007                         sk_wmem_free_skb(sk, skb);
3008                         sk->sk_wmem_queued += nskb->truesize;
3009                         sk_mem_charge(sk, nskb->truesize);
3010                         skb = nskb;
3011                 }
3012 
3013                 TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ACK;
3014                 tcp_ecn_send_synack(sk, skb);
3015         }
3016         return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
3017 }
3018 
3019 /**
3020  * tcp_make_synack - Prepare a SYN-ACK.
3021  * sk: listener socket
3022  * dst: dst entry attached to the SYNACK
3023  * req: request_sock pointer
3024  *
3025  * Allocate one skb and build a SYNACK packet.
3026  * @dst is consumed : Caller should not use it again.
3027  */
3028 struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
3029                                 struct request_sock *req,
3030                                 struct tcp_fastopen_cookie *foc,
3031                                 enum tcp_synack_type synack_type)
3032 {
3033         struct inet_request_sock *ireq = inet_rsk(req);
3034         const struct tcp_sock *tp = tcp_sk(sk);
3035         struct tcp_md5sig_key *md5 = NULL;
3036         struct tcp_out_options opts;
3037         struct sk_buff *skb;
3038         int tcp_header_size;
3039         struct tcphdr *th;
3040         u16 user_mss;
3041         int mss;
3042 
3043         skb = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
3044         if (unlikely(!skb)) {
3045                 dst_release(dst);
3046                 return NULL;
3047         }
3048         /* Reserve space for headers. */
3049         skb_reserve(skb, MAX_TCP_HEADER);
3050 
3051         switch (synack_type) {
3052         case TCP_SYNACK_NORMAL:
3053                 skb_set_owner_w(skb, req_to_sk(req));
3054                 break;
3055         case TCP_SYNACK_COOKIE:
3056                 /* Under synflood, we do not attach skb to a socket,
3057                  * to avoid false sharing.
3058                  */
3059                 break;
3060         case TCP_SYNACK_FASTOPEN:
3061                 /* sk is a const pointer, because we want to express multiple
3062                  * cpu might call us concurrently.
3063                  * sk->sk_wmem_alloc in an atomic, we can promote to rw.
3064                  */
3065                 skb_set_owner_w(skb, (struct sock *)sk);
3066                 break;
3067         }
3068         skb_dst_set(skb, dst);
3069 
3070         mss = dst_metric_advmss(dst);
3071         user_mss = READ_ONCE(tp->rx_opt.user_mss);
3072         if (user_mss && user_mss < mss)
3073                 mss = user_mss;
3074 
3075         memset(&opts, 0, sizeof(opts));
3076 #ifdef CONFIG_SYN_COOKIES
3077         if (unlikely(req->cookie_ts))
3078                 skb->skb_mstamp.stamp_jiffies = cookie_init_timestamp(req);
3079         else
3080 #endif
3081         skb_mstamp_get(&skb->skb_mstamp);
3082 
3083 #ifdef CONFIG_TCP_MD5SIG
3084         rcu_read_lock();
3085         md5 = tcp_rsk(req)->af_specific->req_md5_lookup(sk, req_to_sk(req));
3086 #endif
3087         skb_set_hash(skb, tcp_rsk(req)->txhash, PKT_HASH_TYPE_L4);
3088         tcp_header_size = tcp_synack_options(req, mss, skb, &opts, md5, foc) +
3089                           sizeof(*th);
3090 
3091         skb_push(skb, tcp_header_size);
3092         skb_reset_transport_header(skb);
3093 
3094         th = (struct tcphdr *)skb->data;
3095         memset(th, 0, sizeof(struct tcphdr));
3096         th->syn = 1;
3097         th->ack = 1;
3098         tcp_ecn_make_synack(req, th);
3099         th->source = htons(ireq->ir_num);
3100         th->dest = ireq->ir_rmt_port;
3101         /* Setting of flags are superfluous here for callers (and ECE is
3102          * not even correctly set)
3103          */
3104         tcp_init_nondata_skb(skb, tcp_rsk(req)->snt_isn,
3105                              TCPHDR_SYN | TCPHDR_ACK);
3106 
3107         th->seq = htonl(TCP_SKB_CB(skb)->seq);
3108         /* XXX data is queued and acked as is. No buffer/window check */
3109         th->ack_seq = htonl(tcp_rsk(req)->rcv_nxt);
3110 
3111         /* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
3112         th->window = htons(min(req->rsk_rcv_wnd, 65535U));
3113         tcp_options_write((__be32 *)(th + 1), NULL, &opts);
3114         th->doff = (tcp_header_size >> 2);
3115         __TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS);
3116 
3117 #ifdef CONFIG_TCP_MD5SIG
3118         /* Okay, we have all we need - do the md5 hash if needed */
3119         if (md5)
3120                 tcp_rsk(req)->af_specific->calc_md5_hash(opts.hash_location,
3121                                                md5, req_to_sk(req), skb);
3122         rcu_read_unlock();
3123 #endif
3124 
3125         /* Do not fool tcpdump (if any), clean our debris */
3126         skb->tstamp.tv64 = 0;
3127         return skb;
3128 }
3129 EXPORT_SYMBOL(tcp_make_synack);
3130 
3131 static void tcp_ca_dst_init(struct sock *sk, const struct dst_entry *dst)
3132 {
3133         struct inet_connection_sock *icsk = inet_csk(sk);
3134         const struct tcp_congestion_ops *ca;
3135         u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
3136 
3137         if (ca_key == TCP_CA_UNSPEC)
3138                 return;
3139 
3140         rcu_read_lock();
3141         ca = tcp_ca_find_key(ca_key);
3142         if (likely(ca && try_module_get(ca->owner))) {
3143                 module_put(icsk->icsk_ca_ops->owner);
3144                 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
3145                 icsk->icsk_ca_ops = ca;
3146         }
3147         rcu_read_unlock();
3148 }
3149 
3150 /* Do all connect socket setups that can be done AF independent. */
3151 static void tcp_connect_init(struct sock *sk)
3152 {
3153         const struct dst_entry *dst = __sk_dst_get(sk);
3154         struct tcp_sock *tp = tcp_sk(sk);
3155         __u8 rcv_wscale;
3156 
3157         /* We'll fix this up when we get a response from the other end.
3158          * See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT.
3159          */
3160         tp->tcp_header_len = sizeof(struct tcphdr) +
3161                 (sysctl_tcp_timestamps ? TCPOLEN_TSTAMP_ALIGNED : 0);
3162 
3163 #ifdef CONFIG_TCP_MD5SIG
3164         if (tp->af_specific->md5_lookup(sk, sk))
3165                 tp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
3166 #endif
3167 
3168         /* If user gave his TCP_MAXSEG, record it to clamp */
3169         if (tp->rx_opt.user_mss)
3170                 tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
3171         tp->max_window = 0;
3172         tcp_mtup_init(sk);
3173         tcp_sync_mss(sk, dst_mtu(dst));
3174 
3175         tcp_ca_dst_init(sk, dst);
3176 
3177         if (!tp->window_clamp)
3178                 tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
3179         tp->advmss = dst_metric_advmss(dst);
3180         if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->advmss)
3181                 tp->advmss = tp->rx_opt.user_mss;
3182 
3183         tcp_initialize_rcv_mss(sk);
3184 
3185         /* limit the window selection if the user enforce a smaller rx buffer */
3186         if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
3187             (tp->window_clamp > tcp_full_space(sk) || tp->window_clamp == 0))
3188                 tp->window_clamp = tcp_full_space(sk);
3189 
3190         tcp_select_initial_window(tcp_full_space(sk),
3191                                   tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
3192                                   &tp->rcv_wnd,
3193                                   &tp->window_clamp,
3194                                   sysctl_tcp_window_scaling,
3195                                   &rcv_wscale,
3196                                   dst_metric(dst, RTAX_INITRWND));
3197 
3198         tp->rx_opt.rcv_wscale = rcv_wscale;
3199         tp->rcv_ssthresh = tp->rcv_wnd;
3200 
3201         sk->sk_err = 0;
3202         sock_reset_flag(sk, SOCK_DONE);
3203         tp->snd_wnd = 0;
3204         tcp_init_wl(tp, 0);
3205         tp->snd_una = tp->write_seq;
3206         tp->snd_sml = tp->write_seq;
3207         tp->snd_up = tp->write_seq;
3208         tp->snd_nxt = tp->write_seq;
3209 
3210         if (likely(!tp->repair))
3211                 tp->rcv_nxt = 0;
3212         else
3213                 tp->rcv_tstamp = tcp_time_stamp;
3214         tp->rcv_wup = tp->rcv_nxt;
3215         tp->copied_seq = tp->rcv_nxt;
3216 
3217         inet_csk(sk)->icsk_rto = TCP_TIMEOUT_INIT;
3218         inet_csk(sk)->icsk_retransmits = 0;
3219         tcp_clear_retrans(tp);
3220 }
3221 
3222 static void tcp_connect_queue_skb(struct sock *sk, struct sk_buff *skb)
3223 {
3224         struct tcp_sock *tp = tcp_sk(sk);
3225         struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);
3226 
3227         tcb->end_seq += skb->len;
3228         __skb_header_release(skb);
3229         __tcp_add_write_queue_tail(sk, skb);
3230         sk->sk_wmem_queued += skb->truesize;
3231         sk_mem_charge(sk, skb->truesize);
3232         tp->write_seq = tcb->end_seq;
3233         tp->packets_out += tcp_skb_pcount(skb);
3234 }
3235 
3236 /* Build and send a SYN with data and (cached) Fast Open cookie. However,
3237  * queue a data-only packet after the regular SYN, such that regular SYNs
3238  * are retransmitted on timeouts. Also if the remote SYN-ACK acknowledges
3239  * only the SYN sequence, the data are retransmitted in the first ACK.
3240  * If cookie is not cached or other error occurs, falls back to send a
3241  * regular SYN with Fast Open cookie request option.
3242  */
3243 static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
3244 {
3245         struct tcp_sock *tp = tcp_sk(sk);
3246         struct tcp_fastopen_request *fo = tp->fastopen_req;
3247         int syn_loss = 0, space, err = 0;
3248         unsigned long last_syn_loss = 0;
3249         struct sk_buff *syn_data;
3250 
3251         tp->rx_opt.mss_clamp = tp->advmss;  /* If MSS is not cached */
3252         tcp_fastopen_cache_get(sk, &tp->rx_opt.mss_clamp, &fo->cookie,
3253                                &syn_loss, &last_syn_loss);
3254         /* Recurring FO SYN losses: revert to regular handshake temporarily */
3255         if (syn_loss > 1 &&
3256             time_before(jiffies, last_syn_loss + (60*HZ << syn_loss))) {
3257                 fo->cookie.len = -1;
3258                 goto fallback;
3259         }
3260 
3261         if (sysctl_tcp_fastopen & TFO_CLIENT_NO_COOKIE)
3262                 fo->cookie.len = -1;
3263         else if (fo->cookie.len <= 0)
3264                 goto fallback;
3265 
3266         /* MSS for SYN-data is based on cached MSS and bounded by PMTU and
3267          * user-MSS. Reserve maximum option space for middleboxes that add
3268          * private TCP options. The cost is reduced data space in SYN :(
3269          */
3270         if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->rx_opt.mss_clamp)
3271                 tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
3272         space = __tcp_mtu_to_mss(sk, inet_csk(sk)->icsk_pmtu_cookie) -
3273                 MAX_TCP_OPTION_SPACE;
3274 
3275         space = min_t(size_t, space, fo->size);
3276 
3277         /* limit to order-0 allocations */
3278         space = min_t(size_t, space, SKB_MAX_HEAD(MAX_TCP_HEADER));
3279 
3280         syn_data = sk_stream_alloc_skb(sk, space, sk->sk_allocation, false);
3281         if (!syn_data)
3282                 goto fallback;
3283         syn_data->ip_summed = CHECKSUM_PARTIAL;
3284         memcpy(syn_data->cb, syn->cb, sizeof(syn->cb));
3285         if (space) {
3286                 int copied = copy_from_iter(skb_put(syn_data, space), space,
3287                                             &fo->data->msg_iter);
3288                 if (unlikely(!copied)) {
3289                         kfree_skb(syn_data);
3290                         goto fallback;
3291                 }
3292                 if (copied != space) {
3293                         skb_trim(syn_data, copied);
3294                         space = copied;
3295                 }
3296         }
3297         /* No more data pending in inet_wait_for_connect() */
3298         if (space == fo->size)
3299                 fo->data = NULL;
3300         fo->copied = space;
3301 
3302         tcp_connect_queue_skb(sk, syn_data);
3303 
3304         err = tcp_transmit_skb(sk, syn_data, 1, sk->sk_allocation);
3305 
3306         syn->skb_mstamp = syn_data->skb_mstamp;
3307 
3308         /* Now full SYN+DATA was cloned and sent (or not),
3309          * remove the SYN from the original skb (syn_data)
3310          * we keep in write queue in case of a retransmit, as we
3311          * also have the SYN packet (with no data) in the same queue.
3312          */
3313         TCP_SKB_CB(syn_data)->seq++;
3314         TCP_SKB_CB(syn_data)->tcp_flags = TCPHDR_ACK | TCPHDR_PSH;
3315         if (!err) {
3316                 tp->syn_data = (fo->copied > 0);
3317                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT);
3318                 goto done;
3319         }
3320 
3321 fallback:
3322         /* Send a regular SYN with Fast Open cookie request option */
3323         if (fo->cookie.len > 0)
3324                 fo->cookie.len = 0;
3325         err = tcp_transmit_skb(sk, syn, 1, sk->sk_allocation);
3326         if (err)
3327                 tp->syn_fastopen = 0;
3328 done:
3329         fo->cookie.len = -1;  /* Exclude Fast Open option for SYN retries */
3330         return err;
3331 }
3332 
3333 /* Build a SYN and send it off. */
3334 int tcp_connect(struct sock *sk)
3335 {
3336         struct tcp_sock *tp = tcp_sk(sk);
3337         struct sk_buff *buff;
3338         int err;
3339 
3340         tcp_connect_init(sk);
3341 
3342         if (unlikely(tp->repair)) {
3343                 tcp_finish_connect(sk, NULL);
3344                 return 0;
3345         }
3346 
3347         buff = sk_stream_alloc_skb(sk, 0, sk->sk_allocation, true);
3348         if (unlikely(!buff))
3349                 return -ENOBUFS;
3350 
3351         tcp_init_nondata_skb(buff, tp->write_seq++, TCPHDR_SYN);
3352         tp->retrans_stamp = tcp_time_stamp;
3353         tcp_connect_queue_skb(sk, buff);
3354         tcp_ecn_send_syn(sk, buff);
3355 
3356         /* Send off SYN; include data in Fast Open. */
3357         err = tp->fastopen_req ? tcp_send_syn_data(sk, buff) :
3358               tcp_transmit_skb(sk, buff, 1, sk->sk_allocation);
3359         if (err == -ECONNREFUSED)
3360                 return err;
3361 
3362         /* We change tp->snd_nxt after the tcp_transmit_skb() call
3363          * in order to make this packet get counted in tcpOutSegs.
3364          */
3365         tp->snd_nxt = tp->write_seq;
3366         tp->pushed_seq = tp->write_seq;
3367         TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS);
3368 
3369         /* Timer for repeating the SYN until an answer. */
3370         inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
3371                                   inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
3372         return 0;
3373 }
3374 EXPORT_SYMBOL(tcp_connect);
3375 
3376 /* Send out a delayed ack, the caller does the policy checking
3377  * to see if we should even be here.  See tcp_input.c:tcp_ack_snd_check()
3378  * for details.
3379  */
3380 void tcp_send_delayed_ack(struct sock *sk)
3381 {
3382         struct inet_connection_sock *icsk = inet_csk(sk);
3383         int ato = icsk->icsk_ack.ato;
3384         unsigned long timeout;
3385 
3386         tcp_ca_event(sk, CA_EVENT_DELAYED_ACK);
3387 
3388         if (ato > TCP_DELACK_MIN) {
3389                 const struct tcp_sock *tp = tcp_sk(sk);
3390                 int max_ato = HZ / 2;
3391 
3392                 if (icsk->icsk_ack.pingpong ||
3393                     (icsk->icsk_ack.pending & ICSK_ACK_PUSHED))
3394                         max_ato = TCP_DELACK_MAX;
3395 
3396                 /* Slow path, intersegment interval is "high". */
3397 
3398                 /* If some rtt estimate is known, use it to bound delayed ack.
3399                  * Do not use inet_csk(sk)->icsk_rto here, use results of rtt measurements
3400                  * directly.
3401                  */
3402                 if (tp->srtt_us) {
3403                         int rtt = max_t(int, usecs_to_jiffies(tp->srtt_us >> 3),
3404                                         TCP_DELACK_MIN);
3405 
3406                         if (rtt < max_ato)
3407                                 max_ato = rtt;
3408                 }
3409 
3410                 ato = min(ato, max_ato);
3411         }
3412 
3413         /* Stay within the limit we were given */
3414         timeout = jiffies + ato;
3415 
3416         /* Use new timeout only if there wasn't a older one earlier. */
3417         if (icsk->icsk_ack.pending & ICSK_ACK_TIMER) {
3418                 /* If delack timer was blocked or is about to expire,
3419                  * send ACK now.
3420                  */
3421                 if (icsk->icsk_ack.blocked ||
3422                     time_before_eq(icsk->icsk_ack.timeout, jiffies + (ato >> 2))) {
3423                         tcp_send_ack(sk);
3424                         return;
3425                 }
3426 
3427                 if (!time_before(timeout, icsk->icsk_ack.timeout))
3428                         timeout = icsk->icsk_ack.timeout;
3429         }
3430         icsk->icsk_ack.pending |= ICSK_ACK_SCHED | ICSK_ACK_TIMER;
3431         icsk->icsk_ack.timeout = timeout;
3432         sk_reset_timer(sk, &icsk->icsk_delack_timer, timeout);
3433 }
3434 
3435 /* This routine sends an ack and also updates the window. */
3436 void tcp_send_ack(struct sock *sk)
3437 {
3438         struct sk_buff *buff;
3439 
3440         /* If we have been reset, we may not send again. */
3441         if (sk->sk_state == TCP_CLOSE)
3442                 return;
3443 
3444         tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK);
3445 
3446         /* We are not putting this on the write queue, so
3447          * tcp_transmit_skb() will set the ownership to this
3448          * sock.
3449          */
3450         buff = alloc_skb(MAX_TCP_HEADER,
3451                          sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
3452         if (unlikely(!buff)) {
3453                 inet_csk_schedule_ack(sk);
3454                 inet_csk(sk)->icsk_ack.ato = TCP_ATO_MIN;
3455                 inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
3456                                           TCP_DELACK_MAX, TCP_RTO_MAX);
3457                 return;
3458         }
3459 
3460         /* Reserve space for headers and prepare control bits. */
3461         skb_reserve(buff, MAX_TCP_HEADER);
3462         tcp_init_nondata_skb(buff, tcp_acceptable_seq(sk), TCPHDR_ACK);
3463 
3464         /* We do not want pure acks influencing TCP Small Queues or fq/pacing
3465          * too much.
3466          * SKB_TRUESIZE(max(1 .. 66, MAX_TCP_HEADER)) is unfortunately ~784
3467          * We also avoid tcp_wfree() overhead (cache line miss accessing
3468          * tp->tsq_flags) by using regular sock_wfree()
3469          */
3470         skb_set_tcp_pure_ack(buff);
3471 
3472         /* Send it off, this clears delayed acks for us. */
3473         skb_mstamp_get(&buff->skb_mstamp);
3474         tcp_transmit_skb(sk, buff, 0, (__force gfp_t)0);
3475 }
3476 EXPORT_SYMBOL_GPL(tcp_send_ack);
3477 
3478 /* This routine sends a packet with an out of date sequence
3479  * number. It assumes the other end will try to ack it.
3480  *
3481  * Question: what should we make while urgent mode?
3482  * 4.4BSD forces sending single byte of data. We cannot send
3483  * out of window data, because we have SND.NXT==SND.MAX...
3484  *
3485  * Current solution: to send TWO zero-length segments in urgent mode:
3486  * one is with SEG.SEQ=SND.UNA to deliver urgent pointer, another is
3487  * out-of-date with SND.UNA-1 to probe window.
3488  */
3489 static int tcp_xmit_probe_skb(struct sock *sk, int urgent, int mib)
3490 {
3491         struct tcp_sock *tp = tcp_sk(sk);
3492         struct sk_buff *skb;
3493 
3494         /* We don't queue it, tcp_transmit_skb() sets ownership. */
3495         skb = alloc_skb(MAX_TCP_HEADER,
3496                         sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
3497         if (!skb)
3498                 return -1;
3499 
3500         /* Reserve space for headers and set control bits. */
3501         skb_reserve(skb, MAX_TCP_HEADER);
3502         /* Use a previous sequence.  This should cause the other
3503          * end to send an ack.  Don't queue or clone SKB, just
3504          * send it.
3505          */
3506         tcp_init_nondata_skb(skb, tp->snd_una - !urgent, TCPHDR_ACK);
3507         skb_mstamp_get(&skb->skb_mstamp);
3508         NET_INC_STATS(sock_net(sk), mib);
3509         return tcp_transmit_skb(sk, skb, 0, (__force gfp_t)0);
3510 }
3511 
3512 void tcp_send_window_probe(struct sock *sk)
3513 {
3514         if (sk->sk_state == TCP_ESTABLISHED) {
3515                 tcp_sk(sk)->snd_wl1 = tcp_sk(sk)->rcv_nxt - 1;
3516                 tcp_xmit_probe_skb(sk, 0, LINUX_MIB_TCPWINPROBE);
3517         }
3518 }
3519 
3520 /* Initiate keepalive or window probe from timer. */
3521 int tcp_write_wakeup(struct sock *sk, int mib)
3522 {
3523         struct tcp_sock *tp = tcp_sk(sk);
3524         struct sk_buff *skb;
3525 
3526         if (sk->sk_state == TCP_CLOSE)
3527                 return -1;
3528 
3529         skb = tcp_send_head(sk);
3530         if (skb && before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp))) {
3531                 int err;
3532                 unsigned int mss = tcp_current_mss(sk);
3533                 unsigned int seg_size = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
3534 
3535                 if (before(tp->pushed_seq, TCP_SKB_CB(skb)->end_seq))
3536                         tp->pushed_seq = TCP_SKB_CB(skb)->end_seq;
3537 
3538                 /* We are probing the opening of a window
3539                  * but the window size is != 0
3540                  * must have been a result SWS avoidance ( sender )
3541                  */
3542                 if (seg_size < TCP_SKB_CB(skb)->end_seq - TCP_SKB_CB(skb)->seq ||
3543                     skb->len > mss) {
3544                         seg_size = min(seg_size, mss);
3545                         TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
3546                         if (tcp_fragment(sk, skb, seg_size, mss, GFP_ATOMIC))
3547                                 return -1;
3548                 } else if (!tcp_skb_pcount(skb))
3549                         tcp_set_skb_tso_segs(skb, mss);
3550 
3551                 TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
3552                 err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
3553                 if (!err)
3554                         tcp_event_new_data_sent(sk, skb);
3555                 return err;
3556         } else {
3557                 if (between(tp->snd_up, tp->snd_una + 1, tp->snd_una + 0xFFFF))
3558                         tcp_xmit_probe_skb(sk, 1, mib);
3559                 return tcp_xmit_probe_skb(sk, 0, mib);
3560         }
3561 }
3562 
3563 /* A window probe timeout has occurred.  If window is not closed send
3564  * a partial packet else a zero probe.
3565  */
3566 void tcp_send_probe0(struct sock *sk)
3567 {
3568         struct inet_connection_sock *icsk = inet_csk(sk);
3569         struct tcp_sock *tp = tcp_sk(sk);
3570         struct net *net = sock_net(sk);
3571         unsigned long probe_max;
3572         int err;
3573 
3574         err = tcp_write_wakeup(sk, LINUX_MIB_TCPWINPROBE);
3575 
3576         if (tp->packets_out || !tcp_send_head(sk)) {
3577                 /* Cancel probe timer, if it is not required. */
3578                 icsk->icsk_probes_out = 0;
3579                 icsk->icsk_backoff = 0;
3580                 return;
3581         }
3582 
3583         if (err <= 0) {
3584                 if (icsk->icsk_backoff < net->ipv4.sysctl_tcp_retries2)
3585                         icsk->icsk_backoff++;
3586                 icsk->icsk_probes_out++;
3587                 probe_max = TCP_RTO_MAX;
3588         } else {
3589                 /* If packet was not sent due to local congestion,
3590                  * do not backoff and do not remember icsk_probes_out.
3591                  * Let local senders to fight for local resources.
3592                  *
3593                  * Use accumulated backoff yet.
3594                  */
3595                 if (!icsk->icsk_probes_out)
3596                         icsk->icsk_probes_out = 1;
3597                 probe_max = TCP_RESOURCE_PROBE_INTERVAL;
3598         }
3599         inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
3600                                   tcp_probe0_when(sk, probe_max),
3601                                   TCP_RTO_MAX);
3602 }
3603 
3604 int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
3605 {
3606         const struct tcp_request_sock_ops *af_ops = tcp_rsk(req)->af_specific;
3607         struct flowi fl;
3608         int res;
3609 
3610         tcp_rsk(req)->txhash = net_tx_rndhash();
3611         res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_NORMAL);
3612         if (!res) {
3613                 __TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);
3614                 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
3615                 if (unlikely(tcp_passive_fastopen(sk)))
3616                         tcp_sk(sk)->total_retrans++;
3617         }
3618         return res;
3619 }
3620 EXPORT_SYMBOL(tcp_rtx_synack);
3621 

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