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Linux/kernel/pid.c

  1 /*
  2  * Generic pidhash and scalable, time-bounded PID allocator
  3  *
  4  * (C) 2002-2003 William Irwin, IBM
  5  * (C) 2004 William Irwin, Oracle
  6  * (C) 2002-2004 Ingo Molnar, Red Hat
  7  *
  8  * pid-structures are backing objects for tasks sharing a given ID to chain
  9  * against. There is very little to them aside from hashing them and
 10  * parking tasks using given ID's on a list.
 11  *
 12  * The hash is always changed with the tasklist_lock write-acquired,
 13  * and the hash is only accessed with the tasklist_lock at least
 14  * read-acquired, so there's no additional SMP locking needed here.
 15  *
 16  * We have a list of bitmap pages, which bitmaps represent the PID space.
 17  * Allocating and freeing PIDs is completely lockless. The worst-case
 18  * allocation scenario when all but one out of 1 million PIDs possible are
 19  * allocated already: the scanning of 32 list entries and at most PAGE_SIZE
 20  * bytes. The typical fastpath is a single successful setbit. Freeing is O(1).
 21  *
 22  * Pid namespaces:
 23  *    (C) 2007 Pavel Emelyanov <xemul@openvz.org>, OpenVZ, SWsoft Inc.
 24  *    (C) 2007 Sukadev Bhattiprolu <sukadev@us.ibm.com>, IBM
 25  *     Many thanks to Oleg Nesterov for comments and help
 26  *
 27  */
 28 
 29 #include <linux/mm.h>
 30 #include <linux/module.h>
 31 #include <linux/slab.h>
 32 #include <linux/init.h>
 33 #include <linux/rculist.h>
 34 #include <linux/bootmem.h>
 35 #include <linux/hash.h>
 36 #include <linux/pid_namespace.h>
 37 #include <linux/init_task.h>
 38 #include <linux/syscalls.h>
 39 
 40 #define pid_hashfn(nr, ns)      \
 41         hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
 42 static struct hlist_head *pid_hash;
 43 static int pidhash_shift;
 44 struct pid init_struct_pid = INIT_STRUCT_PID;
 45 
 46 int pid_max = PID_MAX_DEFAULT;
 47 
 48 #define RESERVED_PIDS           300
 49 
 50 int pid_max_min = RESERVED_PIDS + 1;
 51 int pid_max_max = PID_MAX_LIMIT;
 52 
 53 #define BITS_PER_PAGE           (PAGE_SIZE*8)
 54 #define BITS_PER_PAGE_MASK      (BITS_PER_PAGE-1)
 55 
 56 static inline int mk_pid(struct pid_namespace *pid_ns,
 57                 struct pidmap *map, int off)
 58 {
 59         return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;
 60 }
 61 
 62 #define find_next_offset(map, off)                                      \
 63                 find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
 64 
 65 /*
 66  * PID-map pages start out as NULL, they get allocated upon
 67  * first use and are never deallocated. This way a low pid_max
 68  * value does not cause lots of bitmaps to be allocated, but
 69  * the scheme scales to up to 4 million PIDs, runtime.
 70  */
 71 struct pid_namespace init_pid_ns = {
 72         .kref = {
 73                 .refcount       = ATOMIC_INIT(2),
 74         },
 75         .pidmap = {
 76                 [ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
 77         },
 78         .last_pid = 0,
 79         .level = 0,
 80         .child_reaper = &init_task,
 81 };
 82 EXPORT_SYMBOL_GPL(init_pid_ns);
 83 
 84 int is_container_init(struct task_struct *tsk)
 85 {
 86         int ret = 0;
 87         struct pid *pid;
 88 
 89         rcu_read_lock();
 90         pid = task_pid(tsk);
 91         if (pid != NULL && pid->numbers[pid->level].nr == 1)
 92                 ret = 1;
 93         rcu_read_unlock();
 94 
 95         return ret;
 96 }
 97 EXPORT_SYMBOL(is_container_init);
 98 
 99 /*
100  * Note: disable interrupts while the pidmap_lock is held as an
101  * interrupt might come in and do read_lock(&tasklist_lock).
102  *
103  * If we don't disable interrupts there is a nasty deadlock between
104  * detach_pid()->free_pid() and another cpu that does
105  * spin_lock(&pidmap_lock) followed by an interrupt routine that does
106  * read_lock(&tasklist_lock);
107  *
108  * After we clean up the tasklist_lock and know there are no
109  * irq handlers that take it we can leave the interrupts enabled.
110  * For now it is easier to be safe than to prove it can't happen.
111  */
112 
113 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
114 
115 static void free_pidmap(struct upid *upid)
116 {
117         int nr = upid->nr;
118         struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
119         int offset = nr & BITS_PER_PAGE_MASK;
120 
121         clear_bit(offset, map->page);
122         atomic_inc(&map->nr_free);
123 }
124 
125 static int alloc_pidmap(struct pid_namespace *pid_ns)
126 {
127         int i, offset, max_scan, pid, last = pid_ns->last_pid;
128         struct pidmap *map;
129 
130         pid = last + 1;
131         if (pid >= pid_max)
132                 pid = RESERVED_PIDS;
133         offset = pid & BITS_PER_PAGE_MASK;
134         map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
135         max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
136         for (i = 0; i <= max_scan; ++i) {
137                 if (unlikely(!map->page)) {
138                         void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
139                         /*
140                          * Free the page if someone raced with us
141                          * installing it:
142                          */
143                         spin_lock_irq(&pidmap_lock);
144                         if (map->page)
145                                 kfree(page);
146                         else
147                                 map->page = page;
148                         spin_unlock_irq(&pidmap_lock);
149                         if (unlikely(!map->page))
150                                 break;
151                 }
152                 if (likely(atomic_read(&map->nr_free))) {
153                         do {
154                                 if (!test_and_set_bit(offset, map->page)) {
155                                         atomic_dec(&map->nr_free);
156                                         pid_ns->last_pid = pid;
157                                         return pid;
158                                 }
159                                 offset = find_next_offset(map, offset);
160                                 pid = mk_pid(pid_ns, map, offset);
161                         /*
162                          * find_next_offset() found a bit, the pid from it
163                          * is in-bounds, and if we fell back to the last
164                          * bitmap block and the final block was the same
165                          * as the starting point, pid is before last_pid.
166                          */
167                         } while (offset < BITS_PER_PAGE && pid < pid_max &&
168                                         (i != max_scan || pid < last ||
169                                             !((last+1) & BITS_PER_PAGE_MASK)));
170                 }
171                 if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
172                         ++map;
173                         offset = 0;
174                 } else {
175                         map = &pid_ns->pidmap[0];
176                         offset = RESERVED_PIDS;
177                         if (unlikely(last == offset))
178                                 break;
179                 }
180                 pid = mk_pid(pid_ns, map, offset);
181         }
182         return -1;
183 }
184 
185 int next_pidmap(struct pid_namespace *pid_ns, int last)
186 {
187         int offset;
188         struct pidmap *map, *end;
189 
190         offset = (last + 1) & BITS_PER_PAGE_MASK;
191         map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
192         end = &pid_ns->pidmap[PIDMAP_ENTRIES];
193         for (; map < end; map++, offset = 0) {
194                 if (unlikely(!map->page))
195                         continue;
196                 offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
197                 if (offset < BITS_PER_PAGE)
198                         return mk_pid(pid_ns, map, offset);
199         }
200         return -1;
201 }
202 
203 void put_pid(struct pid *pid)
204 {
205         struct pid_namespace *ns;
206 
207         if (!pid)
208                 return;
209 
210         ns = pid->numbers[pid->level].ns;
211         if ((atomic_read(&pid->count) == 1) ||
212              atomic_dec_and_test(&pid->count)) {
213                 kmem_cache_free(ns->pid_cachep, pid);
214                 put_pid_ns(ns);
215         }
216 }
217 EXPORT_SYMBOL_GPL(put_pid);
218 
219 static void delayed_put_pid(struct rcu_head *rhp)
220 {
221         struct pid *pid = container_of(rhp, struct pid, rcu);
222         put_pid(pid);
223 }
224 
225 void free_pid(struct pid *pid)
226 {
227         /* We can be called with write_lock_irq(&tasklist_lock) held */
228         int i;
229         unsigned long flags;
230 
231         spin_lock_irqsave(&pidmap_lock, flags);
232         for (i = 0; i <= pid->level; i++)
233                 hlist_del_rcu(&pid->numbers[i].pid_chain);
234         spin_unlock_irqrestore(&pidmap_lock, flags);
235 
236         for (i = 0; i <= pid->level; i++)
237                 free_pidmap(pid->numbers + i);
238 
239         call_rcu(&pid->rcu, delayed_put_pid);
240 }
241 
242 struct pid *alloc_pid(struct pid_namespace *ns)
243 {
244         struct pid *pid;
245         enum pid_type type;
246         int i, nr;
247         struct pid_namespace *tmp;
248         struct upid *upid;
249 
250         pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
251         if (!pid)
252                 goto out;
253 
254         tmp = ns;
255         for (i = ns->level; i >= 0; i--) {
256                 nr = alloc_pidmap(tmp);
257                 if (nr < 0)
258                         goto out_free;
259 
260                 pid->numbers[i].nr = nr;
261                 pid->numbers[i].ns = tmp;
262                 tmp = tmp->parent;
263         }
264 
265         get_pid_ns(ns);
266         pid->level = ns->level;
267         atomic_set(&pid->count, 1);
268         for (type = 0; type < PIDTYPE_MAX; ++type)
269                 INIT_HLIST_HEAD(&pid->tasks[type]);
270 
271         spin_lock_irq(&pidmap_lock);
272         for (i = ns->level; i >= 0; i--) {
273                 upid = &pid->numbers[i];
274                 hlist_add_head_rcu(&upid->pid_chain,
275                                 &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
276         }
277         spin_unlock_irq(&pidmap_lock);
278 
279 out:
280         return pid;
281 
282 out_free:
283         while (++i <= ns->level)
284                 free_pidmap(pid->numbers + i);
285 
286         kmem_cache_free(ns->pid_cachep, pid);
287         pid = NULL;
288         goto out;
289 }
290 
291 struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
292 {
293         struct hlist_node *elem;
294         struct upid *pnr;
295 
296         hlist_for_each_entry_rcu(pnr, elem,
297                         &pid_hash[pid_hashfn(nr, ns)], pid_chain)
298                 if (pnr->nr == nr && pnr->ns == ns)
299                         return container_of(pnr, struct pid,
300                                         numbers[ns->level]);
301 
302         return NULL;
303 }
304 EXPORT_SYMBOL_GPL(find_pid_ns);
305 
306 struct pid *find_vpid(int nr)
307 {
308         return find_pid_ns(nr, current->nsproxy->pid_ns);
309 }
310 EXPORT_SYMBOL_GPL(find_vpid);
311 
312 /*
313  * attach_pid() must be called with the tasklist_lock write-held.
314  */
315 void attach_pid(struct task_struct *task, enum pid_type type,
316                 struct pid *pid)
317 {
318         struct pid_link *link;
319 
320         link = &task->pids[type];
321         link->pid = pid;
322         hlist_add_head_rcu(&link->node, &pid->tasks[type]);
323 }
324 
325 static void __change_pid(struct task_struct *task, enum pid_type type,
326                         struct pid *new)
327 {
328         struct pid_link *link;
329         struct pid *pid;
330         int tmp;
331 
332         link = &task->pids[type];
333         pid = link->pid;
334 
335         hlist_del_rcu(&link->node);
336         link->pid = new;
337 
338         for (tmp = PIDTYPE_MAX; --tmp >= 0; )
339                 if (!hlist_empty(&pid->tasks[tmp]))
340                         return;
341 
342         free_pid(pid);
343 }
344 
345 void detach_pid(struct task_struct *task, enum pid_type type)
346 {
347         __change_pid(task, type, NULL);
348 }
349 
350 void change_pid(struct task_struct *task, enum pid_type type,
351                 struct pid *pid)
352 {
353         __change_pid(task, type, pid);
354         attach_pid(task, type, pid);
355 }
356 
357 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
358 void transfer_pid(struct task_struct *old, struct task_struct *new,
359                            enum pid_type type)
360 {
361         new->pids[type].pid = old->pids[type].pid;
362         hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
363 }
364 
365 struct task_struct *pid_task(struct pid *pid, enum pid_type type)
366 {
367         struct task_struct *result = NULL;
368         if (pid) {
369                 struct hlist_node *first;
370                 first = rcu_dereference(pid->tasks[type].first);
371                 if (first)
372                         result = hlist_entry(first, struct task_struct, pids[(type)].node);
373         }
374         return result;
375 }
376 EXPORT_SYMBOL(pid_task);
377 
378 /*
379  * Must be called under rcu_read_lock() or with tasklist_lock read-held.
380  */
381 struct task_struct *find_task_by_pid_type_ns(int type, int nr,
382                 struct pid_namespace *ns)
383 {
384         return pid_task(find_pid_ns(nr, ns), type);
385 }
386 
387 EXPORT_SYMBOL(find_task_by_pid_type_ns);
388 
389 struct task_struct *find_task_by_vpid(pid_t vnr)
390 {
391         return find_task_by_pid_type_ns(PIDTYPE_PID, vnr,
392                         current->nsproxy->pid_ns);
393 }
394 EXPORT_SYMBOL(find_task_by_vpid);
395 
396 struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
397 {
398         return find_task_by_pid_type_ns(PIDTYPE_PID, nr, ns);
399 }
400 EXPORT_SYMBOL(find_task_by_pid_ns);
401 
402 struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
403 {
404         struct pid *pid;
405         rcu_read_lock();
406         pid = get_pid(task->pids[type].pid);
407         rcu_read_unlock();
408         return pid;
409 }
410 
411 struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
412 {
413         struct task_struct *result;
414         rcu_read_lock();
415         result = pid_task(pid, type);
416         if (result)
417                 get_task_struct(result);
418         rcu_read_unlock();
419         return result;
420 }
421 
422 struct pid *find_get_pid(pid_t nr)
423 {
424         struct pid *pid;
425 
426         rcu_read_lock();
427         pid = get_pid(find_vpid(nr));
428         rcu_read_unlock();
429 
430         return pid;
431 }
432 EXPORT_SYMBOL_GPL(find_get_pid);
433 
434 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
435 {
436         struct upid *upid;
437         pid_t nr = 0;
438 
439         if (pid && ns->level <= pid->level) {
440                 upid = &pid->numbers[ns->level];
441                 if (upid->ns == ns)
442                         nr = upid->nr;
443         }
444         return nr;
445 }
446 
447 pid_t pid_vnr(struct pid *pid)
448 {
449         return pid_nr_ns(pid, current->nsproxy->pid_ns);
450 }
451 EXPORT_SYMBOL_GPL(pid_vnr);
452 
453 pid_t task_pid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
454 {
455         return pid_nr_ns(task_pid(tsk), ns);
456 }
457 EXPORT_SYMBOL(task_pid_nr_ns);
458 
459 pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
460 {
461         return pid_nr_ns(task_tgid(tsk), ns);
462 }
463 EXPORT_SYMBOL(task_tgid_nr_ns);
464 
465 pid_t task_pgrp_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
466 {
467         return pid_nr_ns(task_pgrp(tsk), ns);
468 }
469 EXPORT_SYMBOL(task_pgrp_nr_ns);
470 
471 pid_t task_session_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
472 {
473         return pid_nr_ns(task_session(tsk), ns);
474 }
475 EXPORT_SYMBOL(task_session_nr_ns);
476 
477 struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
478 {
479         return ns_of_pid(task_pid(tsk));
480 }
481 EXPORT_SYMBOL_GPL(task_active_pid_ns);
482 
483 /*
484  * Used by proc to find the first pid that is greater than or equal to nr.
485  *
486  * If there is a pid at nr this function is exactly the same as find_pid_ns.
487  */
488 struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
489 {
490         struct pid *pid;
491 
492         do {
493                 pid = find_pid_ns(nr, ns);
494                 if (pid)
495                         break;
496                 nr = next_pidmap(ns, nr);
497         } while (nr > 0);
498 
499         return pid;
500 }
501 
502 /*
503  * The pid hash table is scaled according to the amount of memory in the
504  * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
505  * more.
506  */
507 void __init pidhash_init(void)
508 {
509         int i, pidhash_size;
510         unsigned long megabytes = nr_kernel_pages >> (20 - PAGE_SHIFT);
511 
512         pidhash_shift = max(4, fls(megabytes * 4));
513         pidhash_shift = min(12, pidhash_shift);
514         pidhash_size = 1 << pidhash_shift;
515 
516         printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
517                 pidhash_size, pidhash_shift,
518                 pidhash_size * sizeof(struct hlist_head));
519 
520         pid_hash = alloc_bootmem(pidhash_size * sizeof(*(pid_hash)));
521         if (!pid_hash)
522                 panic("Could not alloc pidhash!\n");
523         for (i = 0; i < pidhash_size; i++)
524                 INIT_HLIST_HEAD(&pid_hash[i]);
525 }
526 
527 void __init pidmap_init(void)
528 {
529         init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
530         /* Reserve PID 0. We never call free_pidmap(0) */
531         set_bit(0, init_pid_ns.pidmap[0].page);
532         atomic_dec(&init_pid_ns.pidmap[0].nr_free);
533 
534         init_pid_ns.pid_cachep = KMEM_CACHE(pid,
535                         SLAB_HWCACHE_ALIGN | SLAB_PANIC);
536 }
537 

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