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Linux/kernel/pid.c

  1 /*
  2  * Generic pidhash and scalable, time-bounded PID allocator
  3  *
  4  * (C) 2002-2003 Nadia Yvette Chambers, IBM
  5  * (C) 2004 Nadia Yvette Chambers, 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/export.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 #include <linux/proc_ns.h>
 40 #include <linux/proc_fs.h>
 41 
 42 #define pid_hashfn(nr, ns)      \
 43         hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
 44 static struct hlist_head *pid_hash;
 45 static unsigned int pidhash_shift = 4;
 46 struct pid init_struct_pid = INIT_STRUCT_PID;
 47 
 48 int pid_max = PID_MAX_DEFAULT;
 49 
 50 #define RESERVED_PIDS           300
 51 
 52 int pid_max_min = RESERVED_PIDS + 1;
 53 int pid_max_max = PID_MAX_LIMIT;
 54 
 55 static inline int mk_pid(struct pid_namespace *pid_ns,
 56                 struct pidmap *map, int off)
 57 {
 58         return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;
 59 }
 60 
 61 #define find_next_offset(map, off)                                      \
 62                 find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
 63 
 64 /*
 65  * PID-map pages start out as NULL, they get allocated upon
 66  * first use and are never deallocated. This way a low pid_max
 67  * value does not cause lots of bitmaps to be allocated, but
 68  * the scheme scales to up to 4 million PIDs, runtime.
 69  */
 70 struct pid_namespace init_pid_ns = {
 71         .kref = {
 72                 .refcount       = ATOMIC_INIT(2),
 73         },
 74         .pidmap = {
 75                 [ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
 76         },
 77         .last_pid = 0,
 78         .nr_hashed = PIDNS_HASH_ADDING,
 79         .level = 0,
 80         .child_reaper = &init_task,
 81         .user_ns = &init_user_ns,
 82         .ns.inum = PROC_PID_INIT_INO,
 83 #ifdef CONFIG_PID_NS
 84         .ns.ops = &pidns_operations,
 85 #endif
 86 };
 87 EXPORT_SYMBOL_GPL(init_pid_ns);
 88 
 89 /*
 90  * Note: disable interrupts while the pidmap_lock is held as an
 91  * interrupt might come in and do read_lock(&tasklist_lock).
 92  *
 93  * If we don't disable interrupts there is a nasty deadlock between
 94  * detach_pid()->free_pid() and another cpu that does
 95  * spin_lock(&pidmap_lock) followed by an interrupt routine that does
 96  * read_lock(&tasklist_lock);
 97  *
 98  * After we clean up the tasklist_lock and know there are no
 99  * irq handlers that take it we can leave the interrupts enabled.
100  * For now it is easier to be safe than to prove it can't happen.
101  */
102 
103 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
104 
105 static void free_pidmap(struct upid *upid)
106 {
107         int nr = upid->nr;
108         struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
109         int offset = nr & BITS_PER_PAGE_MASK;
110 
111         clear_bit(offset, map->page);
112         atomic_inc(&map->nr_free);
113 }
114 
115 /*
116  * If we started walking pids at 'base', is 'a' seen before 'b'?
117  */
118 static int pid_before(int base, int a, int b)
119 {
120         /*
121          * This is the same as saying
122          *
123          * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
124          * and that mapping orders 'a' and 'b' with respect to 'base'.
125          */
126         return (unsigned)(a - base) < (unsigned)(b - base);
127 }
128 
129 /*
130  * We might be racing with someone else trying to set pid_ns->last_pid
131  * at the pid allocation time (there's also a sysctl for this, but racing
132  * with this one is OK, see comment in kernel/pid_namespace.c about it).
133  * We want the winner to have the "later" value, because if the
134  * "earlier" value prevails, then a pid may get reused immediately.
135  *
136  * Since pids rollover, it is not sufficient to just pick the bigger
137  * value.  We have to consider where we started counting from.
138  *
139  * 'base' is the value of pid_ns->last_pid that we observed when
140  * we started looking for a pid.
141  *
142  * 'pid' is the pid that we eventually found.
143  */
144 static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
145 {
146         int prev;
147         int last_write = base;
148         do {
149                 prev = last_write;
150                 last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
151         } while ((prev != last_write) && (pid_before(base, last_write, pid)));
152 }
153 
154 static int alloc_pidmap(struct pid_namespace *pid_ns)
155 {
156         int i, offset, max_scan, pid, last = pid_ns->last_pid;
157         struct pidmap *map;
158 
159         pid = last + 1;
160         if (pid >= pid_max)
161                 pid = RESERVED_PIDS;
162         offset = pid & BITS_PER_PAGE_MASK;
163         map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
164         /*
165          * If last_pid points into the middle of the map->page we
166          * want to scan this bitmap block twice, the second time
167          * we start with offset == 0 (or RESERVED_PIDS).
168          */
169         max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
170         for (i = 0; i <= max_scan; ++i) {
171                 if (unlikely(!map->page)) {
172                         void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
173                         /*
174                          * Free the page if someone raced with us
175                          * installing it:
176                          */
177                         spin_lock_irq(&pidmap_lock);
178                         if (!map->page) {
179                                 map->page = page;
180                                 page = NULL;
181                         }
182                         spin_unlock_irq(&pidmap_lock);
183                         kfree(page);
184                         if (unlikely(!map->page))
185                                 return -ENOMEM;
186                 }
187                 if (likely(atomic_read(&map->nr_free))) {
188                         for ( ; ; ) {
189                                 if (!test_and_set_bit(offset, map->page)) {
190                                         atomic_dec(&map->nr_free);
191                                         set_last_pid(pid_ns, last, pid);
192                                         return pid;
193                                 }
194                                 offset = find_next_offset(map, offset);
195                                 if (offset >= BITS_PER_PAGE)
196                                         break;
197                                 pid = mk_pid(pid_ns, map, offset);
198                                 if (pid >= pid_max)
199                                         break;
200                         }
201                 }
202                 if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
203                         ++map;
204                         offset = 0;
205                 } else {
206                         map = &pid_ns->pidmap[0];
207                         offset = RESERVED_PIDS;
208                         if (unlikely(last == offset))
209                                 break;
210                 }
211                 pid = mk_pid(pid_ns, map, offset);
212         }
213         return -EAGAIN;
214 }
215 
216 int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
217 {
218         int offset;
219         struct pidmap *map, *end;
220 
221         if (last >= PID_MAX_LIMIT)
222                 return -1;
223 
224         offset = (last + 1) & BITS_PER_PAGE_MASK;
225         map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
226         end = &pid_ns->pidmap[PIDMAP_ENTRIES];
227         for (; map < end; map++, offset = 0) {
228                 if (unlikely(!map->page))
229                         continue;
230                 offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
231                 if (offset < BITS_PER_PAGE)
232                         return mk_pid(pid_ns, map, offset);
233         }
234         return -1;
235 }
236 
237 void put_pid(struct pid *pid)
238 {
239         struct pid_namespace *ns;
240 
241         if (!pid)
242                 return;
243 
244         ns = pid->numbers[pid->level].ns;
245         if ((atomic_read(&pid->count) == 1) ||
246              atomic_dec_and_test(&pid->count)) {
247                 kmem_cache_free(ns->pid_cachep, pid);
248                 put_pid_ns(ns);
249         }
250 }
251 EXPORT_SYMBOL_GPL(put_pid);
252 
253 static void delayed_put_pid(struct rcu_head *rhp)
254 {
255         struct pid *pid = container_of(rhp, struct pid, rcu);
256         put_pid(pid);
257 }
258 
259 void free_pid(struct pid *pid)
260 {
261         /* We can be called with write_lock_irq(&tasklist_lock) held */
262         int i;
263         unsigned long flags;
264 
265         spin_lock_irqsave(&pidmap_lock, flags);
266         for (i = 0; i <= pid->level; i++) {
267                 struct upid *upid = pid->numbers + i;
268                 struct pid_namespace *ns = upid->ns;
269                 hlist_del_rcu(&upid->pid_chain);
270                 switch(--ns->nr_hashed) {
271                 case 2:
272                 case 1:
273                         /* When all that is left in the pid namespace
274                          * is the reaper wake up the reaper.  The reaper
275                          * may be sleeping in zap_pid_ns_processes().
276                          */
277                         wake_up_process(ns->child_reaper);
278                         break;
279                 case PIDNS_HASH_ADDING:
280                         /* Handle a fork failure of the first process */
281                         WARN_ON(ns->child_reaper);
282                         ns->nr_hashed = 0;
283                         /* fall through */
284                 case 0:
285                         schedule_work(&ns->proc_work);
286                         break;
287                 }
288         }
289         spin_unlock_irqrestore(&pidmap_lock, flags);
290 
291         for (i = 0; i <= pid->level; i++)
292                 free_pidmap(pid->numbers + i);
293 
294         call_rcu(&pid->rcu, delayed_put_pid);
295 }
296 
297 struct pid *alloc_pid(struct pid_namespace *ns)
298 {
299         struct pid *pid;
300         enum pid_type type;
301         int i, nr;
302         struct pid_namespace *tmp;
303         struct upid *upid;
304         int retval = -ENOMEM;
305 
306         pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
307         if (!pid)
308                 return ERR_PTR(retval);
309 
310         tmp = ns;
311         pid->level = ns->level;
312         for (i = ns->level; i >= 0; i--) {
313                 nr = alloc_pidmap(tmp);
314                 if (nr < 0) {
315                         retval = nr;
316                         goto out_free;
317                 }
318 
319                 pid->numbers[i].nr = nr;
320                 pid->numbers[i].ns = tmp;
321                 tmp = tmp->parent;
322         }
323 
324         if (unlikely(is_child_reaper(pid))) {
325                 if (pid_ns_prepare_proc(ns))
326                         goto out_free;
327         }
328 
329         get_pid_ns(ns);
330         atomic_set(&pid->count, 1);
331         for (type = 0; type < PIDTYPE_MAX; ++type)
332                 INIT_HLIST_HEAD(&pid->tasks[type]);
333 
334         upid = pid->numbers + ns->level;
335         spin_lock_irq(&pidmap_lock);
336         if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
337                 goto out_unlock;
338         for ( ; upid >= pid->numbers; --upid) {
339                 hlist_add_head_rcu(&upid->pid_chain,
340                                 &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
341                 upid->ns->nr_hashed++;
342         }
343         spin_unlock_irq(&pidmap_lock);
344 
345         return pid;
346 
347 out_unlock:
348         spin_unlock_irq(&pidmap_lock);
349         put_pid_ns(ns);
350 
351 out_free:
352         while (++i <= ns->level)
353                 free_pidmap(pid->numbers + i);
354 
355         kmem_cache_free(ns->pid_cachep, pid);
356         return ERR_PTR(retval);
357 }
358 
359 void disable_pid_allocation(struct pid_namespace *ns)
360 {
361         spin_lock_irq(&pidmap_lock);
362         ns->nr_hashed &= ~PIDNS_HASH_ADDING;
363         spin_unlock_irq(&pidmap_lock);
364 }
365 
366 struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
367 {
368         struct upid *pnr;
369 
370         hlist_for_each_entry_rcu(pnr,
371                         &pid_hash[pid_hashfn(nr, ns)], pid_chain)
372                 if (pnr->nr == nr && pnr->ns == ns)
373                         return container_of(pnr, struct pid,
374                                         numbers[ns->level]);
375 
376         return NULL;
377 }
378 EXPORT_SYMBOL_GPL(find_pid_ns);
379 
380 struct pid *find_vpid(int nr)
381 {
382         return find_pid_ns(nr, task_active_pid_ns(current));
383 }
384 EXPORT_SYMBOL_GPL(find_vpid);
385 
386 /*
387  * attach_pid() must be called with the tasklist_lock write-held.
388  */
389 void attach_pid(struct task_struct *task, enum pid_type type)
390 {
391         struct pid_link *link = &task->pids[type];
392         hlist_add_head_rcu(&link->node, &link->pid->tasks[type]);
393 }
394 
395 static void __change_pid(struct task_struct *task, enum pid_type type,
396                         struct pid *new)
397 {
398         struct pid_link *link;
399         struct pid *pid;
400         int tmp;
401 
402         link = &task->pids[type];
403         pid = link->pid;
404 
405         hlist_del_rcu(&link->node);
406         link->pid = new;
407 
408         for (tmp = PIDTYPE_MAX; --tmp >= 0; )
409                 if (!hlist_empty(&pid->tasks[tmp]))
410                         return;
411 
412         free_pid(pid);
413 }
414 
415 void detach_pid(struct task_struct *task, enum pid_type type)
416 {
417         __change_pid(task, type, NULL);
418 }
419 
420 void change_pid(struct task_struct *task, enum pid_type type,
421                 struct pid *pid)
422 {
423         __change_pid(task, type, pid);
424         attach_pid(task, type);
425 }
426 
427 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
428 void transfer_pid(struct task_struct *old, struct task_struct *new,
429                            enum pid_type type)
430 {
431         new->pids[type].pid = old->pids[type].pid;
432         hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
433 }
434 
435 struct task_struct *pid_task(struct pid *pid, enum pid_type type)
436 {
437         struct task_struct *result = NULL;
438         if (pid) {
439                 struct hlist_node *first;
440                 first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
441                                               lockdep_tasklist_lock_is_held());
442                 if (first)
443                         result = hlist_entry(first, struct task_struct, pids[(type)].node);
444         }
445         return result;
446 }
447 EXPORT_SYMBOL(pid_task);
448 
449 /*
450  * Must be called under rcu_read_lock().
451  */
452 struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
453 {
454         RCU_LOCKDEP_WARN(!rcu_read_lock_held(),
455                          "find_task_by_pid_ns() needs rcu_read_lock() protection");
456         return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
457 }
458 
459 struct task_struct *find_task_by_vpid(pid_t vnr)
460 {
461         return find_task_by_pid_ns(vnr, task_active_pid_ns(current));
462 }
463 
464 struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
465 {
466         struct pid *pid;
467         rcu_read_lock();
468         if (type != PIDTYPE_PID)
469                 task = task->group_leader;
470         pid = get_pid(rcu_dereference(task->pids[type].pid));
471         rcu_read_unlock();
472         return pid;
473 }
474 EXPORT_SYMBOL_GPL(get_task_pid);
475 
476 struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
477 {
478         struct task_struct *result;
479         rcu_read_lock();
480         result = pid_task(pid, type);
481         if (result)
482                 get_task_struct(result);
483         rcu_read_unlock();
484         return result;
485 }
486 EXPORT_SYMBOL_GPL(get_pid_task);
487 
488 struct pid *find_get_pid(pid_t nr)
489 {
490         struct pid *pid;
491 
492         rcu_read_lock();
493         pid = get_pid(find_vpid(nr));
494         rcu_read_unlock();
495 
496         return pid;
497 }
498 EXPORT_SYMBOL_GPL(find_get_pid);
499 
500 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
501 {
502         struct upid *upid;
503         pid_t nr = 0;
504 
505         if (pid && ns->level <= pid->level) {
506                 upid = &pid->numbers[ns->level];
507                 if (upid->ns == ns)
508                         nr = upid->nr;
509         }
510         return nr;
511 }
512 EXPORT_SYMBOL_GPL(pid_nr_ns);
513 
514 pid_t pid_vnr(struct pid *pid)
515 {
516         return pid_nr_ns(pid, task_active_pid_ns(current));
517 }
518 EXPORT_SYMBOL_GPL(pid_vnr);
519 
520 pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
521                         struct pid_namespace *ns)
522 {
523         pid_t nr = 0;
524 
525         rcu_read_lock();
526         if (!ns)
527                 ns = task_active_pid_ns(current);
528         if (likely(pid_alive(task))) {
529                 if (type != PIDTYPE_PID)
530                         task = task->group_leader;
531                 nr = pid_nr_ns(rcu_dereference(task->pids[type].pid), ns);
532         }
533         rcu_read_unlock();
534 
535         return nr;
536 }
537 EXPORT_SYMBOL(__task_pid_nr_ns);
538 
539 pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
540 {
541         return pid_nr_ns(task_tgid(tsk), ns);
542 }
543 EXPORT_SYMBOL(task_tgid_nr_ns);
544 
545 struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
546 {
547         return ns_of_pid(task_pid(tsk));
548 }
549 EXPORT_SYMBOL_GPL(task_active_pid_ns);
550 
551 /*
552  * Used by proc to find the first pid that is greater than or equal to nr.
553  *
554  * If there is a pid at nr this function is exactly the same as find_pid_ns.
555  */
556 struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
557 {
558         struct pid *pid;
559 
560         do {
561                 pid = find_pid_ns(nr, ns);
562                 if (pid)
563                         break;
564                 nr = next_pidmap(ns, nr);
565         } while (nr > 0);
566 
567         return pid;
568 }
569 
570 /*
571  * The pid hash table is scaled according to the amount of memory in the
572  * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
573  * more.
574  */
575 void __init pidhash_init(void)
576 {
577         unsigned int i, pidhash_size;
578 
579         pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
580                                            HASH_EARLY | HASH_SMALL,
581                                            &pidhash_shift, NULL,
582                                            0, 4096);
583         pidhash_size = 1U << pidhash_shift;
584 
585         for (i = 0; i < pidhash_size; i++)
586                 INIT_HLIST_HEAD(&pid_hash[i]);
587 }
588 
589 void __init pidmap_init(void)
590 {
591         /* Verify no one has done anything silly: */
592         BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_HASH_ADDING);
593 
594         /* bump default and minimum pid_max based on number of cpus */
595         pid_max = min(pid_max_max, max_t(int, pid_max,
596                                 PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
597         pid_max_min = max_t(int, pid_max_min,
598                                 PIDS_PER_CPU_MIN * num_possible_cpus());
599         pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
600 
601         init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
602         /* Reserve PID 0. We never call free_pidmap(0) */
603         set_bit(0, init_pid_ns.pidmap[0].page);
604         atomic_dec(&init_pid_ns.pidmap[0].nr_free);
605 
606         init_pid_ns.pid_cachep = KMEM_CACHE(pid,
607                         SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
608 }
609 

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