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Linux/kernel/pid.c

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

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