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