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                                 break;
 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 -1;
 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
 305         pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
 306         if (!pid)
 307                 goto out;
 308
 309         tmp = ns;
 310         pid->level = ns->level;
 311         for (i = ns->level; i >= 0; i--) {
 312                 nr = alloc_pidmap(tmp);
 313                 if (nr < 0)
 314                         goto out_free;
 315
 316                 pid->numbers[i].nr = nr;
 317                 pid->numbers[i].ns = tmp;
 318                 tmp = tmp->parent;
 319         }
 320
 321         if (unlikely(is_child_reaper(pid))) {
 322                 if (pid_ns_prepare_proc(ns))
 323                         goto out_free;
 324         }
 325
 326         get_pid_ns(ns);
 327         atomic_set(&pid->count, 1);
 328         for (type = 0; type < PIDTYPE_MAX; ++type)
 329                 INIT_HLIST_HEAD(&pid->tasks[type]);
 330
 331         upid = pid->numbers + ns->level;
 332         spin_lock_irq(&pidmap_lock);
 333         if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
 334                 goto out_unlock;
 335         for ( ; upid >= pid->numbers; --upid) {
 336                 hlist_add_head_rcu(&upid->pid_chain,
 337                                 &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
 338                 upid->ns->nr_hashed++;
 339         }
 340         spin_unlock_irq(&pidmap_lock);
 341
 342 out:
 343         return pid;
 344
 345 out_unlock:
 346         spin_unlock_irq(&pidmap_lock);
 347         put_pid_ns(ns);
 348
 349 out_free:
 350         while (++i <= ns->level)
 351                 free_pidmap(pid->numbers + i);
 352
 353         kmem_cache_free(ns->pid_cachep, pid);
 354         pid = NULL;
 355         goto out;
 356 }
 357
 358 void disable_pid_allocation(struct pid_namespace *ns)
 359 {
 360         spin_lock_irq(&pidmap_lock);
 361         ns->nr_hashed &= ~PIDNS_HASH_ADDING;
 362         spin_unlock_irq(&pidmap_lock);
 363 }
 364
 365 struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
 366 {
 367         struct upid *pnr;
 368
 369         hlist_for_each_entry_rcu(pnr,
 370                         &pid_hash[pid_hashfn(nr, ns)], pid_chain)
 371                 if (pnr->nr == nr && pnr->ns == ns)
 372                         return container_of(pnr, struct pid,
 373                                         numbers[ns->level]);
 374
 375         return NULL;
 376 }
 377 EXPORT_SYMBOL_GPL(find_pid_ns);
 378
 379 struct pid *find_vpid(int nr)
 380 {
 381         return find_pid_ns(nr, task_active_pid_ns(current));
 382 }
 383 EXPORT_SYMBOL_GPL(find_vpid);
 384
 385 /*
 386  * attach_pid() must be called with the tasklist_lock write-held.
 387  */
 388 void attach_pid(struct task_struct *task, enum pid_type type)
 389 {
 390         struct pid_link *link = &task->pids[type];
 391         hlist_add_head_rcu(&link->node, &link->pid->tasks[type]);
 392 }
 393
 394 static void __change_pid(struct task_struct *task, enum pid_type type,
 395                         struct pid *new)
 396 {
 397         struct pid_link *link;
 398         struct pid *pid;
 399         int tmp;
 400
 401         link = &task->pids[type];
 402         pid = link->pid;
 403
 404         hlist_del_rcu(&link->node);
 405         link->pid = new;
 406
 407         for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 408                 if (!hlist_empty(&pid->tasks[tmp]))
 409                         return;
 410
 411         free_pid(pid);
 412 }
 413
 414 void detach_pid(struct task_struct *task, enum pid_type type)
 415 {
 416         __change_pid(task, type, NULL);
 417 }
 418
 419 void change_pid(struct task_struct *task, enum pid_type type,
 420                 struct pid *pid)
 421 {
 422         __change_pid(task, type, pid);
 423         attach_pid(task, type);
 424 }
 425
 426 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
 427 void transfer_pid(struct task_struct *old, struct task_struct *new,
 428                            enum pid_type type)
 429 {
 430         new->pids[type].pid = old->pids[type].pid;
 431         hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
 432 }
 433
 434 struct task_struct *pid_task(struct pid *pid, enum pid_type type)
 435 {
 436         struct task_struct *result = NULL;
 437         if (pid) {
 438                 struct hlist_node *first;
 439                 first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
 440                                               lockdep_tasklist_lock_is_held());
 441                 if (first)
 442                         result = hlist_entry(first, struct task_struct, pids[(type)].node);
 443         }
 444         return result;
 445 }
 446 EXPORT_SYMBOL(pid_task);
 447
 448 /*
 449  * Must be called under rcu_read_lock().
 450  */
 451 struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
 452 {
 453         rcu_lockdep_assert(rcu_read_lock_held(),
 454                            "find_task_by_pid_ns() needs rcu_read_lock()"
 455                            " 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(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(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         /* Veryify 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);
 608 }