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_fs.h>
  40
  41 #define pid_hashfn(nr, ns)      \
  42         hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
  43 static struct hlist_head *pid_hash;
  44 static unsigned int pidhash_shift = 4;
  45 struct pid init_struct_pid = INIT_STRUCT_PID;
  46
  47 int pid_max = PID_MAX_DEFAULT;
  48
  49 #define RESERVED_PIDS           300
  50
  51 int pid_max_min = RESERVED_PIDS + 1;
  52 int pid_max_max = PID_MAX_LIMIT;
  53
  54 #define BITS_PER_PAGE           (PAGE_SIZE*8)
  55 #define BITS_PER_PAGE_MASK      (BITS_PER_PAGE-1)
  56
  57 static inline int mk_pid(struct pid_namespace *pid_ns,
  58                 struct pidmap *map, int off)
  59 {
  60         return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;
  61 }
  62
  63 #define find_next_offset(map, off)                                      \
  64                 find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
  65
  66 /*
  67  * PID-map pages start out as NULL, they get allocated upon
  68  * first use and are never deallocated. This way a low pid_max
  69  * value does not cause lots of bitmaps to be allocated, but
  70  * the scheme scales to up to 4 million PIDs, runtime.
  71  */
  72 struct pid_namespace init_pid_ns = {
  73         .kref = {
  74                 .refcount       = ATOMIC_INIT(2),
  75         },
  76         .pidmap = {
  77                 [ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
  78         },
  79         .last_pid = 0,
  80         .level = 0,
  81         .child_reaper = &init_task,
  82         .user_ns = &init_user_ns,
  83         .proc_inum = PROC_PID_INIT_INO,
  84 };
  85 EXPORT_SYMBOL_GPL(init_pid_ns);
  86
  87 /*
  88  * Note: disable interrupts while the pidmap_lock is held as an
  89  * interrupt might come in and do read_lock(&tasklist_lock).
  90  *
  91  * If we don't disable interrupts there is a nasty deadlock between
  92  * detach_pid()->free_pid() and another cpu that does
  93  * spin_lock(&pidmap_lock) followed by an interrupt routine that does
  94  * read_lock(&tasklist_lock);
  95  *
  96  * After we clean up the tasklist_lock and know there are no
  97  * irq handlers that take it we can leave the interrupts enabled.
  98  * For now it is easier to be safe than to prove it can't happen.
  99  */
 100
 101 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
 102
 103 static void free_pidmap(struct upid *upid)
 104 {
 105         int nr = upid->nr;
 106         struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
 107         int offset = nr & BITS_PER_PAGE_MASK;
 108
 109         clear_bit(offset, map->page);
 110         atomic_inc(&map->nr_free);
 111 }
 112
 113 /*
 114  * If we started walking pids at 'base', is 'a' seen before 'b'?
 115  */
 116 static int pid_before(int base, int a, int b)
 117 {
 118         /*
 119          * This is the same as saying
 120          *
 121          * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
 122          * and that mapping orders 'a' and 'b' with respect to 'base'.
 123          */
 124         return (unsigned)(a - base) < (unsigned)(b - base);
 125 }
 126
 127 /*
 128  * We might be racing with someone else trying to set pid_ns->last_pid
 129  * at the pid allocation time (there's also a sysctl for this, but racing
 130  * with this one is OK, see comment in kernel/pid_namespace.c about it).
 131  * We want the winner to have the "later" value, because if the
 132  * "earlier" value prevails, then a pid may get reused immediately.
 133  *
 134  * Since pids rollover, it is not sufficient to just pick the bigger
 135  * value.  We have to consider where we started counting from.
 136  *
 137  * 'base' is the value of pid_ns->last_pid that we observed when
 138  * we started looking for a pid.
 139  *
 140  * 'pid' is the pid that we eventually found.
 141  */
 142 static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
 143 {
 144         int prev;
 145         int last_write = base;
 146         do {
 147                 prev = last_write;
 148                 last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
 149         } while ((prev != last_write) && (pid_before(base, last_write, pid)));
 150 }
 151
 152 static int alloc_pidmap(struct pid_namespace *pid_ns)
 153 {
 154         int i, offset, max_scan, pid, last = pid_ns->last_pid;
 155         struct pidmap *map;
 156
 157         pid = last + 1;
 158         if (pid >= pid_max)
 159                 pid = RESERVED_PIDS;
 160         offset = pid & BITS_PER_PAGE_MASK;
 161         map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
 162         /*
 163          * If last_pid points into the middle of the map->page we
 164          * want to scan this bitmap block twice, the second time
 165          * we start with offset == 0 (or RESERVED_PIDS).
 166          */
 167         max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
 168         for (i = 0; i <= max_scan; ++i) {
 169                 if (unlikely(!map->page)) {
 170                         void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 171                         /*
 172                          * Free the page if someone raced with us
 173                          * installing it:
 174                          */
 175                         spin_lock_irq(&pidmap_lock);
 176                         if (!map->page) {
 177                                 map->page = page;
 178                                 page = NULL;
 179                         }
 180                         spin_unlock_irq(&pidmap_lock);
 181                         kfree(page);
 182                         if (unlikely(!map->page))
 183                                 break;
 184                 }
 185                 if (likely(atomic_read(&map->nr_free))) {
 186                         do {
 187                                 if (!test_and_set_bit(offset, map->page)) {
 188                                         atomic_dec(&map->nr_free);
 189                                         set_last_pid(pid_ns, last, pid);
 190                                         return pid;
 191                                 }
 192                                 offset = find_next_offset(map, offset);
 193                                 pid = mk_pid(pid_ns, map, offset);
 194                         } while (offset < BITS_PER_PAGE && pid < pid_max);
 195                 }
 196                 if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
 197                         ++map;
 198                         offset = 0;
 199                 } else {
 200                         map = &pid_ns->pidmap[0];
 201                         offset = RESERVED_PIDS;
 202                         if (unlikely(last == offset))
 203                                 break;
 204                 }
 205                 pid = mk_pid(pid_ns, map, offset);
 206         }
 207         return -1;
 208 }
 209
 210 int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
 211 {
 212         int offset;
 213         struct pidmap *map, *end;
 214
 215         if (last >= PID_MAX_LIMIT)
 216                 return -1;
 217
 218         offset = (last + 1) & BITS_PER_PAGE_MASK;
 219         map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
 220         end = &pid_ns->pidmap[PIDMAP_ENTRIES];
 221         for (; map < end; map++, offset = 0) {
 222                 if (unlikely(!map->page))
 223                         continue;
 224                 offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
 225                 if (offset < BITS_PER_PAGE)
 226                         return mk_pid(pid_ns, map, offset);
 227         }
 228         return -1;
 229 }
 230
 231 void put_pid(struct pid *pid)
 232 {
 233         struct pid_namespace *ns;
 234
 235         if (!pid)
 236                 return;
 237
 238         ns = pid->numbers[pid->level].ns;
 239         if ((atomic_read(&pid->count) == 1) ||
 240              atomic_dec_and_test(&pid->count)) {
 241                 kmem_cache_free(ns->pid_cachep, pid);
 242                 put_pid_ns(ns);
 243         }
 244 }
 245 EXPORT_SYMBOL_GPL(put_pid);
 246
 247 static void delayed_put_pid(struct rcu_head *rhp)
 248 {
 249         struct pid *pid = container_of(rhp, struct pid, rcu);
 250         put_pid(pid);
 251 }
 252
 253 void free_pid(struct pid *pid)
 254 {
 255         /* We can be called with write_lock_irq(&tasklist_lock) held */
 256         int i;
 257         unsigned long flags;
 258
 259         spin_lock_irqsave(&pidmap_lock, flags);
 260         for (i = 0; i <= pid->level; i++) {
 261                 struct upid *upid = pid->numbers + i;
 262                 struct pid_namespace *ns = upid->ns;
 263                 hlist_del_rcu(&upid->pid_chain);
 264                 switch(--ns->nr_hashed) {
 265                 case 1:
 266                         /* When all that is left in the pid namespace
 267                          * is the reaper wake up the reaper.  The reaper
 268                          * may be sleeping in zap_pid_ns_processes().
 269                          */
 270                         wake_up_process(ns->child_reaper);
 271                         break;
 272                 case 0:
 273                         schedule_work(&ns->proc_work);
 274                         break;
 275                 }
 276         }
 277         spin_unlock_irqrestore(&pidmap_lock, flags);
 278
 279         for (i = 0; i <= pid->level; i++)
 280                 free_pidmap(pid->numbers + i);
 281
 282         call_rcu(&pid->rcu, delayed_put_pid);
 283 }
 284
 285 struct pid *alloc_pid(struct pid_namespace *ns)
 286 {
 287         struct pid *pid;
 288         enum pid_type type;
 289         int i, nr;
 290         struct pid_namespace *tmp;
 291         struct upid *upid;
 292
 293         pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
 294         if (!pid)
 295                 goto out;
 296
 297         tmp = ns;
 298         pid->level = ns->level;
 299         for (i = ns->level; i >= 0; i--) {
 300                 nr = alloc_pidmap(tmp);
 301                 if (nr < 0)
 302                         goto out_free;
 303
 304                 pid->numbers[i].nr = nr;
 305                 pid->numbers[i].ns = tmp;
 306                 tmp = tmp->parent;
 307         }
 308
 309         if (unlikely(is_child_reaper(pid))) {
 310                 if (pid_ns_prepare_proc(ns))
 311                         goto out_free;
 312         }
 313
 314         get_pid_ns(ns);
 315         atomic_set(&pid->count, 1);
 316         for (type = 0; type < PIDTYPE_MAX; ++type)
 317                 INIT_HLIST_HEAD(&pid->tasks[type]);
 318
 319         upid = pid->numbers + ns->level;
 320         spin_lock_irq(&pidmap_lock);
 321         if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
 322                 goto out_unlock;
 323         for ( ; upid >= pid->numbers; --upid) {
 324                 hlist_add_head_rcu(&upid->pid_chain,
 325                                 &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
 326                 upid->ns->nr_hashed++;
 327         }
 328         spin_unlock_irq(&pidmap_lock);
 329
 330 out:
 331         return pid;
 332
 333 out_unlock:
 334         spin_unlock_irq(&pidmap_lock);
 335 out_free:
 336         while (++i <= ns->level)
 337                 free_pidmap(pid->numbers + i);
 338
 339         kmem_cache_free(ns->pid_cachep, pid);
 340         pid = NULL;
 341         goto out;
 342 }
 343
 344 void disable_pid_allocation(struct pid_namespace *ns)
 345 {
 346         spin_lock_irq(&pidmap_lock);
 347         ns->nr_hashed &= ~PIDNS_HASH_ADDING;
 348         spin_unlock_irq(&pidmap_lock);
 349 }
 350
 351 struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
 352 {
 353         struct upid *pnr;
 354
 355         hlist_for_each_entry_rcu(pnr,
 356                         &pid_hash[pid_hashfn(nr, ns)], pid_chain)
 357                 if (pnr->nr == nr && pnr->ns == ns)
 358                         return container_of(pnr, struct pid,
 359                                         numbers[ns->level]);
 360
 361         return NULL;
 362 }
 363 EXPORT_SYMBOL_GPL(find_pid_ns);
 364
 365 struct pid *find_vpid(int nr)
 366 {
 367         return find_pid_ns(nr, task_active_pid_ns(current));
 368 }
 369 EXPORT_SYMBOL_GPL(find_vpid);
 370
 371 /*
 372  * attach_pid() must be called with the tasklist_lock write-held.
 373  */
 374 void attach_pid(struct task_struct *task, enum pid_type type,
 375                 struct pid *pid)
 376 {
 377         struct pid_link *link;
 378
 379         link = &task->pids[type];
 380         link->pid = pid;
 381         hlist_add_head_rcu(&link->node, &pid->tasks[type]);
 382 }
 383
 384 static void __change_pid(struct task_struct *task, enum pid_type type,
 385                         struct pid *new)
 386 {
 387         struct pid_link *link;
 388         struct pid *pid;
 389         int tmp;
 390
 391         link = &task->pids[type];
 392         pid = link->pid;
 393
 394         hlist_del_rcu(&link->node);
 395         link->pid = new;
 396
 397         for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 398                 if (!hlist_empty(&pid->tasks[tmp]))
 399                         return;
 400
 401         free_pid(pid);
 402 }
 403
 404 void detach_pid(struct task_struct *task, enum pid_type type)
 405 {
 406         __change_pid(task, type, NULL);
 407 }
 408
 409 void change_pid(struct task_struct *task, enum pid_type type,
 410                 struct pid *pid)
 411 {
 412         __change_pid(task, type, pid);
 413         attach_pid(task, type, pid);
 414 }
 415
 416 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
 417 void transfer_pid(struct task_struct *old, struct task_struct *new,
 418                            enum pid_type type)
 419 {
 420         new->pids[type].pid = old->pids[type].pid;
 421         hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
 422 }
 423
 424 struct task_struct *pid_task(struct pid *pid, enum pid_type type)
 425 {
 426         struct task_struct *result = NULL;
 427         if (pid) {
 428                 struct hlist_node *first;
 429                 first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
 430                                               lockdep_tasklist_lock_is_held());
 431                 if (first)
 432                         result = hlist_entry(first, struct task_struct, pids[(type)].node);
 433         }
 434         return result;
 435 }
 436 EXPORT_SYMBOL(pid_task);
 437
 438 /*
 439  * Must be called under rcu_read_lock().
 440  */
 441 struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
 442 {
 443         rcu_lockdep_assert(rcu_read_lock_held(),
 444                            "find_task_by_pid_ns() needs rcu_read_lock()"
 445                            " protection");
 446         return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
 447 }
 448
 449 struct task_struct *find_task_by_vpid(pid_t vnr)
 450 {
 451         return find_task_by_pid_ns(vnr, task_active_pid_ns(current));
 452 }
 453
 454 struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 455 {
 456         struct pid *pid;
 457         rcu_read_lock();
 458         if (type != PIDTYPE_PID)
 459                 task = task->group_leader;
 460         pid = get_pid(task->pids[type].pid);
 461         rcu_read_unlock();
 462         return pid;
 463 }
 464 EXPORT_SYMBOL_GPL(get_task_pid);
 465
 466 struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
 467 {
 468         struct task_struct *result;
 469         rcu_read_lock();
 470         result = pid_task(pid, type);
 471         if (result)
 472                 get_task_struct(result);
 473         rcu_read_unlock();
 474         return result;
 475 }
 476 EXPORT_SYMBOL_GPL(get_pid_task);
 477
 478 struct pid *find_get_pid(pid_t nr)
 479 {
 480         struct pid *pid;
 481
 482         rcu_read_lock();
 483         pid = get_pid(find_vpid(nr));
 484         rcu_read_unlock();
 485
 486         return pid;
 487 }
 488 EXPORT_SYMBOL_GPL(find_get_pid);
 489
 490 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
 491 {
 492         struct upid *upid;
 493         pid_t nr = 0;
 494
 495         if (pid && ns->level <= pid->level) {
 496                 upid = &pid->numbers[ns->level];
 497                 if (upid->ns == ns)
 498                         nr = upid->nr;
 499         }
 500         return nr;
 501 }
 502 EXPORT_SYMBOL_GPL(pid_nr_ns);
 503
 504 pid_t pid_vnr(struct pid *pid)
 505 {
 506         return pid_nr_ns(pid, task_active_pid_ns(current));
 507 }
 508 EXPORT_SYMBOL_GPL(pid_vnr);
 509
 510 pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
 511                         struct pid_namespace *ns)
 512 {
 513         pid_t nr = 0;
 514
 515         rcu_read_lock();
 516         if (!ns)
 517                 ns = task_active_pid_ns(current);
 518         if (likely(pid_alive(task))) {
 519                 if (type != PIDTYPE_PID)
 520                         task = task->group_leader;
 521                 nr = pid_nr_ns(task->pids[type].pid, ns);
 522         }
 523         rcu_read_unlock();
 524
 525         return nr;
 526 }
 527 EXPORT_SYMBOL(__task_pid_nr_ns);
 528
 529 pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
 530 {
 531         return pid_nr_ns(task_tgid(tsk), ns);
 532 }
 533 EXPORT_SYMBOL(task_tgid_nr_ns);
 534
 535 struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
 536 {
 537         return ns_of_pid(task_pid(tsk));
 538 }
 539 EXPORT_SYMBOL_GPL(task_active_pid_ns);
 540
 541 /*
 542  * Used by proc to find the first pid that is greater than or equal to nr.
 543  *
 544  * If there is a pid at nr this function is exactly the same as find_pid_ns.
 545  */
 546 struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 547 {
 548         struct pid *pid;
 549
 550         do {
 551                 pid = find_pid_ns(nr, ns);
 552                 if (pid)
 553                         break;
 554                 nr = next_pidmap(ns, nr);
 555         } while (nr > 0);
 556
 557         return pid;
 558 }
 559
 560 /*
 561  * The pid hash table is scaled according to the amount of memory in the
 562  * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 563  * more.
 564  */
 565 void __init pidhash_init(void)
 566 {
 567         unsigned int i, pidhash_size;
 568
 569         pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
 570                                            HASH_EARLY | HASH_SMALL,
 571                                            &pidhash_shift, NULL,
 572                                            0, 4096);
 573         pidhash_size = 1U << pidhash_shift;
 574
 575         for (i = 0; i < pidhash_size; i++)
 576                 INIT_HLIST_HEAD(&pid_hash[i]);
 577 }
 578
 579 void __init pidmap_init(void)
 580 {
 581         /* Veryify no one has done anything silly */
 582         BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_HASH_ADDING);
 583
 584         /* bump default and minimum pid_max based on number of cpus */
 585         pid_max = min(pid_max_max, max_t(int, pid_max,
 586                                 PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
 587         pid_max_min = max_t(int, pid_max_min,
 588                                 PIDS_PER_CPU_MIN * num_possible_cpus());
 589         pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
 590
 591         init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 592         /* Reserve PID 0. We never call free_pidmap(0) */
 593         set_bit(0, init_pid_ns.pidmap[0].page);
 594         atomic_dec(&init_pid_ns.pidmap[0].nr_free);
 595         init_pid_ns.nr_hashed = PIDNS_HASH_ADDING;
 596
 597         init_pid_ns.pid_cachep = KMEM_CACHE(pid,
 598                         SLAB_HWCACHE_ALIGN | SLAB_PANIC);
 599 }