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 (IS_ERR_VALUE(nr)) {
 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_assert(rcu_read_lock_held(),
 455                            "find_task_by_pid_ns() needs rcu_read_lock()"
 456                            " protection");
 457         return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
 458 }
 459
 460 struct task_struct *find_task_by_vpid(pid_t vnr)
 461 {
 462         return find_task_by_pid_ns(vnr, task_active_pid_ns(current));
 463 }
 464
 465 struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 466 {
 467         struct pid *pid;
 468         rcu_read_lock();
 469         if (type != PIDTYPE_PID)
 470                 task = task->group_leader;
 471         pid = get_pid(task->pids[type].pid);
 472         rcu_read_unlock();
 473         return pid;
 474 }
 475 EXPORT_SYMBOL_GPL(get_task_pid);
 476
 477 struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
 478 {
 479         struct task_struct *result;
 480         rcu_read_lock();
 481         result = pid_task(pid, type);
 482         if (result)
 483                 get_task_struct(result);
 484         rcu_read_unlock();
 485         return result;
 486 }
 487 EXPORT_SYMBOL_GPL(get_pid_task);
 488
 489 struct pid *find_get_pid(pid_t nr)
 490 {
 491         struct pid *pid;
 492
 493         rcu_read_lock();
 494         pid = get_pid(find_vpid(nr));
 495         rcu_read_unlock();
 496
 497         return pid;
 498 }
 499 EXPORT_SYMBOL_GPL(find_get_pid);
 500
 501 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
 502 {
 503         struct upid *upid;
 504         pid_t nr = 0;
 505
 506         if (pid && ns->level <= pid->level) {
 507                 upid = &pid->numbers[ns->level];
 508                 if (upid->ns == ns)
 509                         nr = upid->nr;
 510         }
 511         return nr;
 512 }
 513 EXPORT_SYMBOL_GPL(pid_nr_ns);
 514
 515 pid_t pid_vnr(struct pid *pid)
 516 {
 517         return pid_nr_ns(pid, task_active_pid_ns(current));
 518 }
 519 EXPORT_SYMBOL_GPL(pid_vnr);
 520
 521 pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
 522                         struct pid_namespace *ns)
 523 {
 524         pid_t nr = 0;
 525
 526         rcu_read_lock();
 527         if (!ns)
 528                 ns = task_active_pid_ns(current);
 529         if (likely(pid_alive(task))) {
 530                 if (type != PIDTYPE_PID)
 531                         task = task->group_leader;
 532                 nr = pid_nr_ns(task->pids[type].pid, ns);
 533         }
 534         rcu_read_unlock();
 535
 536         return nr;
 537 }
 538 EXPORT_SYMBOL(__task_pid_nr_ns);
 539
 540 pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
 541 {
 542         return pid_nr_ns(task_tgid(tsk), ns);
 543 }
 544 EXPORT_SYMBOL(task_tgid_nr_ns);
 545
 546 struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
 547 {
 548         return ns_of_pid(task_pid(tsk));
 549 }
 550 EXPORT_SYMBOL_GPL(task_active_pid_ns);
 551
 552 /*
 553  * Used by proc to find the first pid that is greater than or equal to nr.
 554  *
 555  * If there is a pid at nr this function is exactly the same as find_pid_ns.
 556  */
 557 struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 558 {
 559         struct pid *pid;
 560
 561         do {
 562                 pid = find_pid_ns(nr, ns);
 563                 if (pid)
 564                         break;
 565                 nr = next_pidmap(ns, nr);
 566         } while (nr > 0);
 567
 568         return pid;
 569 }
 570
 571 /*
 572  * The pid hash table is scaled according to the amount of memory in the
 573  * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 574  * more.
 575  */
 576 void __init pidhash_init(void)
 577 {
 578         unsigned int i, pidhash_size;
 579
 580         pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
 581                                            HASH_EARLY | HASH_SMALL,
 582                                            &pidhash_shift, NULL,
 583                                            0, 4096);
 584         pidhash_size = 1U << pidhash_shift;
 585
 586         for (i = 0; i < pidhash_size; i++)
 587                 INIT_HLIST_HEAD(&pid_hash[i]);
 588 }
 589
 590 void __init pidmap_init(void)
 591 {
 592         /* Veryify no one has done anything silly */
 593         BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_HASH_ADDING);
 594
 595         /* bump default and minimum pid_max based on number of cpus */
 596         pid_max = min(pid_max_max, max_t(int, pid_max,
 597                                 PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
 598         pid_max_min = max_t(int, pid_max_min,
 599                                 PIDS_PER_CPU_MIN * num_possible_cpus());
 600         pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
 601
 602         init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 603         /* Reserve PID 0. We never call free_pidmap(0) */
 604         set_bit(0, init_pid_ns.pidmap[0].page);
 605         atomic_dec(&init_pid_ns.pidmap[0].nr_free);
 606
 607         init_pid_ns.pid_cachep = KMEM_CACHE(pid,
 608                         SLAB_HWCACHE_ALIGN | SLAB_PANIC);
 609 }