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 #define BITS_PER_PAGE           (PAGE_SIZE*8)
  56 #define BITS_PER_PAGE_MASK      (BITS_PER_PAGE-1)
  57
  58 static inline int mk_pid(struct pid_namespace *pid_ns,
  59                 struct pidmap *map, int off)
  60 {
  61         return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;
  62 }
  63
  64 #define find_next_offset(map, off)                                      \
  65                 find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
  66
  67 /*
  68  * PID-map pages start out as NULL, they get allocated upon
  69  * first use and are never deallocated. This way a low pid_max
  70  * value does not cause lots of bitmaps to be allocated, but
  71  * the scheme scales to up to 4 million PIDs, runtime.
  72  */
  73 struct pid_namespace init_pid_ns = {
  74         .kref = {
  75                 .refcount       = ATOMIC_INIT(2),
  76         },
  77         .pidmap = {
  78                 [ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
  79         },
  80         .last_pid = 0,
  81         .level = 0,
  82         .child_reaper = &init_task,
  83         .user_ns = &init_user_ns,
  84         .proc_inum = PROC_PID_INIT_INO,
  85 };
  86 EXPORT_SYMBOL_GPL(init_pid_ns);
  87
  88 /*
  89  * Note: disable interrupts while the pidmap_lock is held as an
  90  * interrupt might come in and do read_lock(&tasklist_lock).
  91  *
  92  * If we don't disable interrupts there is a nasty deadlock between
  93  * detach_pid()->free_pid() and another cpu that does
  94  * spin_lock(&pidmap_lock) followed by an interrupt routine that does
  95  * read_lock(&tasklist_lock);
  96  *
  97  * After we clean up the tasklist_lock and know there are no
  98  * irq handlers that take it we can leave the interrupts enabled.
  99  * For now it is easier to be safe than to prove it can't happen.
 100  */
 101
 102 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
 103
 104 static void free_pidmap(struct upid *upid)
 105 {
 106         int nr = upid->nr;
 107         struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
 108         int offset = nr & BITS_PER_PAGE_MASK;
 109
 110         clear_bit(offset, map->page);
 111         atomic_inc(&map->nr_free);
 112 }
 113
 114 /*
 115  * If we started walking pids at 'base', is 'a' seen before 'b'?
 116  */
 117 static int pid_before(int base, int a, int b)
 118 {
 119         /*
 120          * This is the same as saying
 121          *
 122          * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
 123          * and that mapping orders 'a' and 'b' with respect to 'base'.
 124          */
 125         return (unsigned)(a - base) < (unsigned)(b - base);
 126 }
 127
 128 /*
 129  * We might be racing with someone else trying to set pid_ns->last_pid
 130  * at the pid allocation time (there's also a sysctl for this, but racing
 131  * with this one is OK, see comment in kernel/pid_namespace.c about it).
 132  * We want the winner to have the "later" value, because if the
 133  * "earlier" value prevails, then a pid may get reused immediately.
 134  *
 135  * Since pids rollover, it is not sufficient to just pick the bigger
 136  * value.  We have to consider where we started counting from.
 137  *
 138  * 'base' is the value of pid_ns->last_pid that we observed when
 139  * we started looking for a pid.
 140  *
 141  * 'pid' is the pid that we eventually found.
 142  */
 143 static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
 144 {
 145         int prev;
 146         int last_write = base;
 147         do {
 148                 prev = last_write;
 149                 last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
 150         } while ((prev != last_write) && (pid_before(base, last_write, pid)));
 151 }
 152
 153 static int alloc_pidmap(struct pid_namespace *pid_ns)
 154 {
 155         int i, offset, max_scan, pid, last = pid_ns->last_pid;
 156         struct pidmap *map;
 157
 158         pid = last + 1;
 159         if (pid >= pid_max)
 160                 pid = RESERVED_PIDS;
 161         offset = pid & BITS_PER_PAGE_MASK;
 162         map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
 163         /*
 164          * If last_pid points into the middle of the map->page we
 165          * want to scan this bitmap block twice, the second time
 166          * we start with offset == 0 (or RESERVED_PIDS).
 167          */
 168         max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
 169         for (i = 0; i <= max_scan; ++i) {
 170                 if (unlikely(!map->page)) {
 171                         void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 172                         /*
 173                          * Free the page if someone raced with us
 174                          * installing it:
 175                          */
 176                         spin_lock_irq(&pidmap_lock);
 177                         if (!map->page) {
 178                                 map->page = page;
 179                                 page = NULL;
 180                         }
 181                         spin_unlock_irq(&pidmap_lock);
 182                         kfree(page);
 183                         if (unlikely(!map->page))
 184                                 break;
 185                 }
 186                 if (likely(atomic_read(&map->nr_free))) {
 187                         do {
 188                                 if (!test_and_set_bit(offset, map->page)) {
 189                                         atomic_dec(&map->nr_free);
 190                                         set_last_pid(pid_ns, last, pid);
 191                                         return pid;
 192                                 }
 193                                 offset = find_next_offset(map, offset);
 194                                 pid = mk_pid(pid_ns, map, offset);
 195                         } while (offset < BITS_PER_PAGE && pid < pid_max);
 196                 }
 197                 if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
 198                         ++map;
 199                         offset = 0;
 200                 } else {
 201                         map = &pid_ns->pidmap[0];
 202                         offset = RESERVED_PIDS;
 203                         if (unlikely(last == offset))
 204                                 break;
 205                 }
 206                 pid = mk_pid(pid_ns, map, offset);
 207         }
 208         return -1;
 209 }
 210
 211 int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
 212 {
 213         int offset;
 214         struct pidmap *map, *end;
 215
 216         if (last >= PID_MAX_LIMIT)
 217                 return -1;
 218
 219         offset = (last + 1) & BITS_PER_PAGE_MASK;
 220         map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
 221         end = &pid_ns->pidmap[PIDMAP_ENTRIES];
 222         for (; map < end; map++, offset = 0) {
 223                 if (unlikely(!map->page))
 224                         continue;
 225                 offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
 226                 if (offset < BITS_PER_PAGE)
 227                         return mk_pid(pid_ns, map, offset);
 228         }
 229         return -1;
 230 }
 231
 232 void put_pid(struct pid *pid)
 233 {
 234         struct pid_namespace *ns;
 235
 236         if (!pid)
 237                 return;
 238
 239         ns = pid->numbers[pid->level].ns;
 240         if ((atomic_read(&pid->count) == 1) ||
 241              atomic_dec_and_test(&pid->count)) {
 242                 kmem_cache_free(ns->pid_cachep, pid);
 243                 put_pid_ns(ns);
 244         }
 245 }
 246 EXPORT_SYMBOL_GPL(put_pid);
 247
 248 static void delayed_put_pid(struct rcu_head *rhp)
 249 {
 250         struct pid *pid = container_of(rhp, struct pid, rcu);
 251         put_pid(pid);
 252 }
 253
 254 void free_pid(struct pid *pid)
 255 {
 256         /* We can be called with write_lock_irq(&tasklist_lock) held */
 257         int i;
 258         unsigned long flags;
 259
 260         spin_lock_irqsave(&pidmap_lock, flags);
 261         for (i = 0; i <= pid->level; i++) {
 262                 struct upid *upid = pid->numbers + i;
 263                 struct pid_namespace *ns = upid->ns;
 264                 hlist_del_rcu(&upid->pid_chain);
 265                 switch(--ns->nr_hashed) {
 266                 case 1:
 267                         /* When all that is left in the pid namespace
 268                          * is the reaper wake up the reaper.  The reaper
 269                          * may be sleeping in zap_pid_ns_processes().
 270                          */
 271                         wake_up_process(ns->child_reaper);
 272                         break;
 273                 case 0:
 274                         schedule_work(&ns->proc_work);
 275                         break;
 276                 }
 277         }
 278         spin_unlock_irqrestore(&pidmap_lock, flags);
 279
 280         for (i = 0; i <= pid->level; i++)
 281                 free_pidmap(pid->numbers + i);
 282
 283         call_rcu(&pid->rcu, delayed_put_pid);
 284 }
 285
 286 struct pid *alloc_pid(struct pid_namespace *ns)
 287 {
 288         struct pid *pid;
 289         enum pid_type type;
 290         int i, nr;
 291         struct pid_namespace *tmp;
 292         struct upid *upid;
 293
 294         pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
 295         if (!pid)
 296                 goto out;
 297
 298         tmp = ns;
 299         pid->level = ns->level;
 300         for (i = ns->level; i >= 0; i--) {
 301                 nr = alloc_pidmap(tmp);
 302                 if (nr < 0)
 303                         goto out_free;
 304
 305                 pid->numbers[i].nr = nr;
 306                 pid->numbers[i].ns = tmp;
 307                 tmp = tmp->parent;
 308         }
 309
 310         if (unlikely(is_child_reaper(pid))) {
 311                 if (pid_ns_prepare_proc(ns))
 312                         goto out_free;
 313         }
 314
 315         get_pid_ns(ns);
 316         atomic_set(&pid->count, 1);
 317         for (type = 0; type < PIDTYPE_MAX; ++type)
 318                 INIT_HLIST_HEAD(&pid->tasks[type]);
 319
 320         upid = pid->numbers + ns->level;
 321         spin_lock_irq(&pidmap_lock);
 322         if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
 323                 goto out_unlock;
 324         for ( ; upid >= pid->numbers; --upid) {
 325                 hlist_add_head_rcu(&upid->pid_chain,
 326                                 &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
 327                 upid->ns->nr_hashed++;
 328         }
 329         spin_unlock_irq(&pidmap_lock);
 330
 331 out:
 332         return pid;
 333
 334 out_unlock:
 335         spin_unlock_irq(&pidmap_lock);
 336 out_free:
 337         while (++i <= ns->level)
 338                 free_pidmap(pid->numbers + i);
 339
 340         kmem_cache_free(ns->pid_cachep, pid);
 341         pid = NULL;
 342         goto out;
 343 }
 344
 345 void disable_pid_allocation(struct pid_namespace *ns)
 346 {
 347         spin_lock_irq(&pidmap_lock);
 348         ns->nr_hashed &= ~PIDNS_HASH_ADDING;
 349         spin_unlock_irq(&pidmap_lock);
 350 }
 351
 352 struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
 353 {
 354         struct upid *pnr;
 355
 356         hlist_for_each_entry_rcu(pnr,
 357                         &pid_hash[pid_hashfn(nr, ns)], pid_chain)
 358                 if (pnr->nr == nr && pnr->ns == ns)
 359                         return container_of(pnr, struct pid,
 360                                         numbers[ns->level]);
 361
 362         return NULL;
 363 }
 364 EXPORT_SYMBOL_GPL(find_pid_ns);
 365
 366 struct pid *find_vpid(int nr)
 367 {
 368         return find_pid_ns(nr, task_active_pid_ns(current));
 369 }
 370 EXPORT_SYMBOL_GPL(find_vpid);
 371
 372 /*
 373  * attach_pid() must be called with the tasklist_lock write-held.
 374  */
 375 void attach_pid(struct task_struct *task, enum pid_type type,
 376                 struct pid *pid)
 377 {
 378         struct pid_link *link;
 379
 380         link = &task->pids[type];
 381         link->pid = pid;
 382         hlist_add_head_rcu(&link->node, &pid->tasks[type]);
 383 }
 384
 385 static void __change_pid(struct task_struct *task, enum pid_type type,
 386                         struct pid *new)
 387 {
 388         struct pid_link *link;
 389         struct pid *pid;
 390         int tmp;
 391
 392         link = &task->pids[type];
 393         pid = link->pid;
 394
 395         hlist_del_rcu(&link->node);
 396         link->pid = new;
 397
 398         for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 399                 if (!hlist_empty(&pid->tasks[tmp]))
 400                         return;
 401
 402         free_pid(pid);
 403 }
 404
 405 void detach_pid(struct task_struct *task, enum pid_type type)
 406 {
 407         __change_pid(task, type, NULL);
 408 }
 409
 410 void change_pid(struct task_struct *task, enum pid_type type,
 411                 struct pid *pid)
 412 {
 413         __change_pid(task, type, pid);
 414         attach_pid(task, type, pid);
 415 }
 416
 417 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
 418 void transfer_pid(struct task_struct *old, struct task_struct *new,
 419                            enum pid_type type)
 420 {
 421         new->pids[type].pid = old->pids[type].pid;
 422         hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
 423 }
 424
 425 struct task_struct *pid_task(struct pid *pid, enum pid_type type)
 426 {
 427         struct task_struct *result = NULL;
 428         if (pid) {
 429                 struct hlist_node *first;
 430                 first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
 431                                               lockdep_tasklist_lock_is_held());
 432                 if (first)
 433                         result = hlist_entry(first, struct task_struct, pids[(type)].node);
 434         }
 435         return result;
 436 }
 437 EXPORT_SYMBOL(pid_task);
 438
 439 /*
 440  * Must be called under rcu_read_lock().
 441  */
 442 struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
 443 {
 444         rcu_lockdep_assert(rcu_read_lock_held(),
 445                            "find_task_by_pid_ns() needs rcu_read_lock()"
 446                            " protection");
 447         return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
 448 }
 449
 450 struct task_struct *find_task_by_vpid(pid_t vnr)
 451 {
 452         return find_task_by_pid_ns(vnr, task_active_pid_ns(current));
 453 }
 454
 455 struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 456 {
 457         struct pid *pid;
 458         rcu_read_lock();
 459         if (type != PIDTYPE_PID)
 460                 task = task->group_leader;
 461         pid = get_pid(task->pids[type].pid);
 462         rcu_read_unlock();
 463         return pid;
 464 }
 465 EXPORT_SYMBOL_GPL(get_task_pid);
 466
 467 struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
 468 {
 469         struct task_struct *result;
 470         rcu_read_lock();
 471         result = pid_task(pid, type);
 472         if (result)
 473                 get_task_struct(result);
 474         rcu_read_unlock();
 475         return result;
 476 }
 477 EXPORT_SYMBOL_GPL(get_pid_task);
 478
 479 struct pid *find_get_pid(pid_t nr)
 480 {
 481         struct pid *pid;
 482
 483         rcu_read_lock();
 484         pid = get_pid(find_vpid(nr));
 485         rcu_read_unlock();
 486
 487         return pid;
 488 }
 489 EXPORT_SYMBOL_GPL(find_get_pid);
 490
 491 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
 492 {
 493         struct upid *upid;
 494         pid_t nr = 0;
 495
 496         if (pid && ns->level <= pid->level) {
 497                 upid = &pid->numbers[ns->level];
 498                 if (upid->ns == ns)
 499                         nr = upid->nr;
 500         }
 501         return nr;
 502 }
 503 EXPORT_SYMBOL_GPL(pid_nr_ns);
 504
 505 pid_t pid_vnr(struct pid *pid)
 506 {
 507         return pid_nr_ns(pid, task_active_pid_ns(current));
 508 }
 509 EXPORT_SYMBOL_GPL(pid_vnr);
 510
 511 pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
 512                         struct pid_namespace *ns)
 513 {
 514         pid_t nr = 0;
 515
 516         rcu_read_lock();
 517         if (!ns)
 518                 ns = task_active_pid_ns(current);
 519         if (likely(pid_alive(task))) {
 520                 if (type != PIDTYPE_PID)
 521                         task = task->group_leader;
 522                 nr = pid_nr_ns(task->pids[type].pid, ns);
 523         }
 524         rcu_read_unlock();
 525
 526         return nr;
 527 }
 528 EXPORT_SYMBOL(__task_pid_nr_ns);
 529
 530 pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
 531 {
 532         return pid_nr_ns(task_tgid(tsk), ns);
 533 }
 534 EXPORT_SYMBOL(task_tgid_nr_ns);
 535
 536 struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
 537 {
 538         return ns_of_pid(task_pid(tsk));
 539 }
 540 EXPORT_SYMBOL_GPL(task_active_pid_ns);
 541
 542 /*
 543  * Used by proc to find the first pid that is greater than or equal to nr.
 544  *
 545  * If there is a pid at nr this function is exactly the same as find_pid_ns.
 546  */
 547 struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 548 {
 549         struct pid *pid;
 550
 551         do {
 552                 pid = find_pid_ns(nr, ns);
 553                 if (pid)
 554                         break;
 555                 nr = next_pidmap(ns, nr);
 556         } while (nr > 0);
 557
 558         return pid;
 559 }
 560
 561 /*
 562  * The pid hash table is scaled according to the amount of memory in the
 563  * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 564  * more.
 565  */
 566 void __init pidhash_init(void)
 567 {
 568         unsigned int i, pidhash_size;
 569
 570         pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
 571                                            HASH_EARLY | HASH_SMALL,
 572                                            &pidhash_shift, NULL,
 573                                            0, 4096);
 574         pidhash_size = 1U << pidhash_shift;
 575
 576         for (i = 0; i < pidhash_size; i++)
 577                 INIT_HLIST_HEAD(&pid_hash[i]);
 578 }
 579
 580 void __init pidmap_init(void)
 581 {
 582         /* Veryify no one has done anything silly */
 583         BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_HASH_ADDING);
 584
 585         /* bump default and minimum pid_max based on number of cpus */
 586         pid_max = min(pid_max_max, max_t(int, pid_max,
 587                                 PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
 588         pid_max_min = max_t(int, pid_max_min,
 589                                 PIDS_PER_CPU_MIN * num_possible_cpus());
 590         pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
 591
 592         init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 593         /* Reserve PID 0. We never call free_pidmap(0) */
 594         set_bit(0, init_pid_ns.pidmap[0].page);
 595         atomic_dec(&init_pid_ns.pidmap[0].nr_free);
 596         init_pid_ns.nr_hashed = PIDNS_HASH_ADDING;
 597
 598         init_pid_ns.pid_cachep = KMEM_CACHE(pid,
 599                         SLAB_HWCACHE_ALIGN | SLAB_PANIC);
 600 }