kernel/pid.c

   1 /*
   2  * Generic pidhash and scalable, time-bounded PID allocator
   3  *
   4  * (C) 2002-2003 William Irwin, IBM
   5  * (C) 2004 William Irwin, 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/module.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
  40 #define pid_hashfn(nr, ns)      \
  41         hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
  42 static struct hlist_head *pid_hash;
  43 static unsigned int pidhash_shift = 4;
  44 struct pid init_struct_pid = INIT_STRUCT_PID;
  45
  46 int pid_max = PID_MAX_DEFAULT;
  47
  48 #define RESERVED_PIDS           300
  49
  50 int pid_max_min = RESERVED_PIDS + 1;
  51 int pid_max_max = PID_MAX_LIMIT;
  52
  53 #define BITS_PER_PAGE           (PAGE_SIZE*8)
  54 #define BITS_PER_PAGE_MASK      (BITS_PER_PAGE-1)
  55
  56 static inline int mk_pid(struct pid_namespace *pid_ns,
  57                 struct pidmap *map, int off)
  58 {
  59         return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;
  60 }
  61
  62 #define find_next_offset(map, off)                                      \
  63                 find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
  64
  65 /*
  66  * PID-map pages start out as NULL, they get allocated upon
  67  * first use and are never deallocated. This way a low pid_max
  68  * value does not cause lots of bitmaps to be allocated, but
  69  * the scheme scales to up to 4 million PIDs, runtime.
  70  */
  71 struct pid_namespace init_pid_ns = {
  72         .kref = {
  73                 .refcount       = ATOMIC_INIT(2),
  74         },
  75         .pidmap = {
  76                 [ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
  77         },
  78         .last_pid = 0,
  79         .level = 0,
  80         .child_reaper = &init_task,
  81 };
  82 EXPORT_SYMBOL_GPL(init_pid_ns);
  83
  84 int is_container_init(struct task_struct *tsk)
  85 {
  86         int ret = 0;
  87         struct pid *pid;
  88
  89         rcu_read_lock();
  90         pid = task_pid(tsk);
  91         if (pid != NULL && pid->numbers[pid->level].nr == 1)
  92                 ret = 1;
  93         rcu_read_unlock();
  94
  95         return ret;
  96 }
  97 EXPORT_SYMBOL(is_container_init);
  98
  99 /*
 100  * Note: disable interrupts while the pidmap_lock is held as an
 101  * interrupt might come in and do read_lock(&tasklist_lock).
 102  *
 103  * If we don't disable interrupts there is a nasty deadlock between
 104  * detach_pid()->free_pid() and another cpu that does
 105  * spin_lock(&pidmap_lock) followed by an interrupt routine that does
 106  * read_lock(&tasklist_lock);
 107  *
 108  * After we clean up the tasklist_lock and know there are no
 109  * irq handlers that take it we can leave the interrupts enabled.
 110  * For now it is easier to be safe than to prove it can't happen.
 111  */
 112
 113 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
 114
 115 static void free_pidmap(struct upid *upid)
 116 {
 117         int nr = upid->nr;
 118         struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
 119         int offset = nr & BITS_PER_PAGE_MASK;
 120
 121         clear_bit(offset, map->page);
 122         atomic_inc(&map->nr_free);
 123 }
 124
 125 /*
 126  * If we started walking pids at 'base', is 'a' seen before 'b'?
 127  */
 128 static int pid_before(int base, int a, int b)
 129 {
 130         /*
 131          * This is the same as saying
 132          *
 133          * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
 134          * and that mapping orders 'a' and 'b' with respect to 'base'.
 135          */
 136         return (unsigned)(a - base) < (unsigned)(b - base);
 137 }
 138
 139 /*
 140  * We might be racing with someone else trying to set pid_ns->last_pid.
 141  * We want the winner to have the "later" value, because if the
 142  * "earlier" value prevails, then a pid may get reused immediately.
 143  *
 144  * Since pids rollover, it is not sufficient to just pick the bigger
 145  * value.  We have to consider where we started counting from.
 146  *
 147  * 'base' is the value of pid_ns->last_pid that we observed when
 148  * we started looking for a pid.
 149  *
 150  * 'pid' is the pid that we eventually found.
 151  */
 152 static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
 153 {
 154         int prev;
 155         int last_write = base;
 156         do {
 157                 prev = last_write;
 158                 last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
 159         } while ((prev != last_write) && (pid_before(base, last_write, pid)));
 160 }
 161
 162 static int alloc_pidmap(struct pid_namespace *pid_ns)
 163 {
 164         int i, offset, max_scan, pid, last = pid_ns->last_pid;
 165         struct pidmap *map;
 166
 167         pid = last + 1;
 168         if (pid >= pid_max)
 169                 pid = RESERVED_PIDS;
 170         offset = pid & BITS_PER_PAGE_MASK;
 171         map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
 172         /*
 173          * If last_pid points into the middle of the map->page we
 174          * want to scan this bitmap block twice, the second time
 175          * we start with offset == 0 (or RESERVED_PIDS).
 176          */
 177         max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
 178         for (i = 0; i <= max_scan; ++i) {
 179                 if (unlikely(!map->page)) {
 180                         void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 181                         /*
 182                          * Free the page if someone raced with us
 183                          * installing it:
 184                          */
 185                         spin_lock_irq(&pidmap_lock);
 186                         if (!map->page) {
 187                                 map->page = page;
 188                                 page = NULL;
 189                         }
 190                         spin_unlock_irq(&pidmap_lock);
 191                         kfree(page);
 192                         if (unlikely(!map->page))
 193                                 break;
 194                 }
 195                 if (likely(atomic_read(&map->nr_free))) {
 196                         do {
 197                                 if (!test_and_set_bit(offset, map->page)) {
 198                                         atomic_dec(&map->nr_free);
 199                                         set_last_pid(pid_ns, last, pid);
 200                                         return pid;
 201                                 }
 202                                 offset = find_next_offset(map, offset);
 203                                 pid = mk_pid(pid_ns, map, offset);
 204                         } while (offset < BITS_PER_PAGE && pid < pid_max);
 205                 }
 206                 if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
 207                         ++map;
 208                         offset = 0;
 209                 } else {
 210                         map = &pid_ns->pidmap[0];
 211                         offset = RESERVED_PIDS;
 212                         if (unlikely(last == offset))
 213                                 break;
 214                 }
 215                 pid = mk_pid(pid_ns, map, offset);
 216         }
 217         return -1;
 218 }
 219
 220 int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
 221 {
 222         int offset;
 223         struct pidmap *map, *end;
 224
 225         if (last >= PID_MAX_LIMIT)
 226                 return -1;
 227
 228         offset = (last + 1) & BITS_PER_PAGE_MASK;
 229         map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
 230         end = &pid_ns->pidmap[PIDMAP_ENTRIES];
 231         for (; map < end; map++, offset = 0) {
 232                 if (unlikely(!map->page))
 233                         continue;
 234                 offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
 235                 if (offset < BITS_PER_PAGE)
 236                         return mk_pid(pid_ns, map, offset);
 237         }
 238         return -1;
 239 }
 240
 241 void put_pid(struct pid *pid)
 242 {
 243         struct pid_namespace *ns;
 244
 245         if (!pid)
 246                 return;
 247
 248         ns = pid->numbers[pid->level].ns;
 249         if ((atomic_read(&pid->count) == 1) ||
 250              atomic_dec_and_test(&pid->count)) {
 251                 kmem_cache_free(ns->pid_cachep, pid);
 252                 put_pid_ns(ns);
 253         }
 254 }
 255 EXPORT_SYMBOL_GPL(put_pid);
 256
 257 static void delayed_put_pid(struct rcu_head *rhp)
 258 {
 259         struct pid *pid = container_of(rhp, struct pid, rcu);
 260         put_pid(pid);
 261 }
 262
 263 void free_pid(struct pid *pid)
 264 {
 265         /* We can be called with write_lock_irq(&tasklist_lock) held */
 266         int i;
 267         unsigned long flags;
 268
 269         spin_lock_irqsave(&pidmap_lock, flags);
 270         for (i = 0; i <= pid->level; i++)
 271                 hlist_del_rcu(&pid->numbers[i].pid_chain);
 272         spin_unlock_irqrestore(&pidmap_lock, flags);
 273
 274         for (i = 0; i <= pid->level; i++)
 275                 free_pidmap(pid->numbers + i);
 276
 277         call_rcu(&pid->rcu, delayed_put_pid);
 278 }
 279
 280 struct pid *alloc_pid(struct pid_namespace *ns)
 281 {
 282         struct pid *pid;
 283         enum pid_type type;
 284         int i, nr;
 285         struct pid_namespace *tmp;
 286         struct upid *upid;
 287
 288         pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
 289         if (!pid)
 290                 goto out;
 291
 292         tmp = ns;
 293         for (i = ns->level; i >= 0; i--) {
 294                 nr = alloc_pidmap(tmp);
 295                 if (nr < 0)
 296                         goto out_free;
 297
 298                 pid->numbers[i].nr = nr;
 299                 pid->numbers[i].ns = tmp;
 300                 tmp = tmp->parent;
 301         }
 302
 303         get_pid_ns(ns);
 304         pid->level = ns->level;
 305         atomic_set(&pid->count, 1);
 306         for (type = 0; type < PIDTYPE_MAX; ++type)
 307                 INIT_HLIST_HEAD(&pid->tasks[type]);
 308
 309         upid = pid->numbers + ns->level;
 310         spin_lock_irq(&pidmap_lock);
 311         for ( ; upid >= pid->numbers; --upid)
 312                 hlist_add_head_rcu(&upid->pid_chain,
 313                                 &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
 314         spin_unlock_irq(&pidmap_lock);
 315
 316 out:
 317         return pid;
 318
 319 out_free:
 320         while (++i <= ns->level)
 321                 free_pidmap(pid->numbers + i);
 322
 323         kmem_cache_free(ns->pid_cachep, pid);
 324         pid = NULL;
 325         goto out;
 326 }
 327
 328 struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
 329 {
 330         struct hlist_node *elem;
 331         struct upid *pnr;
 332
 333         hlist_for_each_entry_rcu(pnr, elem,
 334                         &pid_hash[pid_hashfn(nr, ns)], pid_chain)
 335                 if (pnr->nr == nr && pnr->ns == ns)
 336                         return container_of(pnr, struct pid,
 337                                         numbers[ns->level]);
 338
 339         return NULL;
 340 }
 341 EXPORT_SYMBOL_GPL(find_pid_ns);
 342
 343 struct pid *find_vpid(int nr)
 344 {
 345         return find_pid_ns(nr, current->nsproxy->pid_ns);
 346 }
 347 EXPORT_SYMBOL_GPL(find_vpid);
 348
 349 /*
 350  * attach_pid() must be called with the tasklist_lock write-held.
 351  */
 352 void attach_pid(struct task_struct *task, enum pid_type type,
 353                 struct pid *pid)
 354 {
 355         struct pid_link *link;
 356
 357         link = &task->pids[type];
 358         link->pid = pid;
 359         hlist_add_head_rcu(&link->node, &pid->tasks[type]);
 360 }
 361
 362 static void __change_pid(struct task_struct *task, enum pid_type type,
 363                         struct pid *new)
 364 {
 365         struct pid_link *link;
 366         struct pid *pid;
 367         int tmp;
 368
 369         link = &task->pids[type];
 370         pid = link->pid;
 371
 372         hlist_del_rcu(&link->node);
 373         link->pid = new;
 374
 375         for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 376                 if (!hlist_empty(&pid->tasks[tmp]))
 377                         return;
 378
 379         free_pid(pid);
 380 }
 381
 382 void detach_pid(struct task_struct *task, enum pid_type type)
 383 {
 384         __change_pid(task, type, NULL);
 385 }
 386
 387 void change_pid(struct task_struct *task, enum pid_type type,
 388                 struct pid *pid)
 389 {
 390         __change_pid(task, type, pid);
 391         attach_pid(task, type, pid);
 392 }
 393
 394 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
 395 void transfer_pid(struct task_struct *old, struct task_struct *new,
 396                            enum pid_type type)
 397 {
 398         new->pids[type].pid = old->pids[type].pid;
 399         hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
 400 }
 401
 402 struct task_struct *pid_task(struct pid *pid, enum pid_type type)
 403 {
 404         struct task_struct *result = NULL;
 405         if (pid) {
 406                 struct hlist_node *first;
 407                 first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
 408                                               rcu_read_lock_held() ||
 409                                               lockdep_tasklist_lock_is_held());
 410                 if (first)
 411                         result = hlist_entry(first, struct task_struct, pids[(type)].node);
 412         }
 413         return result;
 414 }
 415 EXPORT_SYMBOL(pid_task);
 416
 417 /*
 418  * Must be called under rcu_read_lock().
 419  */
 420 struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
 421 {
 422         rcu_lockdep_assert(rcu_read_lock_held());
 423         return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
 424 }
 425
 426 struct task_struct *find_task_by_vpid(pid_t vnr)
 427 {
 428         return find_task_by_pid_ns(vnr, current->nsproxy->pid_ns);
 429 }
 430
 431 struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 432 {
 433         struct pid *pid;
 434         rcu_read_lock();
 435         if (type != PIDTYPE_PID)
 436                 task = task->group_leader;
 437         pid = get_pid(task->pids[type].pid);
 438         rcu_read_unlock();
 439         return pid;
 440 }
 441 EXPORT_SYMBOL_GPL(get_task_pid);
 442
 443 struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
 444 {
 445         struct task_struct *result;
 446         rcu_read_lock();
 447         result = pid_task(pid, type);
 448         if (result)
 449                 get_task_struct(result);
 450         rcu_read_unlock();
 451         return result;
 452 }
 453 EXPORT_SYMBOL_GPL(get_pid_task);
 454
 455 struct pid *find_get_pid(pid_t nr)
 456 {
 457         struct pid *pid;
 458
 459         rcu_read_lock();
 460         pid = get_pid(find_vpid(nr));
 461         rcu_read_unlock();
 462
 463         return pid;
 464 }
 465 EXPORT_SYMBOL_GPL(find_get_pid);
 466
 467 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
 468 {
 469         struct upid *upid;
 470         pid_t nr = 0;
 471
 472         if (pid && ns->level <= pid->level) {
 473                 upid = &pid->numbers[ns->level];
 474                 if (upid->ns == ns)
 475                         nr = upid->nr;
 476         }
 477         return nr;
 478 }
 479
 480 pid_t pid_vnr(struct pid *pid)
 481 {
 482         return pid_nr_ns(pid, current->nsproxy->pid_ns);
 483 }
 484 EXPORT_SYMBOL_GPL(pid_vnr);
 485
 486 pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
 487                         struct pid_namespace *ns)
 488 {
 489         pid_t nr = 0;
 490
 491         rcu_read_lock();
 492         if (!ns)
 493                 ns = current->nsproxy->pid_ns;
 494         if (likely(pid_alive(task))) {
 495                 if (type != PIDTYPE_PID)
 496                         task = task->group_leader;
 497                 nr = pid_nr_ns(task->pids[type].pid, ns);
 498         }
 499         rcu_read_unlock();
 500
 501         return nr;
 502 }
 503 EXPORT_SYMBOL(__task_pid_nr_ns);
 504
 505 pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
 506 {
 507         return pid_nr_ns(task_tgid(tsk), ns);
 508 }
 509 EXPORT_SYMBOL(task_tgid_nr_ns);
 510
 511 struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
 512 {
 513         return ns_of_pid(task_pid(tsk));
 514 }
 515 EXPORT_SYMBOL_GPL(task_active_pid_ns);
 516
 517 /*
 518  * Used by proc to find the first pid that is greater than or equal to nr.
 519  *
 520  * If there is a pid at nr this function is exactly the same as find_pid_ns.
 521  */
 522 struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 523 {
 524         struct pid *pid;
 525
 526         do {
 527                 pid = find_pid_ns(nr, ns);
 528                 if (pid)
 529                         break;
 530                 nr = next_pidmap(ns, nr);
 531         } while (nr > 0);
 532
 533         return pid;
 534 }
 535
 536 /*
 537  * The pid hash table is scaled according to the amount of memory in the
 538  * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 539  * more.
 540  */
 541 void __init pidhash_init(void)
 542 {
 543         int i, pidhash_size;
 544
 545         pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
 546                                            HASH_EARLY | HASH_SMALL,
 547                                            &pidhash_shift, NULL, 4096);
 548         pidhash_size = 1 << pidhash_shift;
 549
 550         for (i = 0; i < pidhash_size; i++)
 551                 INIT_HLIST_HEAD(&pid_hash[i]);
 552 }
 553
 554 void __init pidmap_init(void)
 555 {
 556         /* bump default and minimum pid_max based on number of cpus */
 557         pid_max = min(pid_max_max, max_t(int, pid_max,
 558                                 PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
 559         pid_max_min = max_t(int, pid_max_min,
 560                                 PIDS_PER_CPU_MIN * num_possible_cpus());
 561         pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
 562
 563         init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 564         /* Reserve PID 0. We never call free_pidmap(0) */
 565         set_bit(0, init_pid_ns.pidmap[0].page);
 566         atomic_dec(&init_pid_ns.pidmap[0].nr_free);
 567
 568         init_pid_ns.pid_cachep = KMEM_CACHE(pid,
 569                         SLAB_HWCACHE_ALIGN | SLAB_PANIC);
 570 }