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         .level = 0,
  79         .child_reaper = &init_task,
  80         .user_ns = &init_user_ns,
  81         .proc_inum = PROC_PID_INIT_INO,
  82 };
  83 EXPORT_SYMBOL_GPL(init_pid_ns);
  84
  85 /*
  86  * Note: disable interrupts while the pidmap_lock is held as an
  87  * interrupt might come in and do read_lock(&tasklist_lock).
  88  *
  89  * If we don't disable interrupts there is a nasty deadlock between
  90  * detach_pid()->free_pid() and another cpu that does
  91  * spin_lock(&pidmap_lock) followed by an interrupt routine that does
  92  * read_lock(&tasklist_lock);
  93  *
  94  * After we clean up the tasklist_lock and know there are no
  95  * irq handlers that take it we can leave the interrupts enabled.
  96  * For now it is easier to be safe than to prove it can't happen.
  97  */
  98
  99 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
 100
 101 static void free_pidmap(struct upid *upid)
 102 {
 103         int nr = upid->nr;
 104         struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
 105         int offset = nr & BITS_PER_PAGE_MASK;
 106
 107         clear_bit(offset, map->page);
 108         atomic_inc(&map->nr_free);
 109 }
 110
 111 /*
 112  * If we started walking pids at 'base', is 'a' seen before 'b'?
 113  */
 114 static int pid_before(int base, int a, int b)
 115 {
 116         /*
 117          * This is the same as saying
 118          *
 119          * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
 120          * and that mapping orders 'a' and 'b' with respect to 'base'.
 121          */
 122         return (unsigned)(a - base) < (unsigned)(b - base);
 123 }
 124
 125 /*
 126  * We might be racing with someone else trying to set pid_ns->last_pid
 127  * at the pid allocation time (there's also a sysctl for this, but racing
 128  * with this one is OK, see comment in kernel/pid_namespace.c about it).
 129  * We want the winner to have the "later" value, because if the
 130  * "earlier" value prevails, then a pid may get reused immediately.
 131  *
 132  * Since pids rollover, it is not sufficient to just pick the bigger
 133  * value.  We have to consider where we started counting from.
 134  *
 135  * 'base' is the value of pid_ns->last_pid that we observed when
 136  * we started looking for a pid.
 137  *
 138  * 'pid' is the pid that we eventually found.
 139  */
 140 static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
 141 {
 142         int prev;
 143         int last_write = base;
 144         do {
 145                 prev = last_write;
 146                 last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
 147         } while ((prev != last_write) && (pid_before(base, last_write, pid)));
 148 }
 149
 150 static int alloc_pidmap(struct pid_namespace *pid_ns)
 151 {
 152         int i, offset, max_scan, pid, last = pid_ns->last_pid;
 153         struct pidmap *map;
 154
 155         pid = last + 1;
 156         if (pid >= pid_max)
 157                 pid = RESERVED_PIDS;
 158         offset = pid & BITS_PER_PAGE_MASK;
 159         map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
 160         /*
 161          * If last_pid points into the middle of the map->page we
 162          * want to scan this bitmap block twice, the second time
 163          * we start with offset == 0 (or RESERVED_PIDS).
 164          */
 165         max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
 166         for (i = 0; i <= max_scan; ++i) {
 167                 if (unlikely(!map->page)) {
 168                         void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 169                         /*
 170                          * Free the page if someone raced with us
 171                          * installing it:
 172                          */
 173                         spin_lock_irq(&pidmap_lock);
 174                         if (!map->page) {
 175                                 map->page = page;
 176                                 page = NULL;
 177                         }
 178                         spin_unlock_irq(&pidmap_lock);
 179                         kfree(page);
 180                         if (unlikely(!map->page))
 181                                 break;
 182                 }
 183                 if (likely(atomic_read(&map->nr_free))) {
 184                         for ( ; ; ) {
 185                                 if (!test_and_set_bit(offset, map->page)) {
 186                                         atomic_dec(&map->nr_free);
 187                                         set_last_pid(pid_ns, last, pid);
 188                                         return pid;
 189                                 }
 190                                 offset = find_next_offset(map, offset);
 191                                 if (offset >= BITS_PER_PAGE)
 192                                         break;
 193                                 pid = mk_pid(pid_ns, map, offset);
 194                                 if (pid >= pid_max)
 195                                         break;
 196                         }
 197                 }
 198                 if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
 199                         ++map;
 200                         offset = 0;
 201                 } else {
 202                         map = &pid_ns->pidmap[0];
 203                         offset = RESERVED_PIDS;
 204                         if (unlikely(last == offset))
 205                                 break;
 206                 }
 207                 pid = mk_pid(pid_ns, map, offset);
 208         }
 209         return -1;
 210 }
 211
 212 int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
 213 {
 214         int offset;
 215         struct pidmap *map, *end;
 216
 217         if (last >= PID_MAX_LIMIT)
 218                 return -1;
 219
 220         offset = (last + 1) & BITS_PER_PAGE_MASK;
 221         map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
 222         end = &pid_ns->pidmap[PIDMAP_ENTRIES];
 223         for (; map < end; map++, offset = 0) {
 224                 if (unlikely(!map->page))
 225                         continue;
 226                 offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
 227                 if (offset < BITS_PER_PAGE)
 228                         return mk_pid(pid_ns, map, offset);
 229         }
 230         return -1;
 231 }
 232
 233 void put_pid(struct pid *pid)
 234 {
 235         struct pid_namespace *ns;
 236
 237         if (!pid)
 238                 return;
 239
 240         ns = pid->numbers[pid->level].ns;
 241         if ((atomic_read(&pid->count) == 1) ||
 242              atomic_dec_and_test(&pid->count)) {
 243                 kmem_cache_free(ns->pid_cachep, pid);
 244                 put_pid_ns(ns);
 245         }
 246 }
 247 EXPORT_SYMBOL_GPL(put_pid);
 248
 249 static void delayed_put_pid(struct rcu_head *rhp)
 250 {
 251         struct pid *pid = container_of(rhp, struct pid, rcu);
 252         put_pid(pid);
 253 }
 254
 255 void free_pid(struct pid *pid)
 256 {
 257         /* We can be called with write_lock_irq(&tasklist_lock) held */
 258         int i;
 259         unsigned long flags;
 260
 261         spin_lock_irqsave(&pidmap_lock, flags);
 262         for (i = 0; i <= pid->level; i++) {
 263                 struct upid *upid = pid->numbers + i;
 264                 struct pid_namespace *ns = upid->ns;
 265                 hlist_del_rcu(&upid->pid_chain);
 266                 switch(--ns->nr_hashed) {
 267                 case 1:
 268                         /* When all that is left in the pid namespace
 269                          * is the reaper wake up the reaper.  The reaper
 270                          * may be sleeping in zap_pid_ns_processes().
 271                          */
 272                         wake_up_process(ns->child_reaper);
 273                         break;
 274                 case 0:
 275                         schedule_work(&ns->proc_work);
 276                         break;
 277                 }
 278         }
 279         spin_unlock_irqrestore(&pidmap_lock, flags);
 280
 281         for (i = 0; i <= pid->level; i++)
 282                 free_pidmap(pid->numbers + i);
 283
 284         call_rcu(&pid->rcu, delayed_put_pid);
 285 }
 286
 287 struct pid *alloc_pid(struct pid_namespace *ns)
 288 {
 289         struct pid *pid;
 290         enum pid_type type;
 291         int i, nr;
 292         struct pid_namespace *tmp;
 293         struct upid *upid;
 294
 295         pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
 296         if (!pid)
 297                 goto out;
 298
 299         tmp = ns;
 300         pid->level = ns->level;
 301         for (i = ns->level; i >= 0; i--) {
 302                 nr = alloc_pidmap(tmp);
 303                 if (nr < 0)
 304                         goto out_free;
 305
 306                 pid->numbers[i].nr = nr;
 307                 pid->numbers[i].ns = tmp;
 308                 tmp = tmp->parent;
 309         }
 310
 311         if (unlikely(is_child_reaper(pid))) {
 312                 if (pid_ns_prepare_proc(ns))
 313                         goto out_free;
 314         }
 315
 316         get_pid_ns(ns);
 317         atomic_set(&pid->count, 1);
 318         for (type = 0; type < PIDTYPE_MAX; ++type)
 319                 INIT_HLIST_HEAD(&pid->tasks[type]);
 320
 321         upid = pid->numbers + ns->level;
 322         spin_lock_irq(&pidmap_lock);
 323         if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
 324                 goto out_unlock;
 325         for ( ; upid >= pid->numbers; --upid) {
 326                 hlist_add_head_rcu(&upid->pid_chain,
 327                                 &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
 328                 upid->ns->nr_hashed++;
 329         }
 330         spin_unlock_irq(&pidmap_lock);
 331
 332 out:
 333         return pid;
 334
 335 out_unlock:
 336         spin_unlock_irq(&pidmap_lock);
 337 out_free:
 338         while (++i <= ns->level)
 339                 free_pidmap(pid->numbers + i);
 340
 341         kmem_cache_free(ns->pid_cachep, pid);
 342         pid = NULL;
 343         goto out;
 344 }
 345
 346 void disable_pid_allocation(struct pid_namespace *ns)
 347 {
 348         spin_lock_irq(&pidmap_lock);
 349         ns->nr_hashed &= ~PIDNS_HASH_ADDING;
 350         spin_unlock_irq(&pidmap_lock);
 351 }
 352
 353 struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
 354 {
 355         struct upid *pnr;
 356
 357         hlist_for_each_entry_rcu(pnr,
 358                         &pid_hash[pid_hashfn(nr, ns)], pid_chain)
 359                 if (pnr->nr == nr && pnr->ns == ns)
 360                         return container_of(pnr, struct pid,
 361                                         numbers[ns->level]);
 362
 363         return NULL;
 364 }
 365 EXPORT_SYMBOL_GPL(find_pid_ns);
 366
 367 struct pid *find_vpid(int nr)
 368 {
 369         return find_pid_ns(nr, task_active_pid_ns(current));
 370 }
 371 EXPORT_SYMBOL_GPL(find_vpid);
 372
 373 /*
 374  * attach_pid() must be called with the tasklist_lock write-held.
 375  */
 376 void attach_pid(struct task_struct *task, enum pid_type type,
 377                 struct pid *pid)
 378 {
 379         struct pid_link *link;
 380
 381         link = &task->pids[type];
 382         link->pid = pid;
 383         hlist_add_head_rcu(&link->node, &pid->tasks[type]);
 384 }
 385
 386 static void __change_pid(struct task_struct *task, enum pid_type type,
 387                         struct pid *new)
 388 {
 389         struct pid_link *link;
 390         struct pid *pid;
 391         int tmp;
 392
 393         link = &task->pids[type];
 394         pid = link->pid;
 395
 396         hlist_del_rcu(&link->node);
 397         link->pid = new;
 398
 399         for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 400                 if (!hlist_empty(&pid->tasks[tmp]))
 401                         return;
 402
 403         free_pid(pid);
 404 }
 405
 406 void detach_pid(struct task_struct *task, enum pid_type type)
 407 {
 408         __change_pid(task, type, NULL);
 409 }
 410
 411 void change_pid(struct task_struct *task, enum pid_type type,
 412                 struct pid *pid)
 413 {
 414         __change_pid(task, type, pid);
 415         attach_pid(task, type, pid);
 416 }
 417
 418 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
 419 void transfer_pid(struct task_struct *old, struct task_struct *new,
 420                            enum pid_type type)
 421 {
 422         new->pids[type].pid = old->pids[type].pid;
 423         hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
 424 }
 425
 426 struct task_struct *pid_task(struct pid *pid, enum pid_type type)
 427 {
 428         struct task_struct *result = NULL;
 429         if (pid) {
 430                 struct hlist_node *first;
 431                 first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
 432                                               lockdep_tasklist_lock_is_held());
 433                 if (first)
 434                         result = hlist_entry(first, struct task_struct, pids[(type)].node);
 435         }
 436         return result;
 437 }
 438 EXPORT_SYMBOL(pid_task);
 439
 440 /*
 441  * Must be called under rcu_read_lock().
 442  */
 443 struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
 444 {
 445         rcu_lockdep_assert(rcu_read_lock_held(),
 446                            "find_task_by_pid_ns() needs rcu_read_lock()"
 447                            " protection");
 448         return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
 449 }
 450
 451 struct task_struct *find_task_by_vpid(pid_t vnr)
 452 {
 453         return find_task_by_pid_ns(vnr, task_active_pid_ns(current));
 454 }
 455
 456 struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 457 {
 458         struct pid *pid;
 459         rcu_read_lock();
 460         if (type != PIDTYPE_PID)
 461                 task = task->group_leader;
 462         pid = get_pid(task->pids[type].pid);
 463         rcu_read_unlock();
 464         return pid;
 465 }
 466 EXPORT_SYMBOL_GPL(get_task_pid);
 467
 468 struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
 469 {
 470         struct task_struct *result;
 471         rcu_read_lock();
 472         result = pid_task(pid, type);
 473         if (result)
 474                 get_task_struct(result);
 475         rcu_read_unlock();
 476         return result;
 477 }
 478 EXPORT_SYMBOL_GPL(get_pid_task);
 479
 480 struct pid *find_get_pid(pid_t nr)
 481 {
 482         struct pid *pid;
 483
 484         rcu_read_lock();
 485         pid = get_pid(find_vpid(nr));
 486         rcu_read_unlock();
 487
 488         return pid;
 489 }
 490 EXPORT_SYMBOL_GPL(find_get_pid);
 491
 492 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
 493 {
 494         struct upid *upid;
 495         pid_t nr = 0;
 496
 497         if (pid && ns->level <= pid->level) {
 498                 upid = &pid->numbers[ns->level];
 499                 if (upid->ns == ns)
 500                         nr = upid->nr;
 501         }
 502         return nr;
 503 }
 504 EXPORT_SYMBOL_GPL(pid_nr_ns);
 505
 506 pid_t pid_vnr(struct pid *pid)
 507 {
 508         return pid_nr_ns(pid, task_active_pid_ns(current));
 509 }
 510 EXPORT_SYMBOL_GPL(pid_vnr);
 511
 512 pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
 513                         struct pid_namespace *ns)
 514 {
 515         pid_t nr = 0;
 516
 517         rcu_read_lock();
 518         if (!ns)
 519                 ns = task_active_pid_ns(current);
 520         if (likely(pid_alive(task))) {
 521                 if (type != PIDTYPE_PID)
 522                         task = task->group_leader;
 523                 nr = pid_nr_ns(task->pids[type].pid, ns);
 524         }
 525         rcu_read_unlock();
 526
 527         return nr;
 528 }
 529 EXPORT_SYMBOL(__task_pid_nr_ns);
 530
 531 pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
 532 {
 533         return pid_nr_ns(task_tgid(tsk), ns);
 534 }
 535 EXPORT_SYMBOL(task_tgid_nr_ns);
 536
 537 struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
 538 {
 539         return ns_of_pid(task_pid(tsk));
 540 }
 541 EXPORT_SYMBOL_GPL(task_active_pid_ns);
 542
 543 /*
 544  * Used by proc to find the first pid that is greater than or equal to nr.
 545  *
 546  * If there is a pid at nr this function is exactly the same as find_pid_ns.
 547  */
 548 struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 549 {
 550         struct pid *pid;
 551
 552         do {
 553                 pid = find_pid_ns(nr, ns);
 554                 if (pid)
 555                         break;
 556                 nr = next_pidmap(ns, nr);
 557         } while (nr > 0);
 558
 559         return pid;
 560 }
 561
 562 /*
 563  * The pid hash table is scaled according to the amount of memory in the
 564  * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 565  * more.
 566  */
 567 void __init pidhash_init(void)
 568 {
 569         unsigned int i, pidhash_size;
 570
 571         pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
 572                                            HASH_EARLY | HASH_SMALL,
 573                                            &pidhash_shift, NULL,
 574                                            0, 4096);
 575         pidhash_size = 1U << pidhash_shift;
 576
 577         for (i = 0; i < pidhash_size; i++)
 578                 INIT_HLIST_HEAD(&pid_hash[i]);
 579 }
 580
 581 void __init pidmap_init(void)
 582 {
 583         /* Veryify no one has done anything silly */
 584         BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_HASH_ADDING);
 585
 586         /* bump default and minimum pid_max based on number of cpus */
 587         pid_max = min(pid_max_max, max_t(int, pid_max,
 588                                 PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
 589         pid_max_min = max_t(int, pid_max_min,
 590                                 PIDS_PER_CPU_MIN * num_possible_cpus());
 591         pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
 592
 593         init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 594         /* Reserve PID 0. We never call free_pidmap(0) */
 595         set_bit(0, init_pid_ns.pidmap[0].page);
 596         atomic_dec(&init_pid_ns.pidmap[0].nr_free);
 597         init_pid_ns.nr_hashed = PIDNS_HASH_ADDING;
 598
 599         init_pid_ns.pid_cachep = KMEM_CACHE(pid,
 600                         SLAB_HWCACHE_ALIGN | SLAB_PANIC);
 601 }