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