linux/kernel/pid.c
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   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)
  44static struct hlist_head *pid_hash;
  45static unsigned int pidhash_shift = 4;
  46struct pid init_struct_pid = INIT_STRUCT_PID;
  47
  48int pid_max = PID_MAX_DEFAULT;
  49
  50#define RESERVED_PIDS           300
  51
  52int pid_max_min = RESERVED_PIDS + 1;
  53int pid_max_max = PID_MAX_LIMIT;
  54
  55static 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 */
  70struct 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        .proc_inum = PROC_PID_INIT_INO,
  83};
  84EXPORT_SYMBOL_GPL(init_pid_ns);
  85
  86/*
  87 * Note: disable interrupts while the pidmap_lock is held as an
  88 * interrupt might come in and do read_lock(&tasklist_lock).
  89 *
  90 * If we don't disable interrupts there is a nasty deadlock between
  91 * detach_pid()->free_pid() and another cpu that does
  92 * spin_lock(&pidmap_lock) followed by an interrupt routine that does
  93 * read_lock(&tasklist_lock);
  94 *
  95 * After we clean up the tasklist_lock and know there are no
  96 * irq handlers that take it we can leave the interrupts enabled.
  97 * For now it is easier to be safe than to prove it can't happen.
  98 */
  99
 100static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
 101
 102static void free_pidmap(struct upid *upid)
 103{
 104        int nr = upid->nr;
 105        struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
 106        int offset = nr & BITS_PER_PAGE_MASK;
 107
 108        clear_bit(offset, map->page);
 109        atomic_inc(&map->nr_free);
 110}
 111
 112/*
 113 * If we started walking pids at 'base', is 'a' seen before 'b'?
 114 */
 115static int pid_before(int base, int a, int b)
 116{
 117        /*
 118         * This is the same as saying
 119         *
 120         * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
 121         * and that mapping orders 'a' and 'b' with respect to 'base'.
 122         */
 123        return (unsigned)(a - base) < (unsigned)(b - base);
 124}
 125
 126/*
 127 * We might be racing with someone else trying to set pid_ns->last_pid
 128 * at the pid allocation time (there's also a sysctl for this, but racing
 129 * with this one is OK, see comment in kernel/pid_namespace.c about it).
 130 * We want the winner to have the "later" value, because if the
 131 * "earlier" value prevails, then a pid may get reused immediately.
 132 *
 133 * Since pids rollover, it is not sufficient to just pick the bigger
 134 * value.  We have to consider where we started counting from.
 135 *
 136 * 'base' is the value of pid_ns->last_pid that we observed when
 137 * we started looking for a pid.
 138 *
 139 * 'pid' is the pid that we eventually found.
 140 */
 141static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
 142{
 143        int prev;
 144        int last_write = base;
 145        do {
 146                prev = last_write;
 147                last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
 148        } while ((prev != last_write) && (pid_before(base, last_write, pid)));
 149}
 150
 151static int alloc_pidmap(struct pid_namespace *pid_ns)
 152{
 153        int i, offset, max_scan, pid, last = pid_ns->last_pid;
 154        struct pidmap *map;
 155
 156        pid = last + 1;
 157        if (pid >= pid_max)
 158                pid = RESERVED_PIDS;
 159        offset = pid & BITS_PER_PAGE_MASK;
 160        map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
 161        /*
 162         * If last_pid points into the middle of the map->page we
 163         * want to scan this bitmap block twice, the second time
 164         * we start with offset == 0 (or RESERVED_PIDS).
 165         */
 166        max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
 167        for (i = 0; i <= max_scan; ++i) {
 168                if (unlikely(!map->page)) {
 169                        void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 170                        /*
 171                         * Free the page if someone raced with us
 172                         * installing it:
 173                         */
 174                        spin_lock_irq(&pidmap_lock);
 175                        if (!map->page) {
 176                                map->page = page;
 177                                page = NULL;
 178                        }
 179                        spin_unlock_irq(&pidmap_lock);
 180                        kfree(page);
 181                        if (unlikely(!map->page))
 182                                break;
 183                }
 184                if (likely(atomic_read(&map->nr_free))) {
 185                        for ( ; ; ) {
 186                                if (!test_and_set_bit(offset, map->page)) {
 187                                        atomic_dec(&map->nr_free);
 188                                        set_last_pid(pid_ns, last, pid);
 189                                        return pid;
 190                                }
 191                                offset = find_next_offset(map, offset);
 192                                if (offset >= BITS_PER_PAGE)
 193                                        break;
 194                                pid = mk_pid(pid_ns, map, offset);
 195                                if (pid >= pid_max)
 196                                        break;
 197                        }
 198                }
 199                if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
 200                        ++map;
 201                        offset = 0;
 202                } else {
 203                        map = &pid_ns->pidmap[0];
 204                        offset = RESERVED_PIDS;
 205                        if (unlikely(last == offset))
 206                                break;
 207                }
 208                pid = mk_pid(pid_ns, map, offset);
 209        }
 210        return -1;
 211}
 212
 213int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
 214{
 215        int offset;
 216        struct pidmap *map, *end;
 217
 218        if (last >= PID_MAX_LIMIT)
 219                return -1;
 220
 221        offset = (last + 1) & BITS_PER_PAGE_MASK;
 222        map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
 223        end = &pid_ns->pidmap[PIDMAP_ENTRIES];
 224        for (; map < end; map++, offset = 0) {
 225                if (unlikely(!map->page))
 226                        continue;
 227                offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
 228                if (offset < BITS_PER_PAGE)
 229                        return mk_pid(pid_ns, map, offset);
 230        }
 231        return -1;
 232}
 233
 234void put_pid(struct pid *pid)
 235{
 236        struct pid_namespace *ns;
 237
 238        if (!pid)
 239                return;
 240
 241        ns = pid->numbers[pid->level].ns;
 242        if ((atomic_read(&pid->count) == 1) ||
 243             atomic_dec_and_test(&pid->count)) {
 244                kmem_cache_free(ns->pid_cachep, pid);
 245                put_pid_ns(ns);
 246        }
 247}
 248EXPORT_SYMBOL_GPL(put_pid);
 249
 250static void delayed_put_pid(struct rcu_head *rhp)
 251{
 252        struct pid *pid = container_of(rhp, struct pid, rcu);
 253        put_pid(pid);
 254}
 255
 256void free_pid(struct pid *pid)
 257{
 258        /* We can be called with write_lock_irq(&tasklist_lock) held */
 259        int i;
 260        unsigned long flags;
 261
 262        spin_lock_irqsave(&pidmap_lock, flags);
 263        for (i = 0; i <= pid->level; i++) {
 264                struct upid *upid = pid->numbers + i;
 265                struct pid_namespace *ns = upid->ns;
 266                hlist_del_rcu(&upid->pid_chain);
 267                switch(--ns->nr_hashed) {
 268                case 2:
 269                case 1:
 270                        /* When all that is left in the pid namespace
 271                         * is the reaper wake up the reaper.  The reaper
 272                         * may be sleeping in zap_pid_ns_processes().
 273                         */
 274                        wake_up_process(ns->child_reaper);
 275                        break;
 276                case PIDNS_HASH_ADDING:
 277                        /* Handle a fork failure of the first process */
 278                        WARN_ON(ns->child_reaper);
 279                        ns->nr_hashed = 0;
 280                        /* fall through */
 281                case 0:
 282                        schedule_work(&ns->proc_work);
 283                        break;
 284                }
 285        }
 286        spin_unlock_irqrestore(&pidmap_lock, flags);
 287
 288        for (i = 0; i <= pid->level; i++)
 289                free_pidmap(pid->numbers + i);
 290
 291        call_rcu(&pid->rcu, delayed_put_pid);
 292}
 293
 294struct pid *alloc_pid(struct pid_namespace *ns)
 295{
 296        struct pid *pid;
 297        enum pid_type type;
 298        int i, nr;
 299        struct pid_namespace *tmp;
 300        struct upid *upid;
 301
 302        pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
 303        if (!pid)
 304                goto out;
 305
 306        tmp = ns;
 307        pid->level = ns->level;
 308        for (i = ns->level; i >= 0; i--) {
 309                nr = alloc_pidmap(tmp);
 310                if (nr < 0)
 311                        goto out_free;
 312
 313                pid->numbers[i].nr = nr;
 314                pid->numbers[i].ns = tmp;
 315                tmp = tmp->parent;
 316        }
 317
 318        if (unlikely(is_child_reaper(pid))) {
 319                if (pid_ns_prepare_proc(ns))
 320                        goto out_free;
 321        }
 322
 323        get_pid_ns(ns);
 324        atomic_set(&pid->count, 1);
 325        for (type = 0; type < PIDTYPE_MAX; ++type)
 326                INIT_HLIST_HEAD(&pid->tasks[type]);
 327
 328        upid = pid->numbers + ns->level;
 329        spin_lock_irq(&pidmap_lock);
 330        if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
 331                goto out_unlock;
 332        for ( ; upid >= pid->numbers; --upid) {
 333                hlist_add_head_rcu(&upid->pid_chain,
 334                                &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
 335                upid->ns->nr_hashed++;
 336        }
 337        spin_unlock_irq(&pidmap_lock);
 338
 339out:
 340        return pid;
 341
 342out_unlock:
 343        spin_unlock_irq(&pidmap_lock);
 344out_free:
 345        while (++i <= ns->level)
 346                free_pidmap(pid->numbers + i);
 347
 348        kmem_cache_free(ns->pid_cachep, pid);
 349        pid = NULL;
 350        goto out;
 351}
 352
 353void disable_pid_allocation(struct pid_namespace *ns)
 354{
 355        spin_lock_irq(&pidmap_lock);
 356        ns->nr_hashed &= ~PIDNS_HASH_ADDING;
 357        spin_unlock_irq(&pidmap_lock);
 358}
 359
 360struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
 361{
 362        struct upid *pnr;
 363
 364        hlist_for_each_entry_rcu(pnr,
 365                        &pid_hash[pid_hashfn(nr, ns)], pid_chain)
 366                if (pnr->nr == nr && pnr->ns == ns)
 367                        return container_of(pnr, struct pid,
 368                                        numbers[ns->level]);
 369
 370        return NULL;
 371}
 372EXPORT_SYMBOL_GPL(find_pid_ns);
 373
 374struct pid *find_vpid(int nr)
 375{
 376        return find_pid_ns(nr, task_active_pid_ns(current));
 377}
 378EXPORT_SYMBOL_GPL(find_vpid);
 379
 380/*
 381 * attach_pid() must be called with the tasklist_lock write-held.
 382 */
 383void attach_pid(struct task_struct *task, enum pid_type type)
 384{
 385        struct pid_link *link = &task->pids[type];
 386        hlist_add_head_rcu(&link->node, &link->pid->tasks[type]);
 387}
 388
 389static void __change_pid(struct task_struct *task, enum pid_type type,
 390                        struct pid *new)
 391{
 392        struct pid_link *link;
 393        struct pid *pid;
 394        int tmp;
 395
 396        link = &task->pids[type];
 397        pid = link->pid;
 398
 399        hlist_del_rcu(&link->node);
 400        link->pid = new;
 401
 402        for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 403                if (!hlist_empty(&pid->tasks[tmp]))
 404                        return;
 405
 406        free_pid(pid);
 407}
 408
 409void detach_pid(struct task_struct *task, enum pid_type type)
 410{
 411        __change_pid(task, type, NULL);
 412}
 413
 414void change_pid(struct task_struct *task, enum pid_type type,
 415                struct pid *pid)
 416{
 417        __change_pid(task, type, pid);
 418        attach_pid(task, type);
 419}
 420
 421/* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
 422void transfer_pid(struct task_struct *old, struct task_struct *new,
 423                           enum pid_type type)
 424{
 425        new->pids[type].pid = old->pids[type].pid;
 426        hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
 427}
 428
 429struct task_struct *pid_task(struct pid *pid, enum pid_type type)
 430{
 431        struct task_struct *result = NULL;
 432        if (pid) {
 433                struct hlist_node *first;
 434                first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
 435                                              lockdep_tasklist_lock_is_held());
 436                if (first)
 437                        result = hlist_entry(first, struct task_struct, pids[(type)].node);
 438        }
 439        return result;
 440}
 441EXPORT_SYMBOL(pid_task);
 442
 443/*
 444 * Must be called under rcu_read_lock().
 445 */
 446struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
 447{
 448        rcu_lockdep_assert(rcu_read_lock_held(),
 449                           "find_task_by_pid_ns() needs rcu_read_lock()"
 450                           " protection");
 451        return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
 452}
 453
 454struct task_struct *find_task_by_vpid(pid_t vnr)
 455{
 456        return find_task_by_pid_ns(vnr, task_active_pid_ns(current));
 457}
 458
 459struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 460{
 461        struct pid *pid;
 462        rcu_read_lock();
 463        if (type != PIDTYPE_PID)
 464                task = task->group_leader;
 465        pid = get_pid(task->pids[type].pid);
 466        rcu_read_unlock();
 467        return pid;
 468}
 469EXPORT_SYMBOL_GPL(get_task_pid);
 470
 471struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
 472{
 473        struct task_struct *result;
 474        rcu_read_lock();
 475        result = pid_task(pid, type);
 476        if (result)
 477                get_task_struct(result);
 478        rcu_read_unlock();
 479        return result;
 480}
 481EXPORT_SYMBOL_GPL(get_pid_task);
 482
 483struct pid *find_get_pid(pid_t nr)
 484{
 485        struct pid *pid;
 486
 487        rcu_read_lock();
 488        pid = get_pid(find_vpid(nr));
 489        rcu_read_unlock();
 490
 491        return pid;
 492}
 493EXPORT_SYMBOL_GPL(find_get_pid);
 494
 495pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
 496{
 497        struct upid *upid;
 498        pid_t nr = 0;
 499
 500        if (pid && ns->level <= pid->level) {
 501                upid = &pid->numbers[ns->level];
 502                if (upid->ns == ns)
 503                        nr = upid->nr;
 504        }
 505        return nr;
 506}
 507EXPORT_SYMBOL_GPL(pid_nr_ns);
 508
 509pid_t pid_vnr(struct pid *pid)
 510{
 511        return pid_nr_ns(pid, task_active_pid_ns(current));
 512}
 513EXPORT_SYMBOL_GPL(pid_vnr);
 514
 515pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
 516                        struct pid_namespace *ns)
 517{
 518        pid_t nr = 0;
 519
 520        rcu_read_lock();
 521        if (!ns)
 522                ns = task_active_pid_ns(current);
 523        if (likely(pid_alive(task))) {
 524                if (type != PIDTYPE_PID)
 525                        task = task->group_leader;
 526                nr = pid_nr_ns(task->pids[type].pid, ns);
 527        }
 528        rcu_read_unlock();
 529
 530        return nr;
 531}
 532EXPORT_SYMBOL(__task_pid_nr_ns);
 533
 534pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
 535{
 536        return pid_nr_ns(task_tgid(tsk), ns);
 537}
 538EXPORT_SYMBOL(task_tgid_nr_ns);
 539
 540struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
 541{
 542        return ns_of_pid(task_pid(tsk));
 543}
 544EXPORT_SYMBOL_GPL(task_active_pid_ns);
 545
 546/*
 547 * Used by proc to find the first pid that is greater than or equal to nr.
 548 *
 549 * If there is a pid at nr this function is exactly the same as find_pid_ns.
 550 */
 551struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 552{
 553        struct pid *pid;
 554
 555        do {
 556                pid = find_pid_ns(nr, ns);
 557                if (pid)
 558                        break;
 559                nr = next_pidmap(ns, nr);
 560        } while (nr > 0);
 561
 562        return pid;
 563}
 564
 565/*
 566 * The pid hash table is scaled according to the amount of memory in the
 567 * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 568 * more.
 569 */
 570void __init pidhash_init(void)
 571{
 572        unsigned int i, pidhash_size;
 573
 574        pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
 575                                           HASH_EARLY | HASH_SMALL,
 576                                           &pidhash_shift, NULL,
 577                                           0, 4096);
 578        pidhash_size = 1U << pidhash_shift;
 579
 580        for (i = 0; i < pidhash_size; i++)
 581                INIT_HLIST_HEAD(&pid_hash[i]);
 582}
 583
 584void __init pidmap_init(void)
 585{
 586        /* Veryify no one has done anything silly */
 587        BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_HASH_ADDING);
 588
 589        /* bump default and minimum pid_max based on number of cpus */
 590        pid_max = min(pid_max_max, max_t(int, pid_max,
 591                                PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
 592        pid_max_min = max_t(int, pid_max_min,
 593                                PIDS_PER_CPU_MIN * num_possible_cpus());
 594        pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
 595
 596        init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 597        /* Reserve PID 0. We never call free_pidmap(0) */
 598        set_bit(0, init_pid_ns.pidmap[0].page);
 599        atomic_dec(&init_pid_ns.pidmap[0].nr_free);
 600
 601        init_pid_ns.pid_cachep = KMEM_CACHE(pid,
 602                        SLAB_HWCACHE_ALIGN | SLAB_PANIC);
 603}
 604
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