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        .level = 0,
  79        .child_reaper = &init_task,
  80        .user_ns = &init_user_ns,
  81        .proc_inum = PROC_PID_INIT_INO,
  82};
  83EXPORT_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
  99static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
 100
 101static 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 */
 114static 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 */
 140static 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
 150static 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
 212int 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
 233void 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}
 247EXPORT_SYMBOL_GPL(put_pid);
 248
 249static 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
 255void 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
 287struct 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
 332out:
 333        return pid;
 334
 335out_unlock:
 336        spin_unlock_irq(&pidmap_lock);
 337out_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
 346void 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
 353struct 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}
 365EXPORT_SYMBOL_GPL(find_pid_ns);
 366
 367struct pid *find_vpid(int nr)
 368{
 369        return find_pid_ns(nr, task_active_pid_ns(current));
 370}
 371EXPORT_SYMBOL_GPL(find_vpid);
 372
 373/*
 374 * attach_pid() must be called with the tasklist_lock write-held.
 375 */
 376void 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
 386static 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
 406void detach_pid(struct task_struct *task, enum pid_type type)
 407{
 408        __change_pid(task, type, NULL);
 409}
 410
 411void 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) */
 419void 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
 426struct 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}
 438EXPORT_SYMBOL(pid_task);
 439
 440/*
 441 * Must be called under rcu_read_lock().
 442 */
 443struct 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
 451struct 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
 456struct 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}
 466EXPORT_SYMBOL_GPL(get_task_pid);
 467
 468struct 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}
 478EXPORT_SYMBOL_GPL(get_pid_task);
 479
 480struct 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}
 490EXPORT_SYMBOL_GPL(find_get_pid);
 491
 492pid_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}
 504EXPORT_SYMBOL_GPL(pid_nr_ns);
 505
 506pid_t pid_vnr(struct pid *pid)
 507{
 508        return pid_nr_ns(pid, task_active_pid_ns(current));
 509}
 510EXPORT_SYMBOL_GPL(pid_vnr);
 511
 512pid_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}
 529EXPORT_SYMBOL(__task_pid_nr_ns);
 530
 531pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
 532{
 533        return pid_nr_ns(task_tgid(tsk), ns);
 534}
 535EXPORT_SYMBOL(task_tgid_nr_ns);
 536
 537struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
 538{
 539        return ns_of_pid(task_pid(tsk));
 540}
 541EXPORT_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 */
 548struct 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 */
 567void __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
 581void __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}
 602
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