linux/kernel/pid.c
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   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)
  42static struct hlist_head *pid_hash;
  43static unsigned int pidhash_shift = 4;
  44struct pid init_struct_pid = INIT_STRUCT_PID;
  45
  46int pid_max = PID_MAX_DEFAULT;
  47
  48#define RESERVED_PIDS           300
  49
  50int pid_max_min = RESERVED_PIDS + 1;
  51int 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
  56static 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 */
  71struct 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};
  82EXPORT_SYMBOL_GPL(init_pid_ns);
  83
  84int 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}
  97EXPORT_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
 113static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
 114
 115static 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 */
 128static 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 */
 154static 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
 164static 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
 222int 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
 243void 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}
 257EXPORT_SYMBOL_GPL(put_pid);
 258
 259static 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
 265void 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
 282struct 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
 318out:
 319        return pid;
 320
 321out_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
 330struct 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}
 343EXPORT_SYMBOL_GPL(find_pid_ns);
 344
 345struct pid *find_vpid(int nr)
 346{
 347        return find_pid_ns(nr, current->nsproxy->pid_ns);
 348}
 349EXPORT_SYMBOL_GPL(find_vpid);
 350
 351/*
 352 * attach_pid() must be called with the tasklist_lock write-held.
 353 */
 354void 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
 364static 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
 384void detach_pid(struct task_struct *task, enum pid_type type)
 385{
 386        __change_pid(task, type, NULL);
 387}
 388
 389void 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) */
 397void 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
 404struct 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}
 416EXPORT_SYMBOL(pid_task);
 417
 418/*
 419 * Must be called under rcu_read_lock().
 420 */
 421struct 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
 429struct task_struct *find_task_by_vpid(pid_t vnr)
 430{
 431        return find_task_by_pid_ns(vnr, current->nsproxy->pid_ns);
 432}
 433
 434struct 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}
 444EXPORT_SYMBOL_GPL(get_task_pid);
 445
 446struct 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}
 456EXPORT_SYMBOL_GPL(get_pid_task);
 457
 458struct 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}
 468EXPORT_SYMBOL_GPL(find_get_pid);
 469
 470pid_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}
 482EXPORT_SYMBOL_GPL(pid_nr_ns);
 483
 484pid_t pid_vnr(struct pid *pid)
 485{
 486        return pid_nr_ns(pid, current->nsproxy->pid_ns);
 487}
 488EXPORT_SYMBOL_GPL(pid_vnr);
 489
 490pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
 491                        struct pid_namespace *ns)
 492{
 493        pid_t nr = 0;
 494
 495        rcu_read_lock();
 496        if (!ns)
 497                ns = current->nsproxy->pid_ns;
 498        if (likely(pid_alive(task))) {
 499                if (type != PIDTYPE_PID)
 500                        task = task->group_leader;
 501                nr = pid_nr_ns(task->pids[type].pid, ns);
 502        }
 503        rcu_read_unlock();
 504
 505        return nr;
 506}
 507EXPORT_SYMBOL(__task_pid_nr_ns);
 508
 509pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
 510{
 511        return pid_nr_ns(task_tgid(tsk), ns);
 512}
 513EXPORT_SYMBOL(task_tgid_nr_ns);
 514
 515struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
 516{
 517        return ns_of_pid(task_pid(tsk));
 518}
 519EXPORT_SYMBOL_GPL(task_active_pid_ns);
 520
 521/*
 522 * Used by proc to find the first pid that is greater than or equal to nr.
 523 *
 524 * If there is a pid at nr this function is exactly the same as find_pid_ns.
 525 */
 526struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 527{
 528        struct pid *pid;
 529
 530        do {
 531                pid = find_pid_ns(nr, ns);
 532                if (pid)
 533                        break;
 534                nr = next_pidmap(ns, nr);
 535        } while (nr > 0);
 536
 537        return pid;
 538}
 539
 540/*
 541 * The pid hash table is scaled according to the amount of memory in the
 542 * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 543 * more.
 544 */
 545void __init pidhash_init(void)
 546{
 547        unsigned int i, pidhash_size;
 548
 549        pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
 550                                           HASH_EARLY | HASH_SMALL,
 551                                           &pidhash_shift, NULL,
 552                                           0, 4096);
 553        pidhash_size = 1U << pidhash_shift;
 554
 555        for (i = 0; i < pidhash_size; i++)
 556                INIT_HLIST_HEAD(&pid_hash[i]);
 557}
 558
 559void __init pidmap_init(void)
 560{
 561        /* bump default and minimum pid_max based on number of cpus */
 562        pid_max = min(pid_max_max, max_t(int, pid_max,
 563                                PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
 564        pid_max_min = max_t(int, pid_max_min,
 565                                PIDS_PER_CPU_MIN * num_possible_cpus());
 566        pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
 567
 568        init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
 569        /* Reserve PID 0. We never call free_pidmap(0) */
 570        set_bit(0, init_pid_ns.pidmap[0].page);
 571        atomic_dec(&init_pid_ns.pidmap[0].nr_free);
 572
 573        init_pid_ns.pid_cachep = KMEM_CACHE(pid,
 574                        SLAB_HWCACHE_ALIGN | SLAB_PANIC);
 575}
 576
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