Root/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)
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}
482
483pid_t pid_vnr(struct pid *pid)
484{
485    return pid_nr_ns(pid, current->nsproxy->pid_ns);
486}
487EXPORT_SYMBOL_GPL(pid_vnr);
488
489pid_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}
506EXPORT_SYMBOL(__task_pid_nr_ns);
507
508pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
509{
510    return pid_nr_ns(task_tgid(tsk), ns);
511}
512EXPORT_SYMBOL(task_tgid_nr_ns);
513
514struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
515{
516    return ns_of_pid(task_pid(tsk));
517}
518EXPORT_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 */
525struct 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 */
544void __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
558void __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}
575

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