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/module.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
125static int alloc_pidmap(struct pid_namespace *pid_ns)
126{
127    int i, offset, max_scan, pid, last = pid_ns->last_pid;
128    struct pidmap *map;
129
130    pid = last + 1;
131    if (pid >= pid_max)
132        pid = RESERVED_PIDS;
133    offset = pid & BITS_PER_PAGE_MASK;
134    map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
135    max_scan = (pid_max + BITS_PER_PAGE - 1)/BITS_PER_PAGE - !offset;
136    for (i = 0; i <= max_scan; ++i) {
137        if (unlikely(!map->page)) {
138            void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
139            /*
140             * Free the page if someone raced with us
141             * installing it:
142             */
143            spin_lock_irq(&pidmap_lock);
144            if (!map->page) {
145                map->page = page;
146                page = NULL;
147            }
148            spin_unlock_irq(&pidmap_lock);
149            kfree(page);
150            if (unlikely(!map->page))
151                break;
152        }
153        if (likely(atomic_read(&map->nr_free))) {
154            do {
155                if (!test_and_set_bit(offset, map->page)) {
156                    atomic_dec(&map->nr_free);
157                    pid_ns->last_pid = pid;
158                    return pid;
159                }
160                offset = find_next_offset(map, offset);
161                pid = mk_pid(pid_ns, map, offset);
162            /*
163             * find_next_offset() found a bit, the pid from it
164             * is in-bounds, and if we fell back to the last
165             * bitmap block and the final block was the same
166             * as the starting point, pid is before last_pid.
167             */
168            } while (offset < BITS_PER_PAGE && pid < pid_max &&
169                    (i != max_scan || pid < last ||
170                        !((last+1) & BITS_PER_PAGE_MASK)));
171        }
172        if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
173            ++map;
174            offset = 0;
175        } else {
176            map = &pid_ns->pidmap[0];
177            offset = RESERVED_PIDS;
178            if (unlikely(last == offset))
179                break;
180        }
181        pid = mk_pid(pid_ns, map, offset);
182    }
183    return -1;
184}
185
186int next_pidmap(struct pid_namespace *pid_ns, int last)
187{
188    int offset;
189    struct pidmap *map, *end;
190
191    offset = (last + 1) & BITS_PER_PAGE_MASK;
192    map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
193    end = &pid_ns->pidmap[PIDMAP_ENTRIES];
194    for (; map < end; map++, offset = 0) {
195        if (unlikely(!map->page))
196            continue;
197        offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
198        if (offset < BITS_PER_PAGE)
199            return mk_pid(pid_ns, map, offset);
200    }
201    return -1;
202}
203
204void put_pid(struct pid *pid)
205{
206    struct pid_namespace *ns;
207
208    if (!pid)
209        return;
210
211    ns = pid->numbers[pid->level].ns;
212    if ((atomic_read(&pid->count) == 1) ||
213         atomic_dec_and_test(&pid->count)) {
214        kmem_cache_free(ns->pid_cachep, pid);
215        put_pid_ns(ns);
216    }
217}
218EXPORT_SYMBOL_GPL(put_pid);
219
220static void delayed_put_pid(struct rcu_head *rhp)
221{
222    struct pid *pid = container_of(rhp, struct pid, rcu);
223    put_pid(pid);
224}
225
226void free_pid(struct pid *pid)
227{
228    /* We can be called with write_lock_irq(&tasklist_lock) held */
229    int i;
230    unsigned long flags;
231
232    spin_lock_irqsave(&pidmap_lock, flags);
233    for (i = 0; i <= pid->level; i++)
234        hlist_del_rcu(&pid->numbers[i].pid_chain);
235    spin_unlock_irqrestore(&pidmap_lock, flags);
236
237    for (i = 0; i <= pid->level; i++)
238        free_pidmap(pid->numbers + i);
239
240    call_rcu(&pid->rcu, delayed_put_pid);
241}
242
243struct pid *alloc_pid(struct pid_namespace *ns)
244{
245    struct pid *pid;
246    enum pid_type type;
247    int i, nr;
248    struct pid_namespace *tmp;
249    struct upid *upid;
250
251    pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
252    if (!pid)
253        goto out;
254
255    tmp = ns;
256    for (i = ns->level; i >= 0; i--) {
257        nr = alloc_pidmap(tmp);
258        if (nr < 0)
259            goto out_free;
260
261        pid->numbers[i].nr = nr;
262        pid->numbers[i].ns = tmp;
263        tmp = tmp->parent;
264    }
265
266    get_pid_ns(ns);
267    pid->level = ns->level;
268    atomic_set(&pid->count, 1);
269    for (type = 0; type < PIDTYPE_MAX; ++type)
270        INIT_HLIST_HEAD(&pid->tasks[type]);
271
272    upid = pid->numbers + ns->level;
273    spin_lock_irq(&pidmap_lock);
274    for ( ; upid >= pid->numbers; --upid)
275        hlist_add_head_rcu(&upid->pid_chain,
276                &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
277    spin_unlock_irq(&pidmap_lock);
278
279out:
280    return pid;
281
282out_free:
283    while (++i <= ns->level)
284        free_pidmap(pid->numbers + i);
285
286    kmem_cache_free(ns->pid_cachep, pid);
287    pid = NULL;
288    goto out;
289}
290
291struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
292{
293    struct hlist_node *elem;
294    struct upid *pnr;
295
296    hlist_for_each_entry_rcu(pnr, elem,
297            &pid_hash[pid_hashfn(nr, ns)], pid_chain)
298        if (pnr->nr == nr && pnr->ns == ns)
299            return container_of(pnr, struct pid,
300                    numbers[ns->level]);
301
302    return NULL;
303}
304EXPORT_SYMBOL_GPL(find_pid_ns);
305
306struct pid *find_vpid(int nr)
307{
308    return find_pid_ns(nr, current->nsproxy->pid_ns);
309}
310EXPORT_SYMBOL_GPL(find_vpid);
311
312/*
313 * attach_pid() must be called with the tasklist_lock write-held.
314 */
315void attach_pid(struct task_struct *task, enum pid_type type,
316        struct pid *pid)
317{
318    struct pid_link *link;
319
320    link = &task->pids[type];
321    link->pid = pid;
322    hlist_add_head_rcu(&link->node, &pid->tasks[type]);
323}
324
325static void __change_pid(struct task_struct *task, enum pid_type type,
326            struct pid *new)
327{
328    struct pid_link *link;
329    struct pid *pid;
330    int tmp;
331
332    link = &task->pids[type];
333    pid = link->pid;
334
335    hlist_del_rcu(&link->node);
336    link->pid = new;
337
338    for (tmp = PIDTYPE_MAX; --tmp >= 0; )
339        if (!hlist_empty(&pid->tasks[tmp]))
340            return;
341
342    free_pid(pid);
343}
344
345void detach_pid(struct task_struct *task, enum pid_type type)
346{
347    __change_pid(task, type, NULL);
348}
349
350void change_pid(struct task_struct *task, enum pid_type type,
351        struct pid *pid)
352{
353    __change_pid(task, type, pid);
354    attach_pid(task, type, pid);
355}
356
357/* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
358void transfer_pid(struct task_struct *old, struct task_struct *new,
359               enum pid_type type)
360{
361    new->pids[type].pid = old->pids[type].pid;
362    hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
363}
364
365struct task_struct *pid_task(struct pid *pid, enum pid_type type)
366{
367    struct task_struct *result = NULL;
368    if (pid) {
369        struct hlist_node *first;
370        first = rcu_dereference_check(pid->tasks[type].first,
371                          rcu_read_lock_held() ||
372                          lockdep_tasklist_lock_is_held());
373        if (first)
374            result = hlist_entry(first, struct task_struct, pids[(type)].node);
375    }
376    return result;
377}
378EXPORT_SYMBOL(pid_task);
379
380/*
381 * Must be called under rcu_read_lock().
382 */
383struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
384{
385    return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
386}
387
388struct task_struct *find_task_by_vpid(pid_t vnr)
389{
390    return find_task_by_pid_ns(vnr, current->nsproxy->pid_ns);
391}
392
393struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
394{
395    struct pid *pid;
396    rcu_read_lock();
397    if (type != PIDTYPE_PID)
398        task = task->group_leader;
399    pid = get_pid(task->pids[type].pid);
400    rcu_read_unlock();
401    return pid;
402}
403
404struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
405{
406    struct task_struct *result;
407    rcu_read_lock();
408    result = pid_task(pid, type);
409    if (result)
410        get_task_struct(result);
411    rcu_read_unlock();
412    return result;
413}
414
415struct pid *find_get_pid(pid_t nr)
416{
417    struct pid *pid;
418
419    rcu_read_lock();
420    pid = get_pid(find_vpid(nr));
421    rcu_read_unlock();
422
423    return pid;
424}
425EXPORT_SYMBOL_GPL(find_get_pid);
426
427pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
428{
429    struct upid *upid;
430    pid_t nr = 0;
431
432    if (pid && ns->level <= pid->level) {
433        upid = &pid->numbers[ns->level];
434        if (upid->ns == ns)
435            nr = upid->nr;
436    }
437    return nr;
438}
439
440pid_t pid_vnr(struct pid *pid)
441{
442    return pid_nr_ns(pid, current->nsproxy->pid_ns);
443}
444EXPORT_SYMBOL_GPL(pid_vnr);
445
446pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
447            struct pid_namespace *ns)
448{
449    pid_t nr = 0;
450
451    rcu_read_lock();
452    if (!ns)
453        ns = current->nsproxy->pid_ns;
454    if (likely(pid_alive(task))) {
455        if (type != PIDTYPE_PID)
456            task = task->group_leader;
457        nr = pid_nr_ns(task->pids[type].pid, ns);
458    }
459    rcu_read_unlock();
460
461    return nr;
462}
463EXPORT_SYMBOL(__task_pid_nr_ns);
464
465pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
466{
467    return pid_nr_ns(task_tgid(tsk), ns);
468}
469EXPORT_SYMBOL(task_tgid_nr_ns);
470
471struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
472{
473    return ns_of_pid(task_pid(tsk));
474}
475EXPORT_SYMBOL_GPL(task_active_pid_ns);
476
477/*
478 * Used by proc to find the first pid that is greater than or equal to nr.
479 *
480 * If there is a pid at nr this function is exactly the same as find_pid_ns.
481 */
482struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
483{
484    struct pid *pid;
485
486    do {
487        pid = find_pid_ns(nr, ns);
488        if (pid)
489            break;
490        nr = next_pidmap(ns, nr);
491    } while (nr > 0);
492
493    return pid;
494}
495
496/*
497 * The pid hash table is scaled according to the amount of memory in the
498 * machine. From a minimum of 16 slots up to 4096 slots at one gigabyte or
499 * more.
500 */
501void __init pidhash_init(void)
502{
503    int i, pidhash_size;
504
505    pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
506                       HASH_EARLY | HASH_SMALL,
507                       &pidhash_shift, NULL, 4096);
508    pidhash_size = 1 << pidhash_shift;
509
510    for (i = 0; i < pidhash_size; i++)
511        INIT_HLIST_HEAD(&pid_hash[i]);
512}
513
514void __init pidmap_init(void)
515{
516    init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
517    /* Reserve PID 0. We never call free_pidmap(0) */
518    set_bit(0, init_pid_ns.pidmap[0].page);
519    atomic_dec(&init_pid_ns.pidmap[0].nr_free);
520
521    init_pid_ns.pid_cachep = KMEM_CACHE(pid,
522            SLAB_HWCACHE_ALIGN | SLAB_PANIC);
523}
524

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