pid.c 15.0 KB
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/*
 * Generic pidhash and scalable, time-bounded PID allocator
 *
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 * (C) 2002-2003 Nadia Yvette Chambers, IBM
 * (C) 2004 Nadia Yvette Chambers, Oracle
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 * (C) 2002-2004 Ingo Molnar, Red Hat
 *
 * pid-structures are backing objects for tasks sharing a given ID to chain
 * against. There is very little to them aside from hashing them and
 * parking tasks using given ID's on a list.
 *
 * The hash is always changed with the tasklist_lock write-acquired,
 * and the hash is only accessed with the tasklist_lock at least
 * read-acquired, so there's no additional SMP locking needed here.
 *
 * We have a list of bitmap pages, which bitmaps represent the PID space.
 * Allocating and freeing PIDs is completely lockless. The worst-case
 * allocation scenario when all but one out of 1 million PIDs possible are
 * allocated already: the scanning of 32 list entries and at most PAGE_SIZE
 * bytes. The typical fastpath is a single successful setbit. Freeing is O(1).
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 *
 * Pid namespaces:
 *    (C) 2007 Pavel Emelyanov <xemul@openvz.org>, OpenVZ, SWsoft Inc.
 *    (C) 2007 Sukadev Bhattiprolu <sukadev@us.ibm.com>, IBM
 *     Many thanks to Oleg Nesterov for comments and help
 *
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 */

#include <linux/mm.h>
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#include <linux/export.h>
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#include <linux/slab.h>
#include <linux/init.h>
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#include <linux/rculist.h>
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#include <linux/bootmem.h>
#include <linux/hash.h>
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#include <linux/pid_namespace.h>
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#include <linux/init_task.h>
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#include <linux/syscalls.h>
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#include <linux/proc_ns.h>
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#include <linux/proc_fs.h>
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#include <linux/sched/task.h>
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#define pid_hashfn(nr, ns)	\
	hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
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static struct hlist_head *pid_hash;
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static unsigned int pidhash_shift = 4;
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struct pid init_struct_pid = INIT_STRUCT_PID;
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int pid_max = PID_MAX_DEFAULT;

#define RESERVED_PIDS		300

int pid_max_min = RESERVED_PIDS + 1;
int pid_max_max = PID_MAX_LIMIT;

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static inline int mk_pid(struct pid_namespace *pid_ns,
		struct pidmap *map, int off)
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{
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	return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;
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}

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#define find_next_offset(map, off)					\
		find_next_zero_bit((map)->page, BITS_PER_PAGE, off)

/*
 * PID-map pages start out as NULL, they get allocated upon
 * first use and are never deallocated. This way a low pid_max
 * value does not cause lots of bitmaps to be allocated, but
 * the scheme scales to up to 4 million PIDs, runtime.
 */
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struct pid_namespace init_pid_ns = {
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	.kref = KREF_INIT(2),
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	.pidmap = {
		[ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
	},
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	.last_pid = 0,
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	.nr_hashed = PIDNS_HASH_ADDING,
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	.level = 0,
	.child_reaper = &init_task,
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	.user_ns = &init_user_ns,
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	.ns.inum = PROC_PID_INIT_INO,
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#ifdef CONFIG_PID_NS
	.ns.ops = &pidns_operations,
#endif
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};
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EXPORT_SYMBOL_GPL(init_pid_ns);
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/*
 * Note: disable interrupts while the pidmap_lock is held as an
 * interrupt might come in and do read_lock(&tasklist_lock).
 *
 * If we don't disable interrupts there is a nasty deadlock between
 * detach_pid()->free_pid() and another cpu that does
 * spin_lock(&pidmap_lock) followed by an interrupt routine that does
 * read_lock(&tasklist_lock);
 *
 * After we clean up the tasklist_lock and know there are no
 * irq handlers that take it we can leave the interrupts enabled.
 * For now it is easier to be safe than to prove it can't happen.
 */
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static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);

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static void free_pidmap(struct upid *upid)
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{
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	int nr = upid->nr;
	struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
	int offset = nr & BITS_PER_PAGE_MASK;
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	clear_bit(offset, map->page);
	atomic_inc(&map->nr_free);
}

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/*
 * If we started walking pids at 'base', is 'a' seen before 'b'?
 */
static int pid_before(int base, int a, int b)
{
	/*
	 * This is the same as saying
	 *
	 * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
	 * and that mapping orders 'a' and 'b' with respect to 'base'.
	 */
	return (unsigned)(a - base) < (unsigned)(b - base);
}

/*
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 * We might be racing with someone else trying to set pid_ns->last_pid
 * at the pid allocation time (there's also a sysctl for this, but racing
 * with this one is OK, see comment in kernel/pid_namespace.c about it).
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 * We want the winner to have the "later" value, because if the
 * "earlier" value prevails, then a pid may get reused immediately.
 *
 * Since pids rollover, it is not sufficient to just pick the bigger
 * value.  We have to consider where we started counting from.
 *
 * 'base' is the value of pid_ns->last_pid that we observed when
 * we started looking for a pid.
 *
 * 'pid' is the pid that we eventually found.
 */
static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
{
	int prev;
	int last_write = base;
	do {
		prev = last_write;
		last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
	} while ((prev != last_write) && (pid_before(base, last_write, pid)));
}

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static int alloc_pidmap(struct pid_namespace *pid_ns)
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{
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	int i, offset, max_scan, pid, last = pid_ns->last_pid;
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	struct pidmap *map;
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	pid = last + 1;
	if (pid >= pid_max)
		pid = RESERVED_PIDS;
	offset = pid & BITS_PER_PAGE_MASK;
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	map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
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	/*
	 * If last_pid points into the middle of the map->page we
	 * want to scan this bitmap block twice, the second time
	 * we start with offset == 0 (or RESERVED_PIDS).
	 */
	max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;
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	for (i = 0; i <= max_scan; ++i) {
		if (unlikely(!map->page)) {
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			void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
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			/*
			 * Free the page if someone raced with us
			 * installing it:
			 */
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			spin_lock_irq(&pidmap_lock);
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			if (!map->page) {
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				map->page = page;
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				page = NULL;
			}
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			spin_unlock_irq(&pidmap_lock);
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			kfree(page);
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			if (unlikely(!map->page))
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				return -ENOMEM;
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		}
		if (likely(atomic_read(&map->nr_free))) {
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			for ( ; ; ) {
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				if (!test_and_set_bit(offset, map->page)) {
					atomic_dec(&map->nr_free);
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					set_last_pid(pid_ns, last, pid);
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					return pid;
				}
				offset = find_next_offset(map, offset);
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				if (offset >= BITS_PER_PAGE)
					break;
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				pid = mk_pid(pid_ns, map, offset);
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				if (pid >= pid_max)
					break;
			}
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		}
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		if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
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			++map;
			offset = 0;
		} else {
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			map = &pid_ns->pidmap[0];
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			offset = RESERVED_PIDS;
			if (unlikely(last == offset))
				break;
		}
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		pid = mk_pid(pid_ns, map, offset);
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	}
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	return -EAGAIN;
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}

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int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
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{
	int offset;
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	struct pidmap *map, *end;
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	if (last >= PID_MAX_LIMIT)
		return -1;

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	offset = (last + 1) & BITS_PER_PAGE_MASK;
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	map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
	end = &pid_ns->pidmap[PIDMAP_ENTRIES];
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	for (; map < end; map++, offset = 0) {
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		if (unlikely(!map->page))
			continue;
		offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
		if (offset < BITS_PER_PAGE)
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			return mk_pid(pid_ns, map, offset);
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	}
	return -1;
}

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void put_pid(struct pid *pid)
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{
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	struct pid_namespace *ns;

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	if (!pid)
		return;
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	ns = pid->numbers[pid->level].ns;
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	if ((atomic_read(&pid->count) == 1) ||
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	     atomic_dec_and_test(&pid->count)) {
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		kmem_cache_free(ns->pid_cachep, pid);
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		put_pid_ns(ns);
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	}
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}
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EXPORT_SYMBOL_GPL(put_pid);
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static void delayed_put_pid(struct rcu_head *rhp)
{
	struct pid *pid = container_of(rhp, struct pid, rcu);
	put_pid(pid);
}

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void free_pid(struct pid *pid)
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{
	/* We can be called with write_lock_irq(&tasklist_lock) held */
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	int i;
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	unsigned long flags;

	spin_lock_irqsave(&pidmap_lock, flags);
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	for (i = 0; i <= pid->level; i++) {
		struct upid *upid = pid->numbers + i;
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		struct pid_namespace *ns = upid->ns;
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		hlist_del_rcu(&upid->pid_chain);
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		switch(--ns->nr_hashed) {
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		case 2:
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		case 1:
			/* When all that is left in the pid namespace
			 * is the reaper wake up the reaper.  The reaper
			 * may be sleeping in zap_pid_ns_processes().
			 */
			wake_up_process(ns->child_reaper);
			break;
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		case PIDNS_HASH_ADDING:
			/* Handle a fork failure of the first process */
			WARN_ON(ns->child_reaper);
			ns->nr_hashed = 0;
			/* fall through */
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		case 0:
			schedule_work(&ns->proc_work);
			break;
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		}
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	}
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	spin_unlock_irqrestore(&pidmap_lock, flags);

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	for (i = 0; i <= pid->level; i++)
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		free_pidmap(pid->numbers + i);
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	call_rcu(&pid->rcu, delayed_put_pid);
}

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struct pid *alloc_pid(struct pid_namespace *ns)
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{
	struct pid *pid;
	enum pid_type type;
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	int i, nr;
	struct pid_namespace *tmp;
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	struct upid *upid;
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	int retval = -ENOMEM;
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	pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
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	if (!pid)
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		return ERR_PTR(retval);
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	tmp = ns;
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	pid->level = ns->level;
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	for (i = ns->level; i >= 0; i--) {
		nr = alloc_pidmap(tmp);
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		if (nr < 0) {
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			retval = nr;
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			goto out_free;
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		}
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		pid->numbers[i].nr = nr;
		pid->numbers[i].ns = tmp;
		tmp = tmp->parent;
	}

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	if (unlikely(is_child_reaper(pid))) {
		if (pid_ns_prepare_proc(ns))
			goto out_free;
	}

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	get_pid_ns(ns);
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	atomic_set(&pid->count, 1);
	for (type = 0; type < PIDTYPE_MAX; ++type)
		INIT_HLIST_HEAD(&pid->tasks[type]);

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	upid = pid->numbers + ns->level;
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	spin_lock_irq(&pidmap_lock);
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	if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
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		goto out_unlock;
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	for ( ; upid >= pid->numbers; --upid) {
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		hlist_add_head_rcu(&upid->pid_chain,
				&pid_hash[pid_hashfn(upid->nr, upid->ns)]);
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		upid->ns->nr_hashed++;
	}
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	spin_unlock_irq(&pidmap_lock);

	return pid;

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out_unlock:
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	spin_unlock_irq(&pidmap_lock);
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	put_pid_ns(ns);

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out_free:
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	while (++i <= ns->level)
		free_pidmap(pid->numbers + i);
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	kmem_cache_free(ns->pid_cachep, pid);
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	return ERR_PTR(retval);
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}

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void disable_pid_allocation(struct pid_namespace *ns)
{
	spin_lock_irq(&pidmap_lock);
	ns->nr_hashed &= ~PIDNS_HASH_ADDING;
	spin_unlock_irq(&pidmap_lock);
}

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struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
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{
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	struct upid *pnr;

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	hlist_for_each_entry_rcu(pnr,
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			&pid_hash[pid_hashfn(nr, ns)], pid_chain)
		if (pnr->nr == nr && pnr->ns == ns)
			return container_of(pnr, struct pid,
					numbers[ns->level]);
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	return NULL;
}
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EXPORT_SYMBOL_GPL(find_pid_ns);
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struct pid *find_vpid(int nr)
{
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	return find_pid_ns(nr, task_active_pid_ns(current));
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}
EXPORT_SYMBOL_GPL(find_vpid);

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/*
 * attach_pid() must be called with the tasklist_lock write-held.
 */
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void attach_pid(struct task_struct *task, enum pid_type type)
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{
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	struct pid_link *link = &task->pids[type];
	hlist_add_head_rcu(&link->node, &link->pid->tasks[type]);
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}

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static void __change_pid(struct task_struct *task, enum pid_type type,
			struct pid *new)
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{
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	struct pid_link *link;
	struct pid *pid;
	int tmp;
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	link = &task->pids[type];
	pid = link->pid;
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	hlist_del_rcu(&link->node);
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	link->pid = new;
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	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
		if (!hlist_empty(&pid->tasks[tmp]))
			return;
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	free_pid(pid);
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}

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void detach_pid(struct task_struct *task, enum pid_type type)
{
	__change_pid(task, type, NULL);
}

void change_pid(struct task_struct *task, enum pid_type type,
		struct pid *pid)
{
	__change_pid(task, type, pid);
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	attach_pid(task, type);
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}

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/* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
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void transfer_pid(struct task_struct *old, struct task_struct *new,
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			   enum pid_type type)
{
	new->pids[type].pid = old->pids[type].pid;
	hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
}

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struct task_struct *pid_task(struct pid *pid, enum pid_type type)
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{
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	struct task_struct *result = NULL;
	if (pid) {
		struct hlist_node *first;
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		first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
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					      lockdep_tasklist_lock_is_held());
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		if (first)
			result = hlist_entry(first, struct task_struct, pids[(type)].node);
	}
	return result;
}
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EXPORT_SYMBOL(pid_task);
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/*
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 * Must be called under rcu_read_lock().
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 */
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struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
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{
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	RCU_LOCKDEP_WARN(!rcu_read_lock_held(),
			 "find_task_by_pid_ns() needs rcu_read_lock() protection");
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	return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
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}
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struct task_struct *find_task_by_vpid(pid_t vnr)
{
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	return find_task_by_pid_ns(vnr, task_active_pid_ns(current));
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}

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struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
{
	struct pid *pid;
	rcu_read_lock();
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	if (type != PIDTYPE_PID)
		task = task->group_leader;
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	pid = get_pid(rcu_dereference(task->pids[type].pid));
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	rcu_read_unlock();
	return pid;
}
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EXPORT_SYMBOL_GPL(get_task_pid);
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struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
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{
	struct task_struct *result;
	rcu_read_lock();
	result = pid_task(pid, type);
	if (result)
		get_task_struct(result);
	rcu_read_unlock();
	return result;
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}
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EXPORT_SYMBOL_GPL(get_pid_task);
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struct pid *find_get_pid(pid_t nr)
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{
	struct pid *pid;

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	rcu_read_lock();
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	pid = get_pid(find_vpid(nr));
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	rcu_read_unlock();
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	return pid;
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}
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EXPORT_SYMBOL_GPL(find_get_pid);
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pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
{
	struct upid *upid;
	pid_t nr = 0;

	if (pid && ns->level <= pid->level) {
		upid = &pid->numbers[ns->level];
		if (upid->ns == ns)
			nr = upid->nr;
	}
	return nr;
}
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EXPORT_SYMBOL_GPL(pid_nr_ns);
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pid_t pid_vnr(struct pid *pid)
{
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	return pid_nr_ns(pid, task_active_pid_ns(current));
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}
EXPORT_SYMBOL_GPL(pid_vnr);

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pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
			struct pid_namespace *ns)
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{
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	pid_t nr = 0;

	rcu_read_lock();
	if (!ns)
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		ns = task_active_pid_ns(current);
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	if (likely(pid_alive(task))) {
		if (type != PIDTYPE_PID)
			task = task->group_leader;
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		nr = pid_nr_ns(rcu_dereference(task->pids[type].pid), ns);
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	}
	rcu_read_unlock();

	return nr;
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}
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EXPORT_SYMBOL(__task_pid_nr_ns);
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pid_t task_tgid_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
{
	return pid_nr_ns(task_tgid(tsk), ns);
}
EXPORT_SYMBOL(task_tgid_nr_ns);

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struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
{
	return ns_of_pid(task_pid(tsk));
}
EXPORT_SYMBOL_GPL(task_active_pid_ns);

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/*
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 * Used by proc to find the first pid that is greater than or equal to nr.
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 *
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 * If there is a pid at nr this function is exactly the same as find_pid_ns.
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 */
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struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
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{
	struct pid *pid;

	do {
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		pid = find_pid_ns(nr, ns);
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		if (pid)
			break;
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		nr = next_pidmap(ns, nr);
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	} while (nr > 0);

	return pid;
}

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/*
 * The pid hash table is scaled according to the amount of memory in the
 * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
 * more.
 */
void __init pidhash_init(void)
{
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	unsigned int i, pidhash_size;
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	pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
					   HASH_EARLY | HASH_SMALL,
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					   &pidhash_shift, NULL,
					   0, 4096);
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	pidhash_size = 1U << pidhash_shift;
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	for (i = 0; i < pidhash_size; i++)
		INIT_HLIST_HEAD(&pid_hash[i]);
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}

void __init pidmap_init(void)
{
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	/* Verify no one has done anything silly: */
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	BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_HASH_ADDING);

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	/* bump default and minimum pid_max based on number of cpus */
	pid_max = min(pid_max_max, max_t(int, pid_max,
				PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
	pid_max_min = max_t(int, pid_max_min,
				PIDS_PER_CPU_MIN * num_possible_cpus());
	pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);

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	init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
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	/* Reserve PID 0. We never call free_pidmap(0) */
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	set_bit(0, init_pid_ns.pidmap[0].page);
	atomic_dec(&init_pid_ns.pidmap[0].nr_free);
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	init_pid_ns.pid_cachep = KMEM_CACHE(pid,
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			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
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}