pid.c

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/*
 * Generic pidhash and scalable, time-bounded PID allocator
 *
 * (C) 2002-2003 William Irwin, IBM
 * (C) 2004 William Irwin, Oracle
 * (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).
 *
 * Pid namespaces:
 *    (C) 2007 Pavel Emelyanov <[email protected]>, OpenVZ, SWsoft Inc.
 *    (C) 2007 Sukadev Bhattiprolu <[email protected]>, IBM
 *     Many thanks to Oleg Nesterov for comments and help
 *
 */

#include <linux/mm.h>
#include <linux/export.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/rculist.h>
#include <linux/bootmem.h>
#include <linux/hash.h>
#include <linux/pid_namespace.h>
#include <linux/init_task.h>
#include <linux/syscalls.h>

#define pid_hashfn(nr, ns)  \
    hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
static struct hlist_head *pid_hash;
static unsigned int pidhash_shift = 4;
struct pid init_struct_pid = INIT_STRUCT_PID;

int pid_max = PID_MAX_DEFAULT;

#define RESERVED_PIDS       300

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

#define BITS_PER_PAGE       (PAGE_SIZE*8)
#define BITS_PER_PAGE_MASK  (BITS_PER_PAGE-1)

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

#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.
 */
struct pid_namespace init_pid_ns = {
    .kref = {
        .refcount       = ATOMIC_INIT(2),
    },
    .pidmap = {
        [ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
    },
    .last_pid = 0,
    .level = 0,
    .child_reaper = &init_task,
};
EXPORT_SYMBOL_GPL(init_pid_ns);

int is_container_init(struct task_struct *tsk)
{
    int ret = 0;
    struct pid *pid;

    rcu_read_lock();
    pid = task_pid(tsk);
    if (pid != NULL && pid->numbers[pid->level].nr == 1)
        ret = 1;
    rcu_read_unlock();

    return ret;
}
EXPORT_SYMBOL(is_container_init);

/*
 * 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.
 */

static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);

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

    clear_bit(offset, map->page);
    atomic_inc(&map->nr_free);
}

/*
 * 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);
}

/*
 * 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).
 * 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)));
}

static int alloc_pidmap(struct pid_namespace *pid_ns)
{
    int i, offset, max_scan, pid, last = pid_ns->last_pid;
    struct pidmap *map;

    pid = last + 1;
    if (pid >= pid_max)
        pid = RESERVED_PIDS;
    offset = pid & BITS_PER_PAGE_MASK;
    map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
    /*
     * 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;
    for (i = 0; i <= max_scan; ++i) {
        if (unlikely(!map->page)) {
            void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);
            /*
             * Free the page if someone raced with us
             * installing it:
             */
            spin_lock_irq(&pidmap_lock);
            if (!map->page) {
                map->page = page;
                page = NULL;
            }
            spin_unlock_irq(&pidmap_lock);
            kfree(page);
            if (unlikely(!map->page))
                break;
        }
        if (likely(atomic_read(&map->nr_free))) {
            do {
                if (!test_and_set_bit(offset, map->page)) {
                    atomic_dec(&map->nr_free);
                    set_last_pid(pid_ns, last, pid);
                    return pid;
                }
                offset = find_next_offset(map, offset);
                pid = mk_pid(pid_ns, map, offset);
            } while (offset < BITS_PER_PAGE && pid < pid_max);
        }
        if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {
            ++map;
            offset = 0;
        } else {
            map = &pid_ns->pidmap[0];
            offset = RESERVED_PIDS;
            if (unlikely(last == offset))
                break;
        }
        pid = mk_pid(pid_ns, map, offset);
    }
    return -1;
}

int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
{
    int offset;
    struct pidmap *map, *end;

    if (last >= PID_MAX_LIMIT)
        return -1;

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

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

    if (!pid)
        return;

    ns = pid->numbers[pid->level].ns;
    if ((atomic_read(&pid->count) == 1) ||
         atomic_dec_and_test(&pid->count)) {
        kmem_cache_free(ns->pid_cachep, pid);
        put_pid_ns(ns);
    }
}
EXPORT_SYMBOL_GPL(put_pid);

static void delayed_put_pid(struct rcu_head *rhp)
{
    struct pid *pid = container_of(rhp, struct pid, rcu);
    put_pid(pid);
}

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

    spin_lock_irqsave(&pidmap_lock, flags);
    for (i = 0; i <= pid->level; i++)
        hlist_del_rcu(&pid->numbers[i].pid_chain);
    spin_unlock_irqrestore(&pidmap_lock, flags);

    for (i = 0; i <= pid->level; i++)
        free_pidmap(pid->numbers + i);

    call_rcu(&pid->rcu, delayed_put_pid);
}

struct pid *alloc_pid(struct pid_namespace *ns)
{
    struct pid *pid;
    enum pid_type type;
    int i, nr;
    struct pid_namespace *tmp;
    struct upid *upid;

    pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
    if (!pid)
        goto out;

    tmp = ns;
    for (i = ns->level; i >= 0; i--) {
        nr = alloc_pidmap(tmp);
        if (nr < 0)
            goto out_free;

        pid->numbers[i].nr = nr;
        pid->numbers[i].ns = tmp;
        tmp = tmp->parent;
    }

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

    upid = pid->numbers + ns->level;
    spin_lock_irq(&pidmap_lock);
    for ( ; upid >= pid->numbers; --upid)
        hlist_add_head_rcu(&upid->pid_chain,
                &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
    spin_unlock_irq(&pidmap_lock);

out:
    return pid;

out_free:
    while (++i <= ns->level)
        free_pidmap(pid->numbers + i);

    kmem_cache_free(ns->pid_cachep, pid);
    pid = NULL;
    goto out;
}

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

    hlist_for_each_entry_rcu(pnr, elem,
            &pid_hash[pid_hashfn(nr, ns)], pid_chain)
        if (pnr->nr == nr && pnr->ns == ns)
            return container_of(pnr, struct pid,
                    numbers[ns->level]);

    return NULL;
}
EXPORT_SYMBOL_GPL(find_pid_ns);

struct pid *find_vpid(int nr)
{
    return find_pid_ns(nr, current->nsproxy->pid_ns);
}
EXPORT_SYMBOL_GPL(find_vpid);

/*
 * attach_pid() must be called with the tasklist_lock write-held.
 */
void attach_pid(struct task_struct *task, enum pid_type type,
        struct pid *pid)
{
    struct pid_link *link;

    link = &task->pids[type];
    link->pid = pid;
    hlist_add_head_rcu(&link->node, &pid->tasks[type]);
}

static void __change_pid(struct task_struct *task, enum pid_type type,
            struct pid *new)
{
    struct pid_link *link;
    struct pid *pid;
    int tmp;

    link = &task->pids[type];
    pid = link->pid;

    hlist_del_rcu(&link->node);
    link->pid = new;

    for (tmp = PIDTYPE_MAX; --tmp >= 0; )
        if (!hlist_empty(&pid->tasks[tmp]))
            return;

    free_pid(pid);
}

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);
    attach_pid(task, type, pid);
}

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

struct task_struct *pid_task(struct pid *pid, enum pid_type type)
{
    struct task_struct *result = NULL;
    if (pid) {
        struct hlist_node *first;
        first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
                          lockdep_tasklist_lock_is_held());
        if (first)
            result = hlist_entry(first, struct task_struct, pids[(type)].node);
    }
    return result;
}
EXPORT_SYMBOL(pid_task);

/*
 * Must be called under rcu_read_lock().
 */
struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
{
    rcu_lockdep_assert(rcu_read_lock_held(),
               "find_task_by_pid_ns() needs rcu_read_lock()"
               " protection");
    return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
}

struct task_struct *find_task_by_vpid(pid_t vnr)
{
    return find_task_by_pid_ns(vnr, current->nsproxy->pid_ns);
}

struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
{
    struct pid *pid;
    rcu_read_lock();
    if (type != PIDTYPE_PID)
        task = task->group_leader;
    pid = get_pid(task->pids[type].pid);
    rcu_read_unlock();
    return pid;
}
EXPORT_SYMBOL_GPL(get_task_pid);

struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
{
    struct task_struct *result;
    rcu_read_lock();
    result = pid_task(pid, type);
    if (result)
        get_task_struct(result);
    rcu_read_unlock();
    return result;
}
EXPORT_SYMBOL_GPL(get_pid_task);

struct pid *find_get_pid(pid_t nr)
{
    struct pid *pid;

    rcu_read_lock();
    pid = get_pid(find_vpid(nr));
    rcu_read_unlock();

    return pid;
}
EXPORT_SYMBOL_GPL(find_get_pid);

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;
}
EXPORT_SYMBOL_GPL(pid_nr_ns);

pid_t pid_vnr(struct pid *pid)
{
    return pid_nr_ns(pid, current->nsproxy->pid_ns);
}
EXPORT_SYMBOL_GPL(pid_vnr);

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

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

    return nr;
}
EXPORT_SYMBOL(__task_pid_nr_ns);

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);

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);

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

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

    return pid;
}

/*
 * 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)
{
    unsigned int i, pidhash_size;

    pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
                       HASH_EARLY | HASH_SMALL,
                       &pidhash_shift, NULL,
                       0, 4096);
    pidhash_size = 1U << pidhash_shift;

    for (i = 0; i < pidhash_size; i++)
        INIT_HLIST_HEAD(&pid_hash[i]);
}

void __init pidmap_init(void)
{
    /* 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);

    init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);
    /* Reserve PID 0. We never call free_pidmap(0) */
    set_bit(0, init_pid_ns.pidmap[0].page);
    atomic_dec(&init_pid_ns.pidmap[0].nr_free);

    init_pid_ns.pid_cachep = KMEM_CACHE(pid,
            SLAB_HWCACHE_ALIGN | SLAB_PANIC);
}