pid namespaces: helpers to find the task by its numerical ids

[mirror_ubuntu-bionic-kernel.git] / kernel / pid.c

/*
 * 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).
 */

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/bootmem.h>
#include <linux/hash.h>
#include <linux/pid_namespace.h>
#include <linux/init_task.h>

#define pid_hashfn(nr, ns)      \
        hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)
static struct hlist_head *pid_hash;
static int pidhash_shift;
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_global_init(struct task_struct *tsk)
{
        return tsk == init_pid_ns.child_reaper;
}

/*
 * 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 fastcall void free_pidmap(struct pid_namespace *pid_ns, int pid)
{
        struct pidmap *map = pid_ns->pidmap + pid / BITS_PER_PAGE;
        int offset = pid & BITS_PER_PAGE_MASK;

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

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];
        max_scan = (pid_max + BITS_PER_PAGE - 1)/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)
                                kfree(page);
                        else
                                map->page = page;
                        spin_unlock_irq(&pidmap_lock);
                        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);
                                        pid_ns->last_pid = pid;
                                        return pid;
                                }
                                offset = find_next_offset(map, offset);
                                pid = mk_pid(pid_ns, map, offset);
                        /*
                         * find_next_offset() found a bit, the pid from it
                         * is in-bounds, and if we fell back to the last
                         * bitmap block and the final block was the same
                         * as the starting point, pid is before last_pid.
                         */
                        } while (offset < BITS_PER_PAGE && pid < pid_max &&
                                        (i != max_scan || pid < last ||
                                            !((last+1) & BITS_PER_PAGE_MASK)));
                }
                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;
}

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

        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;
}

fastcall 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);
                if (ns != &init_pid_ns)
                        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);
}

fastcall 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].ns, pid->numbers[i].nr);

        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;
        }

        if (ns != &init_pid_ns)
                get_pid_ns(ns);

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

        spin_lock_irq(&pidmap_lock);
        for (i = ns->level; i >= 0; i--) {
                upid = &pid->numbers[i];
                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:
        for (i++; i <= ns->level; i++)
                free_pidmap(pid->numbers[i].ns, pid->numbers[i].nr);

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

struct pid * fastcall 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);

/*
 * attach_pid() must be called with the tasklist_lock write-held.
 */
int fastcall 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]);

        return 0;
}

void fastcall detach_pid(struct task_struct *task, enum pid_type type)
{
        struct pid_link *link;
        struct pid *pid;
        int tmp;

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

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

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

        free_pid(pid);
}

/* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
void fastcall 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);
        old->pids[type].pid = NULL;
}

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

/*
 * Must be called under rcu_read_lock() or with tasklist_lock read-held.
 */
struct task_struct *find_task_by_pid_type_ns(int type, int nr,
                struct pid_namespace *ns)
{
        return pid_task(find_pid_ns(nr, ns), type);
}

EXPORT_SYMBOL(find_task_by_pid_type_ns);

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

struct task_struct *fastcall 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;
}

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;
}

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;
}

/*
 * Used by proc to find the first pid that is greater then or equal to nr.
 *
 * If there is a pid at nr this function is exactly the same as find_pid.
 */
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;
}
EXPORT_SYMBOL_GPL(find_get_pid);

struct pid_cache {
        int nr_ids;
        char name[16];
        struct kmem_cache *cachep;
        struct list_head list;
};

static LIST_HEAD(pid_caches_lh);
static DEFINE_MUTEX(pid_caches_mutex);

/*
 * creates the kmem cache to allocate pids from.
 * @nr_ids: the number of numerical ids this pid will have to carry
 */

static struct kmem_cache *create_pid_cachep(int nr_ids)
{
        struct pid_cache *pcache;
        struct kmem_cache *cachep;

        mutex_lock(&pid_caches_mutex);
        list_for_each_entry (pcache, &pid_caches_lh, list)
                if (pcache->nr_ids == nr_ids)
                        goto out;

        pcache = kmalloc(sizeof(struct pid_cache), GFP_KERNEL);
        if (pcache == NULL)
                goto err_alloc;

        snprintf(pcache->name, sizeof(pcache->name), "pid_%d", nr_ids);
        cachep = kmem_cache_create(pcache->name,
                        /* FIXME add numerical ids here */
                        sizeof(struct pid), 0, SLAB_HWCACHE_ALIGN, NULL);
        if (cachep == NULL)
                goto err_cachep;

        pcache->nr_ids = nr_ids;
        pcache->cachep = cachep;
        list_add(&pcache->list, &pid_caches_lh);
out:
        mutex_unlock(&pid_caches_mutex);
        return pcache->cachep;

err_cachep:
        kfree(pcache);
err_alloc:
        mutex_unlock(&pid_caches_mutex);
        return NULL;
}

struct pid_namespace *copy_pid_ns(unsigned long flags, struct pid_namespace *old_ns)
{
        BUG_ON(!old_ns);
        get_pid_ns(old_ns);
        return old_ns;
}

void free_pid_ns(struct kref *kref)
{
        struct pid_namespace *ns;

        ns = container_of(kref, struct pid_namespace, kref);
        kfree(ns);
}

/*
 * 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)
{
        int i, pidhash_size;
        unsigned long megabytes = nr_kernel_pages >> (20 - PAGE_SHIFT);

        pidhash_shift = max(4, fls(megabytes * 4));
        pidhash_shift = min(12, pidhash_shift);
        pidhash_size = 1 << pidhash_shift;

        printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
                pidhash_size, pidhash_shift,
                pidhash_size * sizeof(struct hlist_head));

        pid_hash = alloc_bootmem(pidhash_size * sizeof(*(pid_hash)));
        if (!pid_hash)
                panic("Could not alloc pidhash!\n");
        for (i = 0; i < pidhash_size; i++)
                INIT_HLIST_HEAD(&pid_hash[i]);
}

void __init pidmap_init(void)
{
        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 = create_pid_cachep(1);
        if (init_pid_ns.pid_cachep == NULL)
                panic("Can't create pid_1 cachep\n");
}