/*
 * Generic pidhash and scalable, time-bounded PID allocator
 *
 * (C) 2002-2003 Nadia Yvette Chambers, IBM
 * (C) 2004 Nadia Yvette Chambers, 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 <xemul@openvz.org>, OpenVZ, SWsoft Inc.
 *    (C) 2007 Sukadev Bhattiprolu <sukadev@us.ibm.com>, 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>
#include <linux/proc_ns.h>
#include <linux/proc_fs.h>
#include <linux/sched/task.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;

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 = KREF_INIT(2),
        .pidmap = {
                [ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
        },
        .last_pid = 0,
        .nr_hashed = PIDNS_HASH_ADDING,
        .level = 0,
        .child_reaper = &init_task,
        .user_ns = &init_user_ns,
        .ns.inum = PROC_PID_INIT_INO,
#ifdef CONFIG_PID_NS
        .ns.ops = &pidns_operations,
#endif
};
EXPORT_SYMBOL_GPL(init_pid_ns);

/*
 * 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))
                                return -ENOMEM;
                }
                if (likely(atomic_read(&map->nr_free))) {
                        for ( ; ; ) {
                                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);
                                if (offset >= BITS_PER_PAGE)
                                        break;
                                pid = mk_pid(pid_ns, map, offset);
                                if (pid >= pid_max)
                                        break;
                        }
                }
                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 -EAGAIN;
}

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++) {
                struct upid *upid = pid->numbers + i;
                struct pid_namespace *ns = upid->ns;
                hlist_del_rcu(&upid->pid_chain);
                switch(--ns->nr_hashed) {
                case 2:
                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;
                case PIDNS_HASH_ADDING:
                        /* Handle a fork failure of the first process */
                        WARN_ON(ns->child_reaper);
                        ns->nr_hashed = 0;
                        /* fall through */
                case 0:
                        schedule_work(&ns->proc_work);
                        break;
                }
        }
        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;
        int retval = -ENOMEM;

        pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
        if (!pid)
                return ERR_PTR(retval);

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

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

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

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

        return pid;

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

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

        kmem_cache_free(ns->pid_cachep, pid);
        return ERR_PTR(retval);
}

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

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

        hlist_for_each_entry_rcu(pnr,
                        &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, task_active_pid_ns(current));
}
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_link *link = &task->pids[type];
        hlist_add_head_rcu(&link->node, &link->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);
}

/* 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_WARN(!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, task_active_pid_ns(current));
}

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

        return nr;
}
EXPORT_SYMBOL(__task_pid_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)
{
        pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,
                                           HASH_EARLY | HASH_SMALL | HASH_ZERO,
                                           &pidhash_shift, NULL,
                                           0, 4096);
}

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

        /* 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 | SLAB_ACCOUNT);
}