/*
 * The "user cache".
 *
 * (C) Copyright 1991-2000 Linus Torvalds
 *
 * We have a per-user structure to keep track of how many
 * processes, files etc the user has claimed, in order to be
 * able to have per-user limits for system resources. 
 */

#include <linux/init.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/bitops.h>
#include <linux/key.h>
#include <linux/interrupt.h>
#include <linux/export.h>
#include <linux/user_namespace.h>

/*
 * userns count is 1 for root user, 1 for init_uts_ns,
 * and 1 for... ?
 */
struct user_namespace init_user_ns = {
        .kref = {
                .refcount       = ATOMIC_INIT(3),
        },
        .creator = &root_user,
};
EXPORT_SYMBOL_GPL(init_user_ns);

/*
 * UID task count cache, to get fast user lookup in "alloc_uid"
 * when changing user ID's (ie setuid() and friends).
 */

#define UIDHASH_MASK            (UIDHASH_SZ - 1)
#define __uidhashfn(uid)        (((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK)
#define uidhashentry(ns, uid)   ((ns)->uidhash_table + __uidhashfn((uid)))

static struct kmem_cache *uid_cachep;

/*
 * The uidhash_lock is mostly taken from process context, but it is
 * occasionally also taken from softirq/tasklet context, when
 * task-structs get RCU-freed. Hence all locking must be softirq-safe.
 * But free_uid() is also called with local interrupts disabled, and running
 * local_bh_enable() with local interrupts disabled is an error - we'll run
 * softirq callbacks, and they can unconditionally enable interrupts, and
 * the caller of free_uid() didn't expect that..
 */
static DEFINE_SPINLOCK(uidhash_lock);

/* root_user.__count is 2, 1 for init task cred, 1 for init_user_ns->user_ns */
struct user_struct root_user = {
        .__count        = ATOMIC_INIT(2),
        .processes      = ATOMIC_INIT(1),
        .files          = ATOMIC_INIT(0),
        .sigpending     = ATOMIC_INIT(0),
        .locked_shm     = 0,
        .user_ns        = &init_user_ns,
};

/*
 * These routines must be called with the uidhash spinlock held!
 */
static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent)
{
        hlist_add_head(&up->uidhash_node, hashent);
}

static void uid_hash_remove(struct user_struct *up)
{
        hlist_del_init(&up->uidhash_node);
        put_user_ns(up->user_ns);
}

static struct user_struct *uid_hash_find(uid_t uid, struct hlist_head *hashent)
{
        struct user_struct *user;
        struct hlist_node *h;

        hlist_for_each_entry(user, h, hashent, uidhash_node) {
                if (user->uid == uid) {
                        atomic_inc(&user->__count);
                        return user;
                }
        }

        return NULL;
}

/* IRQs are disabled and uidhash_lock is held upon function entry.
 * IRQ state (as stored in flags) is restored and uidhash_lock released
 * upon function exit.
 */
static void free_user(struct user_struct *up, unsigned long flags)
        __releases(&uidhash_lock)
{
        uid_hash_remove(up);
        spin_unlock_irqrestore(&uidhash_lock, flags);
        key_put(up->uid_keyring);
        key_put(up->session_keyring);
        kmem_cache_free(uid_cachep, up);
}

/*
 * Locate the user_struct for the passed UID.  If found, take a ref on it.  The
 * caller must undo that ref with free_uid().
 *
 * If the user_struct could not be found, return NULL.
 */
struct user_struct *find_user(uid_t uid)
{
        struct user_struct *ret;
        unsigned long flags;
        struct user_namespace *ns = current_user_ns();

        spin_lock_irqsave(&uidhash_lock, flags);
        ret = uid_hash_find(uid, uidhashentry(ns, uid));
        spin_unlock_irqrestore(&uidhash_lock, flags);
        return ret;
}

void free_uid(struct user_struct *up)
{
        unsigned long flags;

        if (!up)
                return;

        local_irq_save(flags);
        if (atomic_dec_and_lock(&up->__count, &uidhash_lock))
                free_user(up, flags);
        else
                local_irq_restore(flags);
}

struct user_struct *alloc_uid(struct user_namespace *ns, uid_t uid)
{
        struct hlist_head *hashent = uidhashentry(ns, uid);
        struct user_struct *up, *new;

        spin_lock_irq(&uidhash_lock);
        up = uid_hash_find(uid, hashent);
        spin_unlock_irq(&uidhash_lock);

        if (!up) {
                new = kmem_cache_zalloc(uid_cachep, GFP_KERNEL);
                if (!new)
                        goto out_unlock;

                new->uid = uid;
                atomic_set(&new->__count, 1);

                new->user_ns = get_user_ns(ns);

                /*
                 * Before adding this, check whether we raced
                 * on adding the same user already..
                 */
                spin_lock_irq(&uidhash_lock);
                up = uid_hash_find(uid, hashent);
                if (up) {
                        put_user_ns(ns);
                        key_put(new->uid_keyring);
                        key_put(new->session_keyring);
                        kmem_cache_free(uid_cachep, new);
                } else {
                        uid_hash_insert(new, hashent);
                        up = new;
                }
                spin_unlock_irq(&uidhash_lock);
        }

        return up;

out_unlock:
        return NULL;
}

static int __init uid_cache_init(void)
{
        int n;

        uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct),
                        0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);

        for(n = 0; n < UIDHASH_SZ; ++n)
                INIT_HLIST_HEAD(init_user_ns.uidhash_table + n);

        /* Insert the root user immediately (init already runs as root) */
        spin_lock_irq(&uidhash_lock);
        uid_hash_insert(&root_user, uidhashentry(&init_user_ns, 0));
        spin_unlock_irq(&uidhash_lock);

        return 0;
}

module_init(uid_cache_init);