kernel/user.c

   1 /*
   2  * The "user cache".
   3  *
   4  * (C) Copyright 1991-2000 Linus Torvalds
   5  *
   6  * We have a per-user structure to keep track of how many
   7  * processes, files etc the user has claimed, in order to be
   8  * able to have per-user limits for system resources.
   9  */
  10
  11 #include <linux/init.h>
  12 #include <linux/sched.h>
  13 #include <linux/slab.h>
  14
  15 /*
  16  * UID task count cache, to get fast user lookup in "alloc_uid"
  17  * when changing user ID's (ie setuid() and friends).
  18  */
  19 #define UIDHASH_BITS            8
  20 #define UIDHASH_SZ              (1 << UIDHASH_BITS)
  21 #define UIDHASH_MASK            (UIDHASH_SZ - 1)
  22 #define __uidhashfn(uid)        (((uid >> UIDHASH_BITS) ^ uid) & UIDHASH_MASK)
  23 #define uidhashentry(uid)       (uidhash_table + __uidhashfn(uid))
  24
  25 static kmem_cache_t *uid_cachep;
  26 static struct user_struct *uidhash_table[UIDHASH_SZ];
  27 static spinlock_t uidhash_lock = SPIN_LOCK_UNLOCKED;
  28
  29 struct user_struct root_user = {
  30         __count:        ATOMIC_INIT(1),
  31         processes:      ATOMIC_INIT(1),
  32         files:          ATOMIC_INIT(0)
  33 };
  34
  35 /*
  36  * These routines must be called with the uidhash spinlock held!
  37  */
  38 static inline void uid_hash_insert(struct user_struct *up, struct user_struct **hashent)
  39 {
  40         struct user_struct *next = *hashent;
  41
  42         up->next = next;
  43         if (next)
  44                 next->pprev = &up->next;
  45         up->pprev = hashent;
  46         *hashent = up;
  47 }
  48
  49 static inline void uid_hash_remove(struct user_struct *up)
  50 {
  51         struct user_struct *next = up->next;
  52         struct user_struct **pprev = up->pprev;
  53
  54         if (next)
  55                 next->pprev = pprev;
  56         *pprev = next;
  57 }
  58
  59 static inline struct user_struct *uid_hash_find(uid_t uid, struct user_struct **hashent)
  60 {
  61         struct user_struct *next;
  62
  63         next = *hashent;
  64         for (;;) {
  65                 struct user_struct *up = next;
  66                 if (next) {
  67                         next = up->next;
  68                         if (up->uid != uid)
  69                                 continue;
  70                         atomic_inc(&up->__count);
  71                 }
  72                 return up;
  73         }
  74 }
  75
  76 void free_uid(struct user_struct *up)
  77 {
  78         if (up && atomic_dec_and_lock(&up->__count, &uidhash_lock)) {
  79                 uid_hash_remove(up);
  80                 kmem_cache_free(uid_cachep, up);
  81                 spin_unlock(&uidhash_lock);
  82         }
  83 }
  84
  85 struct user_struct * alloc_uid(uid_t uid)
  86 {
  87         struct user_struct **hashent = uidhashentry(uid);
  88         struct user_struct *up;
  89
  90         spin_lock(&uidhash_lock);
  91         up = uid_hash_find(uid, hashent);
  92         spin_unlock(&uidhash_lock);
  93
  94         if (!up) {
  95                 struct user_struct *new;
  96
  97                 new = kmem_cache_alloc(uid_cachep, SLAB_KERNEL);
  98                 if (!new)
  99                         return NULL;
 100                 new->uid = uid;
 101                 atomic_set(&new->__count, 1);
 102                 atomic_set(&new->processes, 0);
 103                 atomic_set(&new->files, 0);
 104
 105                 /*
 106                  * Before adding this, check whether we raced
 107                  * on adding the same user already..
 108                  */
 109                 spin_lock(&uidhash_lock);
 110                 up = uid_hash_find(uid, hashent);
 111                 if (up) {
 112                         kmem_cache_free(uid_cachep, new);
 113                 } else {
 114                         uid_hash_insert(new, hashent);
 115                         up = new;
 116                 }
 117                 spin_unlock(&uidhash_lock);
 118
 119         }
 120         return up;
 121 }
 122
 123
 124 static int __init uid_cache_init(void)
 125 {
 126         uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct),
 127                                        0,
 128                                        SLAB_HWCACHE_ALIGN, NULL, NULL);
 129         if(!uid_cachep)
 130                 panic("Cannot create uid taskcount SLAB cache\n");
 131
 132         /* Insert the root user immediately - init already runs with this */
 133         uid_hash_insert(&root_user, uidhashentry(0));
 134         return 0;
 135 }
 136
 137 module_init(uid_cache_init);