The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/kernel/user.c

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    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 #include <linux/bitops.h>
   15 #include <linux/key.h>
   16 #include <linux/interrupt.h>
   17 #include <linux/export.h>
   18 #include <linux/user_namespace.h>
   19 #include <linux/proc_fs.h>
   20 
   21 /*
   22  * userns count is 1 for root user, 1 for init_uts_ns,
   23  * and 1 for... ?
   24  */
   25 struct user_namespace init_user_ns = {
   26         .uid_map = {
   27                 .nr_extents = 1,
   28                 .extent[0] = {
   29                         .first = 0,
   30                         .lower_first = 0,
   31                         .count = 4294967295U,
   32                 },
   33         },
   34         .gid_map = {
   35                 .nr_extents = 1,
   36                 .extent[0] = {
   37                         .first = 0,
   38                         .lower_first = 0,
   39                         .count = 4294967295U,
   40                 },
   41         },
   42         .projid_map = {
   43                 .nr_extents = 1,
   44                 .extent[0] = {
   45                         .first = 0,
   46                         .lower_first = 0,
   47                         .count = 4294967295U,
   48                 },
   49         },
   50         .kref = {
   51                 .refcount       = ATOMIC_INIT(3),
   52         },
   53         .owner = GLOBAL_ROOT_UID,
   54         .group = GLOBAL_ROOT_GID,
   55         .proc_inum = PROC_USER_INIT_INO,
   56 };
   57 EXPORT_SYMBOL_GPL(init_user_ns);
   58 
   59 /*
   60  * UID task count cache, to get fast user lookup in "alloc_uid"
   61  * when changing user ID's (ie setuid() and friends).
   62  */
   63 
   64 #define UIDHASH_BITS    (CONFIG_BASE_SMALL ? 3 : 7)
   65 #define UIDHASH_SZ      (1 << UIDHASH_BITS)
   66 #define UIDHASH_MASK            (UIDHASH_SZ - 1)
   67 #define __uidhashfn(uid)        (((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK)
   68 #define uidhashentry(uid)       (uidhash_table + __uidhashfn((__kuid_val(uid))))
   69 
   70 static struct kmem_cache *uid_cachep;
   71 struct hlist_head uidhash_table[UIDHASH_SZ];
   72 
   73 /*
   74  * The uidhash_lock is mostly taken from process context, but it is
   75  * occasionally also taken from softirq/tasklet context, when
   76  * task-structs get RCU-freed. Hence all locking must be softirq-safe.
   77  * But free_uid() is also called with local interrupts disabled, and running
   78  * local_bh_enable() with local interrupts disabled is an error - we'll run
   79  * softirq callbacks, and they can unconditionally enable interrupts, and
   80  * the caller of free_uid() didn't expect that..
   81  */
   82 static DEFINE_SPINLOCK(uidhash_lock);
   83 
   84 /* root_user.__count is 1, for init task cred */
   85 struct user_struct root_user = {
   86         .__count        = ATOMIC_INIT(1),
   87         .processes      = ATOMIC_INIT(1),
   88         .files          = ATOMIC_INIT(0),
   89         .sigpending     = ATOMIC_INIT(0),
   90         .locked_shm     = 0,
   91         .uid            = GLOBAL_ROOT_UID,
   92 };
   93 
   94 /*
   95  * These routines must be called with the uidhash spinlock held!
   96  */
   97 static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent)
   98 {
   99         hlist_add_head(&up->uidhash_node, hashent);
  100 }
  101 
  102 static void uid_hash_remove(struct user_struct *up)
  103 {
  104         hlist_del_init(&up->uidhash_node);
  105 }
  106 
  107 static struct user_struct *uid_hash_find(kuid_t uid, struct hlist_head *hashent)
  108 {
  109         struct user_struct *user;
  110         struct hlist_node *h;
  111 
  112         hlist_for_each_entry(user, h, hashent, uidhash_node) {
  113                 if (uid_eq(user->uid, uid)) {
  114                         atomic_inc(&user->__count);
  115                         return user;
  116                 }
  117         }
  118 
  119         return NULL;
  120 }
  121 
  122 /* IRQs are disabled and uidhash_lock is held upon function entry.
  123  * IRQ state (as stored in flags) is restored and uidhash_lock released
  124  * upon function exit.
  125  */
  126 static void free_user(struct user_struct *up, unsigned long flags)
  127         __releases(&uidhash_lock)
  128 {
  129         uid_hash_remove(up);
  130         spin_unlock_irqrestore(&uidhash_lock, flags);
  131         key_put(up->uid_keyring);
  132         key_put(up->session_keyring);
  133         kmem_cache_free(uid_cachep, up);
  134 }
  135 
  136 /*
  137  * Locate the user_struct for the passed UID.  If found, take a ref on it.  The
  138  * caller must undo that ref with free_uid().
  139  *
  140  * If the user_struct could not be found, return NULL.
  141  */
  142 struct user_struct *find_user(kuid_t uid)
  143 {
  144         struct user_struct *ret;
  145         unsigned long flags;
  146 
  147         spin_lock_irqsave(&uidhash_lock, flags);
  148         ret = uid_hash_find(uid, uidhashentry(uid));
  149         spin_unlock_irqrestore(&uidhash_lock, flags);
  150         return ret;
  151 }
  152 
  153 void free_uid(struct user_struct *up)
  154 {
  155         unsigned long flags;
  156 
  157         if (!up)
  158                 return;
  159 
  160         local_irq_save(flags);
  161         if (atomic_dec_and_lock(&up->__count, &uidhash_lock))
  162                 free_user(up, flags);
  163         else
  164                 local_irq_restore(flags);
  165 }
  166 
  167 struct user_struct *alloc_uid(kuid_t uid)
  168 {
  169         struct hlist_head *hashent = uidhashentry(uid);
  170         struct user_struct *up, *new;
  171 
  172         spin_lock_irq(&uidhash_lock);
  173         up = uid_hash_find(uid, hashent);
  174         spin_unlock_irq(&uidhash_lock);
  175 
  176         if (!up) {
  177                 new = kmem_cache_zalloc(uid_cachep, GFP_KERNEL);
  178                 if (!new)
  179                         goto out_unlock;
  180 
  181                 new->uid = uid;
  182                 atomic_set(&new->__count, 1);
  183 
  184                 /*
  185                  * Before adding this, check whether we raced
  186                  * on adding the same user already..
  187                  */
  188                 spin_lock_irq(&uidhash_lock);
  189                 up = uid_hash_find(uid, hashent);
  190                 if (up) {
  191                         key_put(new->uid_keyring);
  192                         key_put(new->session_keyring);
  193                         kmem_cache_free(uid_cachep, new);
  194                 } else {
  195                         uid_hash_insert(new, hashent);
  196                         up = new;
  197                 }
  198                 spin_unlock_irq(&uidhash_lock);
  199         }
  200 
  201         return up;
  202 
  203 out_unlock:
  204         return NULL;
  205 }
  206 
  207 static int __init uid_cache_init(void)
  208 {
  209         int n;
  210 
  211         uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct),
  212                         0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
  213 
  214         for(n = 0; n < UIDHASH_SZ; ++n)
  215                 INIT_HLIST_HEAD(uidhash_table + n);
  216 
  217         /* Insert the root user immediately (init already runs as root) */
  218         spin_lock_irq(&uidhash_lock);
  219         uid_hash_insert(&root_user, uidhashentry(GLOBAL_ROOT_UID));
  220         spin_unlock_irq(&uidhash_lock);
  221 
  222         return 0;
  223 }
  224 
  225 module_init(uid_cache_init);

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