Cache user_ns in struct cred

If !CONFIG_USERNS, have current_user_ns() defined to (&init_user_ns).

Get rid of _current_user_ns.  This requires nsown_capable() to be
defined in capability.c rather than as static inline in capability.h,
so do that.

Request_key needs init_user_ns defined at current_user_ns if
!CONFIG_USERNS, so forward-declare that in cred.h if !CONFIG_USERNS
at current_user_ns() define.

Compile-tested with and without CONFIG_USERNS.

Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
[ This makes a huge performance difference for acl_permission_check(),
  up to 30%.  And that is one of the hottest kernel functions for loads
  that are pathname-lookup heavy.  ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 18 insertions, 6 deletions

diff --git a/kernel/capability.c b/kernel/capability.c
index bf0c734d0c1..32a80e08ff4 100644
--- a/kernel/capability.c
+++ b/kernel/capability.c
@@ -399,3 +399,15 @@ bool task_ns_capable(struct task_struct *t, int cap)
         return ns_capable(task_cred_xxx(t, user)->user_ns, cap);
 }
 EXPORT_SYMBOL(task_ns_capable);
+
+/**
+ * nsown_capable - Check superior capability to one's own user_ns
+ * @cap: The capability in question
+ *
+ * Return true if the current task has the given superior capability
+ * targeted at its own user namespace.
+ */
+bool nsown_capable(int cap)
+{
+        return ns_capable(current_user_ns(), cap);
+}
diff --git a/kernel/cred.c b/kernel/cred.c
index 5557b55048d..8093c16b84b 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -54,6 +54,7 @@ struct cred init_cred = {
         .cap_effective          = CAP_INIT_EFF_SET,
         .cap_bset               = CAP_INIT_BSET,
         .user                   = INIT_USER,
+        .user_ns                = &init_user_ns,
         .group_info             = &init_groups,
 #ifdef CONFIG_KEYS
         .tgcred                 = &init_tgcred,
@@ -410,6 +411,11 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
                         goto error_put;
         }
 
+        /* cache user_ns in cred.  Doesn't need a refcount because it will
+         * stay pinned by cred->user
+         */
+        new->user_ns = new->user->user_ns;
+
 #ifdef CONFIG_KEYS
         /* new threads get their own thread keyrings if their parent already
          * had one */
@@ -741,12 +747,6 @@ int set_create_files_as(struct cred *new, struct inode *inode)
 }
 EXPORT_SYMBOL(set_create_files_as);
 
-struct user_namespace *current_user_ns(void)
-{
-        return _current_user_ns();
-}
-EXPORT_SYMBOL(current_user_ns);
-
 #ifdef CONFIG_DEBUG_CREDENTIALS
 
 bool creds_are_invalid(const struct cred *cred)