[RFC v2 2/4] cred: Add a light version of override/revert_creds()

[Vinicius Costa Gomes <vinicius.gomes@intel.com>]

Add a light version of override/revert_creds(), this should only be
used when the credentials in question will outlive the critical
section and the critical section doesn't change the ->usage of the
credentials.

To make their usage less error prone, introduce cleanup guards asto be
used like this:

     guard(cred)(credentials_to_override_and_restore);

or this:

     scoped_guard(cred, credentials_to_override_and_restore) {
             /* with credentials overridden */
     }

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
---
 include/linux/cred.h | 21 +++++++++++++++++++++
 kernel/cred.c        |  6 +++---
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 2976f534a7a3..e9f2237e4bf8 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -172,6 +172,27 @@ static inline bool cap_ambient_invariant_ok(const struct cred *cred)
 					  cred->cap_inheritable));
 }
 
+/*
+ * Override creds without bumping reference count. Caller must ensure
+ * reference remains valid or has taken reference. Almost always not the
+ * interface you want. Use override_creds()/revert_creds() instead.
+ */
+static inline const struct cred *override_creds_light(const struct cred *override_cred)
+{
+	const struct cred *old = current->cred;
+
+	rcu_assign_pointer(current->cred, override_cred);
+	return old;
+}
+
+static inline void revert_creds_light(const struct cred *revert_cred)
+{
+	rcu_assign_pointer(current->cred, revert_cred);
+}
+
+DEFINE_GUARD(cred, const struct cred *, _T = override_creds_light(_T),
+	     revert_creds_light(_T));
+
 /**
  * get_new_cred_many - Get references on a new set of credentials
  * @cred: The new credentials to reference
diff --git a/kernel/cred.c b/kernel/cred.c
index c033a201c808..f95f71e3ac1d 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -485,7 +485,7 @@ EXPORT_SYMBOL(abort_creds);
  */
 const struct cred *override_creds(const struct cred *new)
 {
-	const struct cred *old = current->cred;
+	const struct cred *old;
 
 	kdebug("override_creds(%p{%ld})", new,
 	       atomic_long_read(&new->usage));
@@ -499,7 +499,7 @@ const struct cred *override_creds(const struct cred *new)
 	 * visible to other threads under RCU.
 	 */
 	get_new_cred((struct cred *)new);
-	rcu_assign_pointer(current->cred, new);
+	old = override_creds_light(new);
 
 	kdebug("override_creds() = %p{%ld}", old,
 	       atomic_long_read(&old->usage));
@@ -521,7 +521,7 @@ void revert_creds(const struct cred *old)
 	kdebug("revert_creds(%p{%ld})", old,
 	       atomic_long_read(&old->usage));
 
-	rcu_assign_pointer(current->cred, old);
+	revert_creds_light(old);
 	put_cred(override);
 }
 EXPORT_SYMBOL(revert_creds);
-- 
2.43.0