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.

Suggested-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
---
 include/linux/cred.h | 18 ++++++++++++++++++
 kernel/cred.c        |  6 +++---
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 2976f534a7a3..e4a3155fe409 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -172,6 +172,24 @@ 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);
+}
+
 /**
  * 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 075cfa7c896f..da7da250f7c8 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);
-- 
2.47.0

Hi Vinicius,

kernel test robot noticed the following build warnings:

[auto build test WARNING on next-20241105]
[also build test WARNING on v6.12-rc6]
[cannot apply to brauner-vfs/vfs.all linus/master v6.12-rc6 v6.12-rc5 v6.12-rc4]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Vinicius-Costa-Gomes/cred-Add-a-light-version-of-override-revert_creds/20241106-033748
base:   next-20241105
patch link:    https://lore.kernel.org/r/20241105193514.828616-2-vinicius.gomes%40intel.com
patch subject: [PATCH overlayfs-next v3 1/4] cred: Add a light version of override/revert_creds()
config: x86_64-randconfig-121-20241106 (https://download.01.org/0day-ci/archive/20241107/202411070234.EOrhSGRU-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20241107/202411070234.EOrhSGRU-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202411070234.EOrhSGRU-lkp@intel.com/

sparse warnings: (new ones prefixed by >>)
   kernel/cred.c:104:9: sparse: sparse: incompatible types in comparison expression (different address spaces):
   kernel/cred.c:104:9: sparse:    struct cred *
   kernel/cred.c:104:9: sparse:    struct cred const [noderef] __rcu *
   kernel/cred.c:105:9: sparse: sparse: incompatible types in comparison expression (different address spaces):
   kernel/cred.c:105:9: sparse:    struct cred *
   kernel/cred.c:105:9: sparse:    struct cred const [noderef] __rcu *
   kernel/cred.c:121:9: sparse: sparse: incorrect type in argument 1 (different address spaces) @@     expected struct atomic64_t const [usertype] *v @@     got struct atomic64_t const [noderef] __rcu * @@
   kernel/cred.c:121:9: sparse:     expected struct atomic64_t const [usertype] *v
   kernel/cred.c:121:9: sparse:     got struct atomic64_t const [noderef] __rcu *
   kernel/cred.c:124:22: sparse: sparse: cast removes address space '__rcu' of expression
   kernel/cred.c:127:17: sparse: sparse: cast removes address space '__rcu' of expression
   kernel/cred.c:218:13: sparse: sparse: incorrect type in assignment (different address spaces) @@     expected struct cred const *old @@     got struct cred const [noderef] __rcu *cred @@
   kernel/cred.c:218:13: sparse:     expected struct cred const *old
   kernel/cred.c:218:13: sparse:     got struct cred const [noderef] __rcu *cred
   kernel/cred.c:305:47: sparse: sparse: incorrect type in argument 1 (different address spaces) @@     expected struct cred const *cred @@     got struct cred const [noderef] __rcu *cred @@
   kernel/cred.c:305:47: sparse:     expected struct cred const *cred
   kernel/cred.c:305:47: sparse:     got struct cred const [noderef] __rcu *cred
   kernel/cred.c:305:30: sparse: sparse: incorrect type in assignment (different address spaces) @@     expected struct cred const [noderef] __rcu *real_cred @@     got struct cred const * @@
   kernel/cred.c:305:30: sparse:     expected struct cred const [noderef] __rcu *real_cred
   kernel/cred.c:305:30: sparse:     got struct cred const *
   kernel/cred.c:306:17: sparse: sparse: incorrect type in argument 1 (different address spaces) @@     expected struct atomic64_t const [usertype] *v @@     got struct atomic64_t const [noderef] __rcu * @@
   kernel/cred.c:306:17: sparse:     expected struct atomic64_t const [usertype] *v
   kernel/cred.c:306:17: sparse:     got struct atomic64_t const [noderef] __rcu *
   kernel/cred.c:344:32: sparse: sparse: incorrect type in assignment (different address spaces) @@     expected struct cred const [noderef] __rcu *real_cred @@     got struct cred const * @@
   kernel/cred.c:344:32: sparse:     expected struct cred const [noderef] __rcu *real_cred
   kernel/cred.c:344:32: sparse:     got struct cred const *
   kernel/cred.c:395:38: sparse: sparse: incorrect type in initializer (different address spaces) @@     expected struct cred const *old @@     got struct cred const [noderef] __rcu *real_cred @@
   kernel/cred.c:395:38: sparse:     expected struct cred const *old
   kernel/cred.c:395:38: sparse:     got struct cred const [noderef] __rcu *real_cred
   kernel/cred.c:400:9: sparse: sparse: incompatible types in comparison expression (different address spaces):
   kernel/cred.c:400:9: sparse:    struct cred const [noderef] __rcu *
   kernel/cred.c:400:9: sparse:    struct cred const *
   kernel/cred.c:519:46: sparse: sparse: incorrect type in initializer (different address spaces) @@     expected struct cred const *override @@     got struct cred const [noderef] __rcu *cred @@
   kernel/cred.c:519:46: sparse:     expected struct cred const *override
   kernel/cred.c:519:46: sparse:     got struct cred const [noderef] __rcu *cred
   kernel/cred.c:301:19: sparse: sparse: dereference of noderef expression
   kernel/cred.c: note: in included file:
>> include/linux/cred.h:182:41: sparse: sparse: incorrect type in initializer (different address spaces) @@     expected struct cred const *old @@     got struct cred const [noderef] __rcu *cred @@
   include/linux/cred.h:182:41: sparse:     expected struct cred const *old
   include/linux/cred.h:182:41: sparse:     got struct cred const [noderef] __rcu *cred

vim +182 include/linux/cred.h

   174	
   175	/*
   176	 * Override creds without bumping reference count. Caller must ensure
   177	 * reference remains valid or has taken reference. Almost always not the
   178	 * interface you want. Use override_creds()/revert_creds() instead.
   179	 */
   180	static inline const struct cred *override_creds_light(const struct cred *override_cred)
   181	{
 > 182		const struct cred *old = current->cred;
   183	
   184		rcu_assign_pointer(current->cred, override_cred);
   185		return old;
   186	}
   187	

-- 
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https://github.com/intel/lkp-tests/wiki