[PATCH] [v2] landlock: work around gcc-16 -Wuninitialized warning

From: Arnd Bergmann <arnd@arndb.de>

gcc has a bug with -ftrivial-auto-var-init=pattern that produces a
warning for correct code that uses sparse bitfields:

security/landlock/fs.c: In function 'is_access_to_paths_allowed.isra':
security/landlock/fs.c:767:28: error: '_layer_masks_child1' is used uninitialized [-Werror=uninitialized]
  767 |         struct layer_masks _layer_masks_child1, _layer_masks_child2;
      |                            ^~~~~~~~~~~~~~~~~~~
security/landlock/fs.c:767:28: note: '_layer_masks_child1' declared here
  767 |         struct layer_masks _layer_masks_child1, _layer_masks_child2;
      |                            ^~~~~~~~~~~~~~~~~~~
security/landlock/fs.c: In function 'hook_unix_find':
security/landlock/fs.c:1649:28: error: 'layer_masks' is used uninitialized [-Werror=uninitialized]
 1649 |         struct layer_masks layer_masks;
      |                            ^~~~~~~~~~~
security/landlock/fs.c:1649:28: note: 'layer_masks' declared here
 1649 |         struct layer_masks layer_masks;
      |                            ^~~~~~~~~~~

To work around this, change the definition of struct layer_mask to
use an explictit padding field. This also avoids the extra attributes
for aligning the structure.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110743
Link: https://lore.kernel.org/all/20260619082133.3504146-1-arnd@kernel.org/ (local)
Fixes: a260c0055665 ("landlock: Add a place for flags to layer rules")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
v2: rebased to linux-next-20260914

I originally sent this in June, but got no reply and this still
happens with gcc-16.2, which is the latest stable release.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 security/landlock/access.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/security/landlock/access.h b/security/landlock/access.h
index bbbb41f41147..f41ac3fc6476 100644
--- a/security/landlock/access.h
+++ b/security/landlock/access.h

@@ -81,6 +81,9 @@ struct layer_mask {
 	 */
 	access_mask_t quiet : 1;
 #endif /* CONFIG_SECURITY_LANDLOCK_LOG */
+	access_mask_t __pad : ((sizeof(access_mask_t) * 8) -
+				LANDLOCK_NUM_ACCESS_MAX -
+				IS_ENABLED(CONFIG_SECURITY_LANDLOCK_LOG));
 } __packed __aligned(sizeof(access_mask_t));
 
 /*

-- 
2.53.0