[PATCH 7.2.y 5/5] landlock: Work around gcc-16 -Wuninitialized warning

[Sasha Levin <sashal@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

[ Upstream commit c4e941bb7654bcbdfb0b6f3341dc2acfdf235c8d ]

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.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110743
Link: https://lore.kernel.org/all/20260619082133.3504146-1-arnd@kernel.org/
Fixes: a260c0055665 ("landlock: Add a place for flags to layer rules")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20260915201036.3527935-1-arnd@kernel.org
[mic: Use BITS_PER_TYPE(), fix kdoc warnings, fix commit message
according to v2 changes]
Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 security/landlock/access.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/security/landlock/access.h b/security/landlock/access.h
index bbbb41f41147c..f843835851d01 100644
--- a/security/landlock/access.h
+++ b/security/landlock/access.h
@@ -61,6 +61,10 @@ union access_masks_all {
 static_assert(sizeof(typeof_member(union access_masks_all, masks)) ==
 	      sizeof(typeof_member(union access_masks_all, all)));
 
+#define _LANDLOCK_LAYER_MASK_PADDING                              \
+	(BITS_PER_TYPE(access_mask_t) - LANDLOCK_NUM_ACCESS_MAX - \
+	 IS_ENABLED(CONFIG_SECURITY_LANDLOCK_LOG))
+
 /**
  * struct layer_mask - The access rights and rule flags for a layer.
  *
@@ -81,6 +85,10 @@ struct layer_mask {
 	 */
 	access_mask_t quiet : 1;
 #endif /* CONFIG_SECURITY_LANDLOCK_LOG */
+	/**
+	 * @__pad: Padding for the compiler's bitfield initialization.
+	 */
+	access_mask_t __pad : _LANDLOCK_LAYER_MASK_PADDING;
 } __packed __aligned(sizeof(access_mask_t));
 
 /*
-- 
2.53.0