linux-kernel - [PATCH] ext4: fix ext4_tune_sb

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Message-Id: <20251204101914.1037148-1-arnd@kernel.org>
Date: Thu,  4 Dec 2025 11:19:10 +0100
From: Arnd Bergmann <arnd@...nel.org>
To: "Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Jan Kara <jack@...e.cz>,
	"Darrick J. Wong" <djwong@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>,
	linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] ext4: fix ext4_tune_sb_params padding

From: Arnd Bergmann <arnd@...db.de>

The padding at the end of struct ext4_tune_sb_params is architecture
specific and in particular is different between x86-32 and x86-64,
since the __u64 member only enforces struct alignment on the latter.

This shows up as a new warning when test-building the headers with
-Wpadded:

include/linux/ext4.h:144:1: error: padding struct size to alignment boundary with 4 bytes [-Werror=padded]

All members inside the structure are naturally aligned, so the only
difference here is the amount of padding at the end. Make the padding
explicit, to have a consistent sizeof(struct ext4_tune_sb_params) of
232 on all architectures and avoid adding compat ioctl handling for
EXT4_IOC_GET_TUNE_SB_PARAM/EXT4_IOC_SET_TUNE_SB_PARAM.

This is an ABI break on x86-32 but hopefully this can go into 6.18.y early
enough as a fixup so no actual users will be affected.  Alternatively, the
kernel could handle the ioctl commands for both sizes (232 and 228 bytes)
on all architectures.

Fixes: 04a91570ac67 ("ext4: implemet new ioctls to set and get superblock parameters")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 include/uapi/linux/ext4.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/uapi/linux/ext4.h b/include/uapi/linux/ext4.h
index 411dcc1e4a35..9c683991c32f 100644
--- a/include/uapi/linux/ext4.h
+++ b/include/uapi/linux/ext4.h
@@ -139,7 +139,7 @@ struct ext4_tune_sb_params {
 	__u32 clear_feature_incompat_mask;
 	__u32 clear_feature_ro_compat_mask;
 	__u8  mount_opts[64];
-	__u8  pad[64];
+	__u8  pad[68];
 };

 #define EXT4_TUNE_FL_ERRORS_BEHAVIOR	0x00000001
-- 
2.39.5