linux-kernel - [PATCH] [v2] ext4: fix ext4_tune_sb

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Message-Id: <20251205111906.1247452-1-arnd@kernel.org>
Date: Fri,  5 Dec 2025 12:19:00 +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] [v2] 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.

Add explicit padding to mount_opts[] to keep the struct members compatible
with the original version and also keep the pad[64] member 8-byte
aligned for future extensions.  This gives a consistent sizeof(struct
ext4_tune_sb_params) of 232 on all architectures and avoids 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.

Fixes: 04a91570ac67 ("ext4: implemet new ioctls to set and get superblock parameters")
Reviewed-by: Jan Kara <jack@...e.cz>
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
v2: extend mount_opts[] instead of pad[], as suggested by Andreas Dilger
---
 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 6829d6f1497d..1c7cdcdb7dca 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_compat_mask;
 	__u32 clear_feature_incompat_mask;
 	__u32 clear_feature_ro_compat_mask;
-	__u8  mount_opts[64];
+	__u8  mount_opts[68];
 	__u8  pad[64];
 };

-- 
2.39.5