linux-kernel - [PATCH][next] fanotify: Avoid a couple of -Wflex-array-member-not-at-end warnings

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Message-ID: <aBqdlxlBtb9s7ydc@kspp>
Date: Tue, 6 May 2025 17:39:03 -0600
From: "Gustavo A. R. Silva" <gustavoars@...nel.org>
To: Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
	Matthew Bobrowski <repnop@...gle.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	linux-hardening@...r.kernel.org
Subject: [PATCH][next] fanotify: Avoid a couple of
 -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Modify FANOTIFY_INLINE_FH() macro, which defines a struct containing a
flexible-array member in the middle (struct fanotify_fh::buf), to use
struct_size_t() to pre-allocate space for both struct fanotify_fh and
its flexible-array member. Replace the struct with a union and relocate
the flexible structure (struct fanotify_fh) to the end.

See the memory layout of struct fanotify_fid_event before and after
changes below.

pahole -C fanotify_fid_event fs/notify/fanotify/fanotify.o

BEFORE:
struct fanotify_fid_event {
        struct fanotify_event      fae;                  /*     0    48 */
        __kernel_fsid_t            fsid;                 /*    48     8 */
        struct {
                struct fanotify_fh object_fh;            /*    56     4 */
                unsigned char      _inline_fh_buf[12];   /*    60    12 */
        };                                               /*    56    16 */

        /* size: 72, cachelines: 2, members: 3 */
        /* last cacheline: 8 bytes */
};

AFTER:
struct fanotify_fid_event {
        struct fanotify_event      fae;                  /*     0    48 */
        __kernel_fsid_t            fsid;                 /*    48     8 */
        union {
                unsigned char      _inline_fh_buf[16];   /*    56    16 */
                struct fanotify_fh object_fh __attribute__((__aligned__(1))); /*    56     4 */
        } __attribute__((__aligned__(1)));               /*    56    16 */

        /* size: 72, cachelines: 2, members: 3 */
        /* forced alignments: 1 */
        /* last cacheline: 8 bytes */
} __attribute__((__aligned__(8)));

So, with these changes, fix the following warnings:

fs/notify/fanotify/fanotify.h:317:28: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
fs/notify/fanotify/fanotify.h:289:28: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
---
 fs/notify/fanotify/fanotify.h | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/notify/fanotify/fanotify.h b/fs/notify/fanotify/fanotify.h
index b44e70e44be6..91c26b1c1d32 100644
--- a/fs/notify/fanotify/fanotify.h
+++ b/fs/notify/fanotify/fanotify.h
@@ -275,12 +275,12 @@ static inline void fanotify_init_event(struct fanotify_event *event,
 	event->pid = NULL;
 }
 
-#define FANOTIFY_INLINE_FH(name, size)					\
-struct {								\
-	struct fanotify_fh name;					\
-	/* Space for object_fh.buf[] - access with fanotify_fh_buf() */	\
-	unsigned char _inline_fh_buf[size];				\
-}
+#define FANOTIFY_INLINE_FH(name, size)						      \
+union {										      \
+	/* Space for object_fh and object_fh.buf[] - access with fanotify_fh_buf() */ \
+	unsigned char _inline_fh_buf[struct_size_t(struct fanotify_fh, buf, size)];   \
+	struct fanotify_fh name;						      \
+} __packed
 
 struct fanotify_fid_event {
 	struct fanotify_event fae;
-- 
2.43.0