On Wed, May 7, 2025 at 1:39 AM Gustavo A. R. Silva
<gustavoars@xxxxxxxxxx> wrote:
>
> -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 */
I'm not that familiar with pahole, but I find it surprising to see this member
aligned(1), when struct fanotify_fh is defined as __aligned(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@xxxxxxxxxx>
> ---
> 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)]; \
The name _inline_fh_buf is confusing in this setting
better use bytes[] as in DEFINE_FLEX() or maybe even consider
a generic helper DEFINE_FLEX_MEMBER() to use instead of
FANOTIFY_INLINE_FH(), because this is not fanotify specific,
except maybe for alignment (see below).
> + struct fanotify_fh name; \
> +} __packed
Why added __packed?
The fact that struct fanotify_fh is 4 bytes aligned could end up with less
bytes reserved for the inline buffer if the union is not also 4 bytes aligned.
So maybe something like this:
#define FANOTIFY_INLINE_FH(name, size) \
DEFINE_FLEX_MEMBER(struct fanotify_fh, name, size) __aligned(4)
Thanks,
Amir.
