On Wed 07-05-25 07:56:21, Amir Goldstein wrote:
> On Wed, May 7, 2025 at 1:39 AM Gustavo A. R. Silva
> <gustavo...@kernel.org> 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).
Yeah.
> > } __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 <gustavo...@kernel.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)]; \
>
> 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).
Yes, I guess a generic helper for this would be nice but if fanotify is the
only place that plays these tricks, we can keep it specific for now. I
agree naming the "space-buffer" field "bytes" would be less confusing.
>
> > + 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)
I guess you need to provide the "member" information to
DEFINE_FLEX_MEMBER() somewhere as well.
Honza
--
Jan Kara <j...@suse.com>
SUSE Labs, CR
