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

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Message-ID: <42nltwupsu4567oc5hioa4djga5yoqqoq3h7j3dj6vjr6hv4kt@54wdcs2wwefj>
Date: Wed, 7 May 2025 13:08:23 +0200
From: Jan Kara <jack@...e.cz>
To: Amir Goldstein <amir73il@...il.com>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>, Jan Kara <jack@...e.cz>, 
	Matthew Bobrowski <repnop@...gle.com>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] fanotify: Avoid a couple of
 -Wflex-array-member-not-at-end warnings

On Wed 07-05-25 07:56:21, Amir Goldstein wrote:
> On Wed, May 7, 2025 at 1:39 AM Gustavo A. R. Silva
> <gustavoars@...nel.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 <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)];   \
> 
> 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 <jack@...e.com>
SUSE Labs, CR