fs/notify/fanotify/fanotify.h | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-)
-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@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)]; \
+ struct fanotify_fh name; \
+} __packed
struct fanotify_fid_event {
struct fanotify_event fae;
--
2.43.0Hi Gustavo,
kernel test robot noticed the following build warnings:
[auto build test WARNING on jack-fs/fsnotify]
[also build test WARNING on linus/master v6.15-rc5 next-20250507]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]
url: https://github.com/intel-lab-lkp/linux/commits/Gustavo-A-R-Silva/fanotify-Avoid-a-couple-of-Wflex-array-member-not-at-end-warnings/20250507-074110
base: https://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs.git fsnotify
patch link: https://lore.kernel.org/r/aBqdlxlBtb9s7ydc%40kspp
patch subject: [PATCH][next] fanotify: Avoid a couple of -Wflex-array-member-not-at-end warnings
config: x86_64-defconfig (https://download.01.org/0day-ci/archive/20250508/202505081249.CUDWsu7Z-lkp@intel.com/config)
compiler: gcc-11 (Debian 11.3.0-12) 11.3.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250508/202505081249.CUDWsu7Z-lkp@intel.com/reproduce)
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202505081249.CUDWsu7Z-lkp@intel.com/
All warnings (new ones prefixed by >>):
In file included from fs/notify/fdinfo.c:17:
>> fs/notify/fanotify/fanotify.h:280:16: warning: alignment 1 of 'union <anonymous>' is less than 4 [-Wpacked-not-aligned]
280 | struct fanotify_fh name; \
| ^~~~~~~~~~~
fs/notify/fanotify/fanotify.h:287:9: note: in expansion of macro 'FANOTIFY_INLINE_FH'
287 | FANOTIFY_INLINE_FH(object_fh, FANOTIFY_INLINE_FH_LEN);
| ^~~~~~~~~~~~~~~~~~
>> fs/notify/fanotify/fanotify.h:280:16: warning: alignment 1 of 'union <anonymous>' is less than 4 [-Wpacked-not-aligned]
280 | struct fanotify_fh name; \
| ^~~~~~~~~~~
fs/notify/fanotify/fanotify.h:315:9: note: in expansion of macro 'FANOTIFY_INLINE_FH'
315 | FANOTIFY_INLINE_FH(object_fh, MAX_HANDLE_SZ);
| ^~~~~~~~~~~~~~~~~~
vim +280 fs/notify/fanotify/fanotify.h
b8a6c3a2f0ae4d Amir Goldstein 2020-07-08 275
2c5069433a3adc Gabriel Krisman Bertazi 2021-10-25 276 #define FANOTIFY_INLINE_FH(name, size) \
e3725b8a2ecdf6 Gustavo A. R. Silva 2025-05-06 277 union { \
e3725b8a2ecdf6 Gustavo A. R. Silva 2025-05-06 278 /* Space for object_fh and object_fh.buf[] - access with fanotify_fh_buf() */ \
e3725b8a2ecdf6 Gustavo A. R. Silva 2025-05-06 279 unsigned char _inline_fh_buf[struct_size_t(struct fanotify_fh, buf, size)]; \
1758cd2e95d31b Alexey Dobriyan 2023-10-10 @280 struct fanotify_fh name; \
e3725b8a2ecdf6 Gustavo A. R. Silva 2025-05-06 281 } __packed
2c5069433a3adc Gabriel Krisman Bertazi 2021-10-25 282
--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
On Wed, May 7, 2025 at 1:39 AM Gustavo A. R. Silva
<gustavoars@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).
> } __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@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).
> + 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.
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@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 <gustavoars@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 <jack@suse.com>
SUSE Labs, CR
On 07/05/25 05:08, Jan Kara wrote:
> 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@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.
Yep, gotcha.
>
>>> } __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@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.
I can send v2 with this change.
>
>>
>>> + 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.
Yeah, I can write something like that as well.
I'll come back with v2, shortly.
Thanks for the feedback, folks! :)
--
Gustavo
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