* [PATCH][next] bcache: Avoid -Wflex-array-member-not-at-end warning @ 2025-11-10 10:58 Gustavo A. R. Silva 2025-11-11 13:17 ` Coly Li 0 siblings, 1 reply; 6+ messages in thread From: Gustavo A. R. Silva @ 2025-11-10 10:58 UTC (permalink / raw) To: Coly Li, Andrew Morton Cc: linux-bcache, linux-kernel, Gustavo A. R. Silva, linux-hardening -Wflex-array-member-not-at-end was introduced in GCC-14, and we are getting ready to enable it, globally. Use the new TRAILING_OVERLAP() helper to fix the following warning: drivers/md/bcache/bset.h:330:27: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] This helper creates a union between a flexible-array member (FAM) and a set of MEMBERS that would otherwise follow it. This overlays the trailing MEMBER struct btree_iter_set stack_data[MAX_BSETS]; onto the FAM struct btree_iter::data[], while keeping the FAM and the start of MEMBER aligned. The static_assert() ensures this alignment remains, and it's intentionally placed inmediately after the corresponding structures --no blank line in between. Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org> --- drivers/md/bcache/bset.h | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/drivers/md/bcache/bset.h b/drivers/md/bcache/bset.h index 011f6062c4c0..6ee2c6a506a2 100644 --- a/drivers/md/bcache/bset.h +++ b/drivers/md/bcache/bset.h @@ -327,9 +327,13 @@ struct btree_iter { /* Fixed-size btree_iter that can be allocated on the stack */ struct btree_iter_stack { - struct btree_iter iter; - struct btree_iter_set stack_data[MAX_BSETS]; + /* Must be last as it ends in a flexible-array member. */ + TRAILING_OVERLAP(struct btree_iter, iter, data, + struct btree_iter_set stack_data[MAX_BSETS]; + ); }; +static_assert(offsetof(struct btree_iter_stack, iter.data) == + offsetof(struct btree_iter_stack, stack_data)); typedef bool (*ptr_filter_fn)(struct btree_keys *b, const struct bkey *k); -- 2.43.0 ^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH][next] bcache: Avoid -Wflex-array-member-not-at-end warning 2025-11-10 10:58 [PATCH][next] bcache: Avoid -Wflex-array-member-not-at-end warning Gustavo A. R. Silva @ 2025-11-11 13:17 ` Coly Li 2025-11-11 13:28 ` Gustavo A. R. Silva 0 siblings, 1 reply; 6+ messages in thread From: Coly Li @ 2025-11-11 13:17 UTC (permalink / raw) To: Gustavo A. R. Silva, Andrew Morton Cc: Andrew Morton, linux-bcache, linux-kernel, linux-hardening On Mon, Nov 10, 2025 at 07:58:58PM +0800, Gustavo A. R. Silva wrote: > -Wflex-array-member-not-at-end was introduced in GCC-14, and we are > getting ready to enable it, globally. > > Use the new TRAILING_OVERLAP() helper to fix the following warning: > > drivers/md/bcache/bset.h:330:27: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] > > This helper creates a union between a flexible-array member (FAM) and a > set of MEMBERS that would otherwise follow it. > > This overlays the trailing MEMBER struct btree_iter_set stack_data[MAX_BSETS]; > onto the FAM struct btree_iter::data[], while keeping the FAM and the start > of MEMBER aligned. > > The static_assert() ensures this alignment remains, and it's > intentionally placed inmediately after the corresponding structures --no > blank line in between. > > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org> > --- > drivers/md/bcache/bset.h | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > > diff --git a/drivers/md/bcache/bset.h b/drivers/md/bcache/bset.h > index 011f6062c4c0..6ee2c6a506a2 100644 > --- a/drivers/md/bcache/bset.h > +++ b/drivers/md/bcache/bset.h > @@ -327,9 +327,13 @@ struct btree_iter { > /* Fixed-size btree_iter that can be allocated on the stack */ > > struct btree_iter_stack { > - struct btree_iter iter; > - struct btree_iter_set stack_data[MAX_BSETS]; > + /* Must be last as it ends in a flexible-array member. */ > + TRAILING_OVERLAP(struct btree_iter, iter, data, > + struct btree_iter_set stack_data[MAX_BSETS]; > + ); > }; > +static_assert(offsetof(struct btree_iter_stack, iter.data) == > + offsetof(struct btree_iter_stack, stack_data)); > I have to say this is ugly. Not the patch, but the gcc 14 warning option of such coding style. Look at TRAILING_OVERLAP() usage here, this is not C, this is something to fix a gcc bug which cannot handle FAM properly. Gustavo, this complain is not to you, just I feel a bit sad how GCC makes the code comes to such an ugly way, and it makes things much complicated. For anyone doesn't have deep understanding of TRAILING_OVERLAP(), I highly suspect whether he or she can understand what happens here. Andrew and Gustavo, is this a mandatary to fix FAM in such way? If yes I take the patch and keep my own opinion. If not, I'd like to see gcc fixes its bug, for the this code I don't see the author does things wrong. Coly Li ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH][next] bcache: Avoid -Wflex-array-member-not-at-end warning 2025-11-11 13:17 ` Coly Li @ 2025-11-11 13:28 ` Gustavo A. R. Silva 2025-11-11 13:43 ` Coly Li 0 siblings, 1 reply; 6+ messages in thread From: Gustavo A. R. Silva @ 2025-11-11 13:28 UTC (permalink / raw) To: Coly Li, Gustavo A. R. Silva, Andrew Morton Cc: linux-bcache, linux-kernel, linux-hardening On 11/11/25 22:17, Coly Li wrote: > On Mon, Nov 10, 2025 at 07:58:58PM +0800, Gustavo A. R. Silva wrote: >> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are >> getting ready to enable it, globally. >> >> Use the new TRAILING_OVERLAP() helper to fix the following warning: >> >> drivers/md/bcache/bset.h:330:27: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] >> >> This helper creates a union between a flexible-array member (FAM) and a >> set of MEMBERS that would otherwise follow it. >> >> This overlays the trailing MEMBER struct btree_iter_set stack_data[MAX_BSETS]; >> onto the FAM struct btree_iter::data[], while keeping the FAM and the start >> of MEMBER aligned. >> >> The static_assert() ensures this alignment remains, and it's >> intentionally placed inmediately after the corresponding structures --no >> blank line in between. >> >> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org> >> --- >> drivers/md/bcache/bset.h | 8 ++++++-- >> 1 file changed, 6 insertions(+), 2 deletions(-) >> >> diff --git a/drivers/md/bcache/bset.h b/drivers/md/bcache/bset.h >> index 011f6062c4c0..6ee2c6a506a2 100644 >> --- a/drivers/md/bcache/bset.h >> +++ b/drivers/md/bcache/bset.h >> @@ -327,9 +327,13 @@ struct btree_iter { >> /* Fixed-size btree_iter that can be allocated on the stack */ >> >> struct btree_iter_stack { >> - struct btree_iter iter; >> - struct btree_iter_set stack_data[MAX_BSETS]; >> + /* Must be last as it ends in a flexible-array member. */ >> + TRAILING_OVERLAP(struct btree_iter, iter, data, >> + struct btree_iter_set stack_data[MAX_BSETS]; >> + ); >> }; >> +static_assert(offsetof(struct btree_iter_stack, iter.data) == >> + offsetof(struct btree_iter_stack, stack_data)); >> > > I have to say this is ugly. Not the patch, but the gcc 14 warning option > of such coding style. Look at TRAILING_OVERLAP() usage here, this is not > C, this is something to fix a gcc bug which cannot handle FAM properly. This is not a GCC bug. > > Gustavo, this complain is not to you, just I feel a bit sad how GCC makes > the code comes to such an ugly way, and it makes things much complicated. > For anyone doesn't have deep understanding of TRAILING_OVERLAP(), I > highly suspect whether he or she can understand what happens here. > > Andrew and Gustavo, is this a mandatary to fix FAM in such way? If yes > I take the patch and keep my own opinion. If not, I'd like to see gcc > fixes its bug, for the this code I don't see the author does things > wrong. This is a false positive that needs to be addressed in some way in order to enable -Wflex-array-member-not-at-end in mainline. Here you can take a look at the patches I (and others) have submitted to modify similar code over the last year: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/log/?qt=grep&q=-Wflex-array-member-not-at-end Thanks -Gustavo ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH][next] bcache: Avoid -Wflex-array-member-not-at-end warning 2025-11-11 13:28 ` Gustavo A. R. Silva @ 2025-11-11 13:43 ` Coly Li 2025-11-13 4:24 ` Gustavo A. R. Silva 0 siblings, 1 reply; 6+ messages in thread From: Coly Li @ 2025-11-11 13:43 UTC (permalink / raw) To: Gustavo A. R. Silva Cc: Gustavo A. R. Silva, Andrew Morton, linux-bcache, linux-kernel, linux-hardening On Tue, Nov 11, 2025 at 10:28:57PM +0800, Gustavo A. R. Silva wrote: > > > On 11/11/25 22:17, Coly Li wrote: > > On Mon, Nov 10, 2025 at 07:58:58PM +0800, Gustavo A. R. Silva wrote: > > > -Wflex-array-member-not-at-end was introduced in GCC-14, and we are > > > getting ready to enable it, globally. > > > > > > Use the new TRAILING_OVERLAP() helper to fix the following warning: > > > > > > drivers/md/bcache/bset.h:330:27: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end] > > > > > > This helper creates a union between a flexible-array member (FAM) and a > > > set of MEMBERS that would otherwise follow it. > > > > > > This overlays the trailing MEMBER struct btree_iter_set stack_data[MAX_BSETS]; > > > onto the FAM struct btree_iter::data[], while keeping the FAM and the start > > > of MEMBER aligned. > > > > > > The static_assert() ensures this alignment remains, and it's > > > intentionally placed inmediately after the corresponding structures --no > > > blank line in between. > > > > > > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org> > > > --- > > > drivers/md/bcache/bset.h | 8 ++++++-- > > > 1 file changed, 6 insertions(+), 2 deletions(-) > > > > > > diff --git a/drivers/md/bcache/bset.h b/drivers/md/bcache/bset.h > > > index 011f6062c4c0..6ee2c6a506a2 100644 > > > --- a/drivers/md/bcache/bset.h > > > +++ b/drivers/md/bcache/bset.h > > > @@ -327,9 +327,13 @@ struct btree_iter { > > > /* Fixed-size btree_iter that can be allocated on the stack */ > > > struct btree_iter_stack { > > > - struct btree_iter iter; > > > - struct btree_iter_set stack_data[MAX_BSETS]; > > > + /* Must be last as it ends in a flexible-array member. */ > > > + TRAILING_OVERLAP(struct btree_iter, iter, data, > > > + struct btree_iter_set stack_data[MAX_BSETS]; > > > + ); > > > }; > > > +static_assert(offsetof(struct btree_iter_stack, iter.data) == > > > + offsetof(struct btree_iter_stack, stack_data)); > > > > > > > I have to say this is ugly. Not the patch, but the gcc 14 warning option > > of such coding style. Look at TRAILING_OVERLAP() usage here, this is not > > C, this is something to fix a gcc bug which cannot handle FAM properly. > > This is not a GCC bug. > > > > > Gustavo, this complain is not to you, just I feel a bit sad how GCC makes > > the code comes to such an ugly way, and it makes things much complicated. > > For anyone doesn't have deep understanding of TRAILING_OVERLAP(), I > > highly suspect whether he or she can understand what happens here. > > > > Andrew and Gustavo, is this a mandatary to fix FAM in such way? If yes > > I take the patch and keep my own opinion. If not, I'd like to see gcc > > fixes its bug, for the this code I don't see the author does things > > wrong. > > This is a false positive that needs to be addressed in some way in order to > enable -Wflex-array-member-not-at-end in mainline. > > Here you can take a look at the patches I (and others) have submitted to > modify similar code over the last year: > > https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/log/?qt=grep&q=-Wflex-array-member-not-at-end > I see. I take this patch, with the above complain... Coly Li ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH][next] bcache: Avoid -Wflex-array-member-not-at-end warning 2025-11-11 13:43 ` Coly Li @ 2025-11-13 4:24 ` Gustavo A. R. Silva 2025-11-24 8:34 ` Gustavo A. R. Silva 0 siblings, 1 reply; 6+ messages in thread From: Gustavo A. R. Silva @ 2025-11-13 4:24 UTC (permalink / raw) To: Coly Li Cc: Gustavo A. R. Silva, Andrew Morton, linux-bcache, linux-kernel, linux-hardening > I see. I take this patch, with the above complain... Thanks, Coly. -Gustavo ^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH][next] bcache: Avoid -Wflex-array-member-not-at-end warning 2025-11-13 4:24 ` Gustavo A. R. Silva @ 2025-11-24 8:34 ` Gustavo A. R. Silva 0 siblings, 0 replies; 6+ messages in thread From: Gustavo A. R. Silva @ 2025-11-24 8:34 UTC (permalink / raw) To: Coly Li Cc: Gustavo A. R. Silva, Andrew Morton, linux-bcache, linux-kernel, linux-hardening On 11/13/25 13:24, Gustavo A. R. Silva wrote: > >> I see. I take this patch, with the above complain... > > Thanks, Coly. BTW, I'm currently looking into the following struct, and I wonder if it's okay to move struct bkey end; to the end. drivers/md/bcache/bset.h:157:struct bset_tree { drivers/md/bcache/bset.h-158- /* drivers/md/bcache/bset.h-159- * We construct a binary tree in an array as if the array drivers/md/bcache/bset.h-160- * started at 1, so that things line up on the same cachelines drivers/md/bcache/bset.h-161- * better: see comments in bset.c at cacheline_to_bkey() for drivers/md/bcache/bset.h-162- * details drivers/md/bcache/bset.h-163- */ drivers/md/bcache/bset.h-164- drivers/md/bcache/bset.h-165- /* size of the binary tree and prev array */ drivers/md/bcache/bset.h-166- unsigned int size; drivers/md/bcache/bset.h-167- drivers/md/bcache/bset.h-168- /* function of size - precalculated for to_inorder() */ drivers/md/bcache/bset.h-169- unsigned int extra; drivers/md/bcache/bset.h-170- drivers/md/bcache/bset.h-171- /* copy of the last key in the set */ drivers/md/bcache/bset.h-172- struct bkey end; drivers/md/bcache/bset.h-173- struct bkey_float *tree; drivers/md/bcache/bset.h-174- drivers/md/bcache/bset.h-175- /* drivers/md/bcache/bset.h-176- * The nodes in the bset tree point to specific keys - this drivers/md/bcache/bset.h-177- * array holds the sizes of the previous key. drivers/md/bcache/bset.h-178- * drivers/md/bcache/bset.h-179- * Conceptually it's a member of struct bkey_float, but we want drivers/md/bcache/bset.h-180- * to keep bkey_float to 4 bytes and prev isn't used in the fast drivers/md/bcache/bset.h-181- * path. drivers/md/bcache/bset.h-182- */ drivers/md/bcache/bset.h-183- uint8_t *prev; drivers/md/bcache/bset.h-184- drivers/md/bcache/bset.h-185- /* The actual btree node, with pointers to each sorted set */ drivers/md/bcache/bset.h-186- struct bset *data; drivers/md/bcache/bset.h-187-}; Something like this: diff --git a/drivers/md/bcache/bset.h b/drivers/md/bcache/bset.h index 6ee2c6a506a2..e6daeb0edc02 100644 --- a/drivers/md/bcache/bset.h +++ b/drivers/md/bcache/bset.h @@ -169,7 +169,6 @@ struct bset_tree { unsigned int extra; /* copy of the last key in the set */ - struct bkey end; struct bkey_float *tree; /* @@ -184,6 +183,9 @@ struct bset_tree { /* The actual btree node, with pointers to each sorted set */ struct bset *data; + + /* Must be last as it ends in a flexible-array member. */ + struct bkey end; }; This would fix 15 more -Wflex-array-member-not-at-end warnings. Thanks -Gustavo ^ permalink raw reply related [flat|nested] 6+ messages in thread
end of thread, other threads:[~2025-11-24 8:34 UTC | newest] Thread overview: 6+ messages (download: mbox.gz follow: Atom feed -- links below jump to the message on this page -- 2025-11-10 10:58 [PATCH][next] bcache: Avoid -Wflex-array-member-not-at-end warning Gustavo A. R. Silva 2025-11-11 13:17 ` Coly Li 2025-11-11 13:28 ` Gustavo A. R. Silva 2025-11-11 13:43 ` Coly Li 2025-11-13 4:24 ` Gustavo A. R. Silva 2025-11-24 8:34 ` Gustavo A. R. Silva
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