Re: [PATCH][next] bcache: Avoid -Wflex-array-member-not-at-end warning

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 <[email protected]>
>> ---
>>   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