Re: [PATCH][next] RDMA/rxe: Avoid -Wflex-array-member-not-at-end warnings

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 11/11/25 20:56, Leon Romanovsky wrote:
> On Tue, Nov 11, 2025 at 12:35:02PM +0900, 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:
>>
>> 21 drivers/infiniband/sw/rxe/rxe_verbs.h:271:33: 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 ib_sge sge[RXE_MAX_SGE]; onto
>> the FAM struct rxe_recv_wqe::dma.sge, 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 related structure --no
>> blank line in between.
>>
>> Lastly, move the conflicting declaration struct rxe_resp_info resp;
>> to the end of the corresponding structure.
>>
>> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
>> ---
>>   drivers/infiniband/sw/rxe/rxe_verbs.h | 18 +++++++++++-------
>>   1 file changed, 11 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/infiniband/sw/rxe/rxe_verbs.h b/drivers/infiniband/sw/rxe/rxe_verbs.h
>> index fd48075810dd..6498d61e8956 100644
>> --- a/drivers/infiniband/sw/rxe/rxe_verbs.h
>> +++ b/drivers/infiniband/sw/rxe/rxe_verbs.h
>> @@ -219,12 +219,6 @@ struct rxe_resp_info {
>>   	u32			rkey;
>>   	u32			length;
>>   
>> -	/* SRQ only */
>> -	struct {
>> -		struct rxe_recv_wqe	wqe;
>> -		struct ib_sge		sge[RXE_MAX_SGE];
>> -	} srq_wqe;
>> -
>>   	/* Responder resources. It's a circular list where the oldest
>>   	 * resource is dropped first.
>>   	 */
>> @@ -232,7 +226,15 @@ struct rxe_resp_info {
>>   	unsigned int		res_head;
>>   	unsigned int		res_tail;
>>   	struct resp_res		*res;
>> +
>> +	/* SRQ only */
>> +	/* Must be last as it ends in a flexible-array member. */
>> +	TRAILING_OVERLAP(struct rxe_recv_wqe, wqe, dma.sge,
>> +		struct ib_sge		sge[RXE_MAX_SGE];
>> +	) srq_wqe;
> 
> Will this change be enough?
> 
> diff --git a/drivers/infiniband/sw/rxe/rxe_verbs.h b/drivers/infiniband/sw/rxe/rxe_verbs.h
> index fd48075810dd..9ab11421a585 100644
> --- a/drivers/infiniband/sw/rxe/rxe_verbs.h
> +++ b/drivers/infiniband/sw/rxe/rxe_verbs.h
> @@ -219,12 +219,6 @@ struct rxe_resp_info {
>          u32                     rkey;
>          u32                     length;
>   
> -       /* SRQ only */
> -       struct {
> -               struct rxe_recv_wqe     wqe;
> -               struct ib_sge           sge[RXE_MAX_SGE];
> -       } srq_wqe;
> -
>          /* Responder resources. It's a circular list where the oldest
>           * resource is dropped first.
>           */
> @@ -232,6 +226,12 @@ struct rxe_resp_info {
>          unsigned int            res_head;
>          unsigned int            res_tail;
>          struct resp_res         *res;
> +
> +       /* SRQ only */
> +       struct {
> +               struct ib_sge           sge[RXE_MAX_SGE];
> +               struct rxe_recv_wqe     wqe;
> +       } srq_wqe;
>   };

The question is if this is really what you want?

sge[RXE_MAX_SGE] is of the following type:

struct ib_sge {
         u64     addr;
         u32     length;
         u32     lkey;
};

and struct rxe_recv_wqe::dma.sge[] is of type:

struct rxe_sge {
         __aligned_u64 addr;
         __u32   length;
         __u32   lkey;
};

Both types are basically the same, and the original code looks
pretty much like what people do when they want to pre-allocate
a number of elements (of the same element type as the flex array)
for a flexible-array member.

Based on the above, the change you suggest seems a bit suspicious,
and I'm not sure that's actually what you want?

Thanks
-Gustavo