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

Gustavo A. R. Silva
2025-11-11T12:14:16Z

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