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

Leon Romanovsky
2025-11-11T14:19:49Z

On Tue, Nov 11, 2025 at 09:14:05PM +0900, Gustavo A. R. Silva wrote:
>
>
> 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?

You wrote about this error: "warning: structure containing a flexible array
member is not at the end of another structure".

My suggestion was simply to move that flex array to be the last element
and save us from the need to have some complex, magic macro in RXE.

Thanks

>
> Thanks
> -Gustavo
>
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