Re: [PATCH] selftest: net: fix variable sized type not at the end of struct warnings

[Ankit Khushwaha]

On Tue, Oct 28, 2025 at 05:23:12PM +0000, Simon Horman wrote:
> On Mon, Oct 27, 2025 at 10:38:56AM +0530, Ankit Khushwaha wrote:
> > Some network selftests defined variable-sized types defined at the end of
> > struct causing -Wgnu-variable-sized-type-not-at-end warning.
> > 
> > warning:
> > timestamping.c:285:18: warning: field 'cm' with variable sized type 
> > 'struct cmsghdr' not at the end of a struct or class is a GNU 
> > extension [-Wgnu-variable-sized-type-not-at-end]
> >   285 |                 struct cmsghdr cm;
> >       |                                ^
> > 
> > ipsec.c:835:5: warning: field 'u' with variable sized type 'union 
> > (unnamed union at ipsec.c:831:3)' not at the end of a struct or class 
> > is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
> >   835 |                 } u;
> >       |                   ^
> > 
> > This patch move these field at the end of struct to fix these warnings.
> > 
> > Signed-off-by: Ankit Khushwaha <ankitkhushwaha.li...@gmail.com>
> 
> Hi Ankit,
> 
> I don't believe this change is correct.
> 
> I think that the intention of the code is the char arrays (buf and control)
> provide the buffer space for the variable-length trailing field
> of the preceding structure. Where we basically have a header followed
> by data. But your patch would place the before the header.
>
Hi Simon,
So if buf and control providing the buffer space, then i think it is
better to suppress `-Wgnu-variable-sized-type-not-at-end` warning 
within this block of code.

        #pragma GCC diagnostic push
        #pragma GCC diagnostic ignored "-Wgnu-variable-sized-type-not-at-end"

        struct {
                union {
                        struct xfrm_algo        alg;
                        struct xfrm_algo_aead   aead;
                        struct xfrm_algo_auth   auth;
                } u;
                char buf[XFRM_ALGO_KEY_BUF_SIZE];
        } alg = {};

        #pragma GCC diagnostic pop

I think this would be fine.

Thanks
-- Ankit