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

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Message-ID: <aRNOfir6X9FmaUrd@horms.kernel.org>
Date: Tue, 11 Nov 2025 14:55:58 +0000
From: Simon Horman <horms@...nel.org>
To: Ankit Khushwaha <ankitkhushwaha.linux@...il.com>
Cc: Steffen Klassert <steffen.klassert@...unet.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Shuah Khan <shuah@...nel.org>,
	Willem de Bruijn <willemdebruijn.kernel@...il.com>,
	Jason Xing <kernelxing@...cent.com>, netdev@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH] selftest: net: fix variable sized type not at the end of
 struct warnings

On Sat, Nov 08, 2025 at 10:59:59PM +0530, Ankit Khushwaha wrote:
> 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.linux@...il.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"

I'm unsure of the attitude towards using #pragma like this in kernel code,
but certainly it would be a new one for me.

> 
> 	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.

In my view, the most promising approach I am aware of is using
TRAILING_OVERLAP(), as illustrated in [1]. However, that
approach was recently rejected, so I guess that means
it doesn't have much promise after all.

[1] https://lore.kernel.org/netdev/aPdx4iPK4-KIhjFq@kspp/