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

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Message-ID: <aQD8AOZduY4Fit3k@horms.kernel.org>
Date: Tue, 28 Oct 2025 17:23:12 +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 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.

-- 
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