linux-kernel - Re: [PATCH v2 2/2] selftests/lib.mk: silence some clang warnings that gcc already ignores

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Date: Mon, 3 Jun 2024 14:06:19 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Shuah Khan <shuah@...nel.org>, Muhammad Usama Anjum
	<usama.anjum@...labora.com>
CC: Beau Belgrave <beaub@...ux.microsoft.com>, Steven Rostedt
	<rostedt@...dmis.org>, Mark Brown <broonie@...nel.org>, Naresh Kamboju
	<naresh.kamboju@...aro.org>, Nick Desaulniers <ndesaulniers@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, Bill Wendling <morbo@...gle.com>,
	sunliming <sunliming@...inos.cn>, Masami Hiramatsu <mhiramat@...nel.org>,
	Valentin Obst <kernel@...entinobst.de>, <linux-kselftest@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, <llvm@...ts.linux.dev>, "Nathan
 Chancellor" <nathan@...nel.org>
Subject: Re: [PATCH v2 2/2] selftests/lib.mk: silence some clang warnings that
 gcc already ignores

On 5/31/24 11:37 AM, John Hubbard wrote:
> gcc defaults to silence (off) for the following warnings, but clang
> defaults to the opposite. The warnings are not useful for the kernel
> itself, which is why they have remained disabled in gcc for the main
> kernel build. And it is only due to including kernel data structures in
> the selftests, that we get the warnings from clang.
> 
>      -Waddress-of-packed-member
>      -Wgnu-variable-sized-type-not-at-end

Even if patch 1/1 here is not merged, I would still like to get this
one reviewed and merged. It still solves the problem for LLVM=1 builds.

thanks,
-- 
John Hubbard
NVIDIA

> 
> In other words, the warnings are not unique to the selftests: there is
> nothing that the selftests' code does that triggers these warnings,
> other than the act of including the kernel's data structures. Therefore,
> silence them for the clang builds as well.
> 
> This eliminates warnings for the net/ and user_events/ kselftest
> subsystems, in these files:
> 
>      ./net/af_unix/scm_rights.c
>      ./net/timestamping.c
>      ./net/ipsec.c
>      ./user_events/perf_test.c
> 
> Cc: Nathan Chancellor <nathan@...nel.org>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>   tools/testing/selftests/lib.mk | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
> index 2902787b89b2..c179c02281e9 100644
> --- a/tools/testing/selftests/lib.mk
> +++ b/tools/testing/selftests/lib.mk
> @@ -50,6 +50,14 @@ else
>   CLANG_FLAGS     += --target=$(notdir $(CROSS_COMPILE:%-=%))
>   endif # CROSS_COMPILE
>   
> +# gcc defaults to silence (off) for the following warnings, but clang defaults
> +# to the opposite. The warnings are not useful for the kernel itself, which is
> +# why they have remained disabled in gcc for the main kernel build. And it is
> +# only due to including kernel data structures in the selftests, that we get the
> +# warnings from clang. Therefore, disable the warnings for clang builds.
> +CFLAGS += -Wno-address-of-packed-member
> +CFLAGS += -Wno-gnu-variable-sized-type-not-at-end
> +
>   CC := $(CLANG) $(CLANG_FLAGS) -fintegrated-as
>   else
>   CC := $(CROSS_COMPILE)gcc