linux-kernel - Re: [PATCH v2] selftests/user_events: silence a clang warning: address of packed member

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Date: Tue, 28 May 2024 13:28:33 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: John Hubbard <jhubbard@...dia.com>
Cc: Shuah Khan <shuah@...nel.org>,
	Beau Belgrave <beaub@...ux.microsoft.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mark Brown <broonie@...nel.org>,
	Naresh Kamboju <naresh.kamboju@...aro.org>,
	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
Subject: Re: [PATCH v2] selftests/user_events: silence a clang warning:
 address of packed member

Hi John,

On Mon, May 27, 2024 at 02:47:04PM -0700, John Hubbard wrote:
> When building with clang, via:
> 
>     make LLVM=1 -C tools/testing/selftest
> 
> ...clang warns about "taking address of packed member 'write_index' ".
> This is not particularly helpful, because the test code really wants to
> write to exactly this location, and if it ends up being unaligned, then
> the test won't work (and may segfault, depending on the CPU type).
> 
> If that ever comes up, it will be obvious and can be fixed. But all it's
> doing now is prevent a clean clang build, so disable the warning.
> 
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
> 
> Changes since the first version:
> 
> 1) Rebased onto Linux 6.10-rc1
> 
> 
> thanks,
> John Hubbard
> 
> 
>  tools/testing/selftests/user_events/Makefile | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/tools/testing/selftests/user_events/Makefile b/tools/testing/selftests/user_events/Makefile
> index 10fcd0066203..617e94344711 100644
> --- a/tools/testing/selftests/user_events/Makefile
> +++ b/tools/testing/selftests/user_events/Makefile
> @@ -1,5 +1,10 @@
>  # SPDX-License-Identifier: GPL-2.0
>  CFLAGS += -Wl,-no-as-needed -Wall $(KHDR_INCLUDES)
> +
> +ifneq ($(LLVM),)

Perhaps it would be better if this were

  ifeq ($(CC),clang)

as that would catch both CC=clang and LLVM=1 users? I haven't tested
this though.

Additionally, I think it would be good to mention that
-Wno-address-of-packed-member is GCC's default, whereas Clang enables
-Waddress-of-packed-member by default.

> +    CFLAGS += -Wno-address-of-packed-member
> +endif
> +
>  LDLIBS += -lrt -lpthread -lm
>  
>  TEST_GEN_PROGS = ftrace_test dyn_test perf_test abi_test
> 
> base-commit: 2bfcfd584ff5ccc8bb7acde19b42570414bf880b
> -- 
> 2.45.1
> 
>