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 13:35:15 -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

On Tue, May 28, 2024 at 01:28:33PM -0700, Nathan Chancellor wrote:
> 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.

Hmmm, now that I am actually looking at tools/testing/selftests/lib.mk,
it seems like CC is only set to clang when $(LLVM) is set, so keeping it
the way it is now is probably best. Sorry about that, I am not too
familiar with the tools build system.

> 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
> > 
> > 
>