From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C6E7930EF86; Wed, 4 Mar 2026 01:28:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772587691; cv=none; b=drf5aovjjP74he3uQmUvGYEoXpIrQtwwKkCtlk4Dkxew0W/kJQ5Y8d+gCQp8g+iZeThAMaDBxXJEztEcbfpnwytrw/RSUo9sg3UgSFWIanYXJwsCb0tpf1yXorYL1ncWT+l5wVZSuo5/1P33xvmGuEHijlf3KzHpFzqi4rja4FE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772587691; c=relaxed/simple; bh=tlL/8WdJD0d9jG2rBAaeDXxofN7B8fJpA+xw0D0xTeU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=GA0JrODmqKt5SnXSpKsRGMM1rF969/Kj7BNjughXQorTU/NnOe54106tlhBUnrmu4Vt2ycKHTeiVeOZ9rH8GLSDaiF797j5TMlTGDz7WPA720h6Kfbi9cRbHzNL7ET28VUTopkfMp3B85AoY/1/3sn1/02kj4yL2dar2Y7jMF4Q= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GlqDVp8B; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="GlqDVp8B" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EB70EC2BC9E; Wed, 4 Mar 2026 01:28:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772587691; bh=tlL/8WdJD0d9jG2rBAaeDXxofN7B8fJpA+xw0D0xTeU=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=GlqDVp8BB7stucd97wuW1mNl2D/oP5MR74esP9luZpfEr7cCLvEL8xo4rGPfhWJgJ 5pyi4cbwQO6TfEVjg3z1HxkTkcD0gsmdgP0DU9g+z5/kXmgm/6GJebBEEjZs2mNO2f jKO1mWagp4X0LBy07vkmsZ6HGMt3C7sqzj2C9tAnfIqh6hZQkmayH3txCJirP0QOZo rqfAUYt7SQ9zx0eCOvQsS6ELbnW2h7jmZd/vNI382pX8C3piW7eLLJdlFNeuuh/6H9 uQtQQv9blsTN0j03QBG19sbBJSwGmrf/9ulKnWE8m5oZS/NKzTwc9WlQx/n36Knh6o TWzNj2GUzklAQ== Message-ID: <2ff9897c-3822-4e31-acfb-884690448af0@kernel.org> Date: Wed, 4 Mar 2026 02:28:05 +0100 Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RESEND v2] tools build: Use -fzero-init-padding-bits=all To: Namhyung Kim Cc: Leo Yan , Nathan Chancellor , Nicolas Schier , Nick Desaulniers , Bill Wendling , Justin Stitt , Arnaldo Carvalho de Melo , Ian Rogers , James Clark , Kees Cook , linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev, bpf@vger.kernel.org References: <20260224-tools_build_fix_zero_init-v2-1-b1acc817a01e@arm.com> <99e7fe4e-72de-4b55-9a9a-ae51718a0e73@kernel.org> <20260227103611.GA1098637@e132581.arm.com> From: Quentin Monnet Content-Language: en-GB In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 2026-03-03 17:14 UTC-0800 ~ Namhyung Kim > Hello, > > On Fri, Feb 27, 2026 at 11:52:38AM +0000, Quentin Monnet wrote: >> 2026-02-27 10:36 UTC+0000 ~ Leo Yan >>> On Thu, Feb 26, 2026 at 10:52:01PM +0000, Quentin Monnet wrote: >>>> 2026-02-26 10:38 UTC-0800 ~ Namhyung Kim >>>>> Adding bpftool maintainer. >>>>> >>>>> On Tue, Feb 24, 2026 at 12:16:40PM +0000, Leo Yan wrote: >>>>>> GCC-15 release claims [1]: >>>>>> >>>>>> {0} initializer in C or C++ for unions no longer guarantees clearing >>>>>> of the whole union (except for static storage duration initialization), >>>>>> it just initializes the first union member to zero. If initialization >>>>>> of the whole union including padding bits is desirable, use {} (valid >>>>>> in C23 or C++) or use -fzero-init-padding-bits=unions option to >>>>>> restore old GCC behavior. >>>>>> >>>>>> As a result, this new behaviour might cause unexpected data when we >>>>>> initialize a union with using the '{ 0 }' initializer. >>>>>> >>>>>> Since commit dce4aab8441d ("kbuild: Use -fzero-init-padding-bits=all"), >>>>>> the kernel has enabled -fzero-init-padding-bits=all to zero padding bits >>>>>> in unions and structures. This commit applies the same option for tools >>>>>> building. >>>>>> >>>>>> The option is not supported neither by any version older than GCC 15 and >>>>>> is also not supported by LLVM, this patch adds the cc-option function to >>>>>> dynamically detect the compiler option. >>>>>> >>>>>> [1] https://gcc.gnu.org/gcc-15/changes.html >>>>>> >>>>>> Signed-off-by: Leo Yan >>>> >>>> >>>> Thank you Namhyung for the Cc. >>>> >>>> I built bpftool with the patch, with gcc 13 (which didn't get the flag, >>>> as expected) and gcc 15, and it's fine with both. As far as I can tell, >>>> bpftool does not initialise any union with "{0}" anyway. >>> >>> Thanks a lot for testing! >>> >>>> One potential concern (I didn't try) could be for cross-compilation: >>>> bpftool's Makefile sets HOST_CFLAGS based on $(CFLAGS), but $(HOSTCC) >>>> and $(CC) could be different versions of gcc, for example. >>> >>> To avoid confusion, we can use EXTRA_CFLAGS and HOST_EXTRACFLAGS instead >>> in Makefile.include: >>> >>> ----- >>> >>> # cc-option >>> # Usage: CFLAGS += $(call cc-option,-march=winchip-c6,-march=i586) >>> cc-option = $(call try-run, \ >>> $(CC) -Werror $(1) -c -x c /dev/null -o "$$TMP",$(1),$(2)) >>> >>> host-cc-option = $(call try-run, \ >>> $(HOSTCC) -Werror $(1) -c -x c /dev/null -o "$$TMP",$(1),$(2)) >>> >>> # Explicitly clear padding bits with the initializer '{ 0 }' >>> EXTRA_CFLAGS += $(call cc-option,-fzero-init-padding-bits=all) >>> HOST_EXTRACFLAGS += $(call host-cc-option,-fzero-init-padding-bits=all) >>> >>> ----- >>> >>> Then, in a project, its Makefile can append EXTRA_CFLAGS and >>> HOST_EXTRACFLAGS to CFLAGS and HOSTCFLAGS respectively. >> >> >> This sounds like it should work for bpftool as long as we += and don't >> overwrite the EXTRA_CFLAGS passed from command line. In bpftool's >> Makefile we'd have to move HOST_CFLAGS's CFLAGS-based defintion higher >> up, before we add the EXTRA_CFLAGS to CFLAGS; and if anything needs to >> be passed to the host binary, users will have to specify >> HOST_EXTRACFLAGS (or HOST_EXTRA_CFLAGS?) independently, which is acceptable. > > Quentin, do you want v2 with this or just ok for v1? > > Thanks, > Namhyung Hi Namhyung (I'm not entirely sure what v1/v2 refer to, this one was tagged v2 and I suspect v1 was the first post before the resend - I suppose you mean this one is v1 and a v2 would be with an additional host variable.) I don't want bpftool's HOST_CFLAGS to inherit -fzero-init-padding-bits=all if the compiler doesn't support it, which may happen with the current version of the patch. I'd prefer a version with separate EXTRA_CFLAGS and HOST_EXTRA_CFLAGS, as proposed by Leo and discussed above, to address the cross-compilation issue. Thanks, Quentin