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 CF19A1A6825; Wed, 4 Mar 2026 01:35:06 +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=1772588106; cv=none; b=h+D9suxua1hu96aoaXXDcPoDjTUQExw3xuWN8dwHsES+7B0StEn2nvr3dXE8YCEHtssGzII85vcYfnHDlUBPoaNFJFKQ7YFcW7U2GHXkrouk+92M58DBu3YxuTd3T50nweG61a505G4Y66r7wpnP34cmrbjPtO5fauTc4Re1D/o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772588106; c=relaxed/simple; bh=PWqQw6QkHOEToxV7Zpy3OPRtRc1QebdYN5ooe3pSOeo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=hw1xqtOpHxGuY4zjj85lzDSncUm+pqZcpm9EYc2FgIMxvms4k6kiKi5gBGxIw9ve40aUhIGq6nEh1JEqQ34rqcnGXw9gwZqDuKjtAryHBUhO5Wl3Ay/e3WsDrq8XluPjSBO5MmNmGHYF1yLrz3GT1JmfkE0R9KKYNDIMdXQtca0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=nKItcNT2; 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="nKItcNT2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F0B32C116C6; Wed, 4 Mar 2026 01:35:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772588106; bh=PWqQw6QkHOEToxV7Zpy3OPRtRc1QebdYN5ooe3pSOeo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=nKItcNT2isfPirMIuyaBmc/eLBbLP3r2Qce8i86fifUGaozLyDIRkAPEF4PTYtN8W vAxNvQyu036666sZl7b6prIDZySssrmhXjfjym71KAjET10kYsWUqcG1dtdFKeZDhy ReF8t7Gw8YYiStL2xpNfv920MIo/SlmhiWm8SFBIkvIjNrNFPFEyVpFMIWgQkvLhDw JtAYUurPDkgfxgrH+EXJeUHYknvy953OMnI3STtfNNLGTKomRXe8Qs7xIoFKaIb6hk e0fceYL7BkQOaTRdH5jvkhA0s/M6J0ihxOAg1FyKlAG7etUHxp2LeLWPFvWUjj30nN PKz9UTTHl5EXQ== Date: Tue, 3 Mar 2026 17:35:04 -0800 From: Namhyung Kim To: Quentin Monnet 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 Subject: Re: [PATCH RESEND v2] tools build: Use -fzero-init-padding-bits=all Message-ID: References: <20260224-tools_build_fix_zero_init-v2-1-b1acc817a01e@arm.com> <99e7fe4e-72de-4b55-9a9a-ae51718a0e73@kernel.org> <20260227103611.GA1098637@e132581.arm.com> <2ff9897c-3822-4e31-acfb-884690448af0@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <2ff9897c-3822-4e31-acfb-884690448af0@kernel.org> On Wed, Mar 04, 2026 at 02:28:05AM +0100, Quentin Monnet wrote: > 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.) Oops, right. They should be v2 and v3. > > 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. Got it. Leo, can you please update the patch? Thanks, Namhyung