linux-kernel - Re: [PATCH v2] tools build: Use -fzero-init-padding-bits=all

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Message-ID: <20250422075826.GA28953@e132581.arm.com>
Date: Tue, 22 Apr 2025 08:58:26 +0100
From: Leo Yan <leo.yan@....com>
To: Nick Desaulniers <ndesaulniers@...gle.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>, Namhyung Kim <namhyung@...nel.org>,
	James Clark <james.clark@...aro.org>, Kees Cook <kees@...nel.org>,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v2] tools build: Use -fzero-init-padding-bits=all

On Wed, Apr 02, 2025 at 06:30:56PM +0100, 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.

Gentle ping.

Thanks,
Leo