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

[Nathan Chancellor]

Hi Leo,

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 <leo.yan@xxxxxxx>
> ---
> Resent to linux-kbuild mailing list.

Kbuild does not maintain/touch tools/. This should go via another tree
like perf or something. It does not look like
tools/scripts/Makefile.include has a clear owner, perf and bpf tend to
be the ones who touch it the most.

> ---
>  tools/scripts/Makefile.include | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
> index b5ecf137febcae59f506e107a7f2e2ad72f4bef4..73f6aef4f3fda0cda7ee8f4b9a3b7ff7d956070d 100644
> --- a/tools/scripts/Makefile.include
> +++ b/tools/scripts/Makefile.include
> @@ -40,6 +40,30 @@ EXTRA_WARNINGS += -Wwrite-strings
>  EXTRA_WARNINGS += -Wformat
>  EXTRA_WARNINGS += -Wno-type-limits
>  
> +# output directory for tests below
> +TMPOUT = .tmp_$$$$
> +
> +# try-run
> +# Usage: option = $(call try-run, $(CC)...-o "$$TMP",option-ok,otherwise)
> +# Exit code chooses option. "$$TMP" serves as a temporary file and is
> +# automatically cleaned up.
> +try-run = $(shell set -e;		\
> +	TMP=$(TMPOUT)/tmp;		\
> +	trap "rm -rf $(TMPOUT)" EXIT;	\
> +	mkdir -p $(TMPOUT);		\
> +	if ($(1)) >/dev/null 2>&1;	\
> +	then echo "$(2)";		\
> +	else echo "$(3)";		\
> +	fi)
> +
> +# cc-option
> +# Usage: CFLAGS += $(call cc-option,-march=winchip-c6,-march=i586)
> +cc-option = $(call try-run, \
> +	$(CC) -Werror $(1) -c -x c /dev/null -o "$$TMP",$(1),$(2))
> +
> +# Explicitly clear padding bits with the initializer '{ 0 }'
> +CFLAGS += $(call cc-option,-fzero-init-padding-bits=all)
> +
>  # Makefiles suck: This macro sets a default value of $(2) for the
>  # variable named by $(1), unless the variable has been set by
>  # environment or command line. This is necessary for CC and AR
> 
> ---
> base-commit: 7dff99b354601dd01829e1511711846e04340a69
> change-id: 20260224-tools_build_fix_zero_init-dc5261bd8b8b
> 
> Best regards,
> -- 
> Leo Yan <leo.yan@xxxxxxx>
>