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

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Message-ID: <CAP-5=fU5y_xKaWnh8tumyjTfJZdr2i+tPUaECyPs_vdhKdYWLg@mail.gmail.com>
Date: Fri, 28 Mar 2025 09:39:41 -0700
From: Ian Rogers <irogers@...gle.com>
To: Leo Yan <leo.yan@....com>
Cc: 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>, Namhyung Kim <namhyung@...nel.org>, 
	James Clark <james.clark@...aro.org>, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, 
	Kees Cook <kees@...nel.org>
Subject: Re: [PATCH] tools build: Use -fzero-init-padding-bits=all

On Fri, Mar 28, 2025 at 6:52 AM Leo Yan <leo.yan@....com> 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@....com>
> ---
>
> This patch was originally from [1]. After consideration, the top level
> of the tools directory is a better place to accommodate this option
> rather than perf folder.
>
> [1] https://lore.kernel.org/linux-perf-users/20250320105235.3498106-1-leo.yan@arm.com/
>
>  tools/scripts/Makefile.include | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/tools/scripts/Makefile.include b/tools/scripts/Makefile.include
> index 0aa4005017c7..e912a10afd89 100644
> --- a/tools/scripts/Makefile.include
> +++ b/tools/scripts/Makefile.include
> @@ -40,6 +40,19 @@ EXTRA_WARNINGS += -Wwrite-strings
>  EXTRA_WARNINGS += -Wformat
>  EXTRA_WARNINGS += -Wno-type-limits
>

It'd be nice to bring in the comment for try-run that's in
./scripts/Makefile.compiler:
```
# 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 = $(shell set -e;              \
> +       if ($(1)) >/dev/null 2>&1;      \
> +       then echo "$(2)";               \
> +       else echo "$(3)";               \
> +       fi)
> +
> +__cc-option = $(call try-run,\
> +       $(1) -Werror $(2) -c -x c /dev/null -o /dev/null,$(2),)
> +cc-option = $(call __cc-option, $(CC),$(1))
> +

I see differences with the ./scripts/Makefile.compiler version of
these functions:
```
# __cc-option
# Usage: MY_CFLAGS += $(call
__cc-option,$(CC),$(MY_CFLAGS),-march=winchip-c6,-march=i586)
__cc-option = $(call try-run,\
       $(1) -Werror $(2) $(3) -c -x c /dev/null -o "$$TMP",$(3),$(4))

cc-option = $(call __cc-option, $(CC),\
       $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS),$(1),$(2))
```
I'm just wondering why as if we need to update these in the future
it'd be easier if the two were identical.

Thanks,
Ian

> +# 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
> --
> 2.34.1
>