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

[Kees Cook]

On Tue, Feb 24, 2026 at 10:19:56AM -0700, Nathan Chancellor wrote:
> 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.

You could claim it! ;)

Regardless, I like to see cc-option available here, as I doubt this will
be the last conditional option for tool builds. (Actually, are there
other conditional options that could use this today in the tools
Makefiles?)

-Kees

> 
> > ---
> >  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>
> > 

-- 
Kees Cook