Re: [PATCH] kbuild: Use -fzero-init-padding-bits=all

[Nathan Chancellor <nathan@kernel.org>]

On Mon, Jan 20, 2025 at 04:36:53PM -0800, Kees Cook wrote:
> Enable -fzero-init-padding-bits=all when available (GCC 15+). This will
> correctly zero padding bits in structs and unions that might have been
> left uninitialized, and will make sure there is no regression in union
> initializations[1]. This fixes the following stackinit KUnit selftest
> cases that had been XFAIL until now because there was no compiler
> support:
> 
>     ok 38 test_small_hole_static_all # SKIP XFAIL uninit bytes: 3
>     ok 39 test_big_hole_static_all # SKIP XFAIL uninit bytes: 124
>     ok 40 test_trailing_hole_static_all # SKIP XFAIL uninit bytes: 7
>     ok 42 test_small_hole_dynamic_all # SKIP XFAIL uninit bytes: 3
>     ok 43 test_big_hole_dynamic_all # SKIP XFAIL uninit bytes: 124
>     ok 44 test_trailing_hole_dynamic_all # SKIP XFAIL uninit bytes: 7
>     ok 58 test_small_hole_assigned_static_all # SKIP XFAIL uninit bytes: 3
>     ok 59 test_big_hole_assigned_static_all # SKIP XFAIL uninit bytes: 124
>     ok 60 test_trailing_hole_assigned_static_all # SKIP XFAIL uninit bytes: 7
>     ok 62 test_small_hole_assigned_dynamic_all # SKIP XFAIL uninit bytes: 3
>     ok 63 test_big_hole_assigned_dynamic_all # SKIP XFAIL uninit bytes: 124
>     ok 64 test_trailing_hole_assigned_dynamic_all # SKIP XFAIL uninit bytes: 7
> 
> All of the above now pass when built under GCC 15:
> 
>     ./tools/testing/kunit/kunit.py run stackinit --arch=x86_64 \
>         --make_option CC=gcc-15
> 
> Suggested-by: Jakub Jelinek <jakub@redhat.com>
> Link: https://lore.kernel.org/linux-toolchains/Z0hRrrNU3Q+ro2T7@tucnak/ [1]
> Signed-off-by: Kees Cook <kees@kernel.org>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

As far as I understand it, clang has decided to do this unconditionally:

https://github.com/llvm/llvm-project/commit/7a086e1b2dc05f54afae3591614feede727601fa

> ---
> Cc: Masahiro Yamada <masahiroy@kernel.org>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Nicolas Schier <nicolas@fjasle.eu>
> Cc: linux-kbuild@vger.kernel.org
> ---
>  scripts/Makefile.extrawarn | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 1d13cecc7cc7..eb719f6d8d53 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -77,6 +77,9 @@ KBUILD_CFLAGS += $(call cc-option,-Werror=designated-init)
>  # Warn if there is an enum types mismatch
>  KBUILD_CFLAGS += $(call cc-option,-Wenum-conversion)
>  
> +# Explicitly clear padding bits during variable initialization
> +KBUILD_CFLAGS += $(call cc-option,-fzero-init-padding-bits=all)
> +
>  KBUILD_CFLAGS += -Wextra
>  KBUILD_CFLAGS += -Wunused
>  
> -- 
> 2.34.1
>