On Tue, Jan 28, 2025 at 4:10 AM Kees Cook <kees@xxxxxxxxxx> wrote:
>
> GCC 15 introduces a regression in "= { 0 }" style initialization of
> unions that Linux has depended on for eliminating uninitialized variable
> contents. GCC does not seem likely to fix it[1], instead suggesting[2]
> that affected projects start using -fzero-init-padding-bits=unions.
>
> To avoid future surprises beyond just the current situation with unions,
> enable -fzero-init-padding-bits=all when available (GCC 15+). This will
> correctly zero padding bits in unions and structs that might have been
> left uninitialized, and will make sure there is no immediate regression
> in union initializations. As seen in the stackinit KUnit selftest union
> cases, which were passing before, were failing under GCC 15:
>
> not ok 18 test_small_start_old_zero
> ok 29 test_small_start_dynamic_partial # SKIP XFAIL uninit bytes: 63
> ok 32 test_small_start_assigned_dynamic_partial # SKIP XFAIL uninit bytes: 63
> ok 67 test_small_start_static_partial # SKIP XFAIL uninit bytes: 63
> ok 70 test_small_start_static_all # SKIP XFAIL uninit bytes: 56
> ok 73 test_small_start_dynamic_all # SKIP XFAIL uninit bytes: 56
> ok 82 test_small_start_assigned_static_partial # SKIP XFAIL uninit bytes: 63
> ok 85 test_small_start_assigned_static_all # SKIP XFAIL uninit bytes: 56
> ok 88 test_small_start_assigned_dynamic_all # SKIP XFAIL uninit bytes: 56
>
> The above all now pass again with -fzero-init-padding-bits=all added.
>
> This also fixes the following cases for struct initialization that had
> been XFAIL until now because there was no compiler support beyond the
> larger "-ftrivial-auto-var-init=zero" option:
>
> 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. Tests can be seen
> with:
>
> ./tools/testing/kunit/kunit.py run stackinit --arch=x86_64 \
> --make_option CC=gcc-15
>
> Clang continues to fully initialize these kinds of variables[3] with
> additional flags.
>
> Suggested-by: Jakub Jelinek <jakub@xxxxxxxxxx>
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118403 [1]
> Link: https://lore.kernel.org/linux-toolchains/Z0hRrrNU3Q+ro2T7@tucnak/ [2]
> Link: https://github.com/llvm/llvm-project/commit/7a086e1b2dc05f54afae3591614feede727601fa [3]
> Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
> ---
Acked-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> Cc: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> Cc: Nicolas Schier <nicolas@xxxxxxxxx>
> Cc: linux-kbuild@xxxxxxxxxxxxxxx
> ---
> 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
>
--
Best Regards
Masahiro Yamada
