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

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

[Ahmad Fatoum]

This is a port of Linux commit dce4aab8441d285b9a78b33753e0bf583c1320ee:

|   Author:     Kees Cook <kees@kernel.org>
|   AuthorDate: Mon Jan 27 11:10:28 2025 -0800
|
|   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] without
|   additional flags.
|
|   Suggested-by: Jakub Jelinek <jakub@redhat.com>
|   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@kernel.org>
|   Acked-by: Masahiro Yamada <masahiroy@kernel.org>
|   Link: https://lore.kernel.org/r/20250127191031.245214-3-kees@kernel.org
|   Signed-off-by: Kees Cook <kees@kernel.org>

A quick look at { 0 } usage in barebox shows that initializations of
struct nvme_command in drivers/nvme/host/core.c might be affected by
this, so better play it safe.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Makefile b/Makefile
index 58335c249a72..2e3931ed7b37 100644
--- a/Makefile
+++ b/Makefile
@@ -759,6 +759,9 @@ endif
 # disable invalid "can't wrap" optimizations for signed / pointers
 KBUILD_CFLAGS	+= $(call cc-option,-fno-strict-overflow)
 
+# Explicitly clear padding bits during variable initialization
+KBUILD_CFLAGS += $(call cc-option,-fzero-init-padding-bits=all)
+
 # Make sure -fstack-check isn't enabled (like gentoo apparently did)
 KBUILD_CFLAGS  += $(call cc-option,-fno-stack-check)
 
-- 
2.39.5

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

[Sascha Hauer]

On Fri, 25 Apr 2025 15:38:49 +0200, Ahmad Fatoum wrote:
> This is a port of Linux commit dce4aab8441d285b9a78b33753e0bf583c1320ee:
> 
> |   Author:     Kees Cook <kees@kernel.org>
> |   AuthorDate: Mon Jan 27 11:10:28 2025 -0800
> |
> |   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] without
> |   additional flags.
> |
> |   Suggested-by: Jakub Jelinek <jakub@redhat.com>
> |   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@kernel.org>
> |   Acked-by: Masahiro Yamada <masahiroy@kernel.org>
> |   Link: https://lore.kernel.org/r/20250127191031.245214-3-kees@kernel.org
> |   Signed-off-by: Kees Cook <kees@kernel.org>
> 
> [...]

Applied, thanks!

[1/1] kbuild: Use -fzero-init-padding-bits=all
      https://git.pengutronix.de/cgit/barebox/commit/?id=56baf9bea6ba (link may not be stable)

Best regards,
-- 
Sascha Hauer <s.hauer@pengutronix.de>