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

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Message-Id: <20250121003646.work.168-kees@kernel.org>
Date: Mon, 20 Jan 2025 16:36:53 -0800
From: Kees Cook <kees@...nel.org>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Kees Cook <kees@...nel.org>,
	Jakub Jelinek <jakub@...hat.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>,
	linux-kbuild@...r.kernel.org,
	Stephen Hemminger <stephen@...workplumber.org>,
	linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH] kbuild: Use -fzero-init-padding-bits=all

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@...hat.com>
Link: https://lore.kernel.org/linux-toolchains/Z0hRrrNU3Q+ro2T7@tucnak/ [1]
Signed-off-by: Kees Cook <kees@...nel.org>
---
Cc: Masahiro Yamada <masahiroy@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>
Cc: Nicolas Schier <nicolas@...sle.eu>
Cc: linux-kbuild@...r.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