kbuild: Use -fzero-init-padding-bits=all

PATCH Superseded Kees Cook 2 participants 2 messages Jan 21, 2025 Kbuild

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Kees Cook

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2025-01-21 00:37 UTC #0 lore raw

PATCH 1/1 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

Link https://lore.kernel.org/linux-toolchains/Z0hRrrNU3Q+ro2T7@tucnak/ [1] open here

Signed-off-by Kees Cook <[email protected]>

---
Cc: Masahiro Yamada <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Nicolas Schier <[email protected]>
Cc: [email protected]

---
 scripts/Makefile.extrawarn | 3 +++
 1 file changed, 3 insertions(+)

scripts/Makefile.extrawarn +3 -0

index 1d13cecc7cc7..eb719f6d8d53 100644
@@ -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

Signature · 1 line

2.34.1

Nathan Chancellor

maintainer Reviewed-by on v1 1/1

2025-01-21 21:51 UTC #1 lore raw

Quoted 20 lines · Kees Cook

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

Quoted 8 lines

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 <[email protected]>
Link https://lore.kernel.org/linux-toolchains/Z0hRrrNU3Q+ro2T7@tucnak/ [1] open here
Signed-off-by Kees Cook <[email protected]>

Reviewed-by Nathan Chancellor <[email protected]>

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

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

Quoted 26 lines

---
Cc: Masahiro Yamada <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Nicolas Schier <[email protected]>
Cc: [email protected]

---
 scripts/Makefile.extrawarn | 3 +++
 1 file changed, 3 insertions(+)

scripts/Makefile.extrawarn +3 -0

index 1d13cecc7cc7..eb719f6d8d53 100644
@@ -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

Signature · 1 line

2.34.1