]> www.infradead.org Git - users/dwmw2/linux.git/commit
kbuild: Use -fzero-init-padding-bits=all
authorKees Cook <kees@kernel.org>
Mon, 27 Jan 2025 19:10:28 +0000 (11:10 -0800)
committerKees Cook <kees@kernel.org>
Thu, 30 Jan 2025 16:48:34 +0000 (08:48 -0800)
commitdce4aab8441d285b9a78b33753e0bf583c1320ee
treeb1add6d22a9b8853e302f7cb70c4ebe03844d7cb
parente71a29db79da194678630ebfcc53ff2aecc9d441
kbuild: Use -fzero-init-padding-bits=all

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
Link: https://lore.kernel.org/linux-toolchains/Z0hRrrNU3Q+ro2T7@tucnak/
Link: https://github.com/llvm/llvm-project/commit/7a086e1b2dc05f54afae3591614feede727601fa
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>
scripts/Makefile.extrawarn