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linux/scripts/Makefile.extrawarn
Kees Cook 1352bb3c51 kbuild: Use -fzero-init-padding-bits=all 
[ Upstream commit dce4aab844 ]

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: 7a086e1b2d [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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
2025-02-21 14:10:58 +01:00

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# SPDX-License-Identifier: GPL-2.0
# ==========================================================================
# make W=... settings
#
# There are four warning groups enabled by W=1, W=2, W=3, and W=e
# They are independent, and can be combined like W=12 or W=123e.
# ==========================================================================
# Default set of warnings, always enabled
KBUILD_CFLAGS += -Wall
KBUILD_CFLAGS += -Wundef
KBUILD_CFLAGS += -Werror=implicit-function-declaration
KBUILD_CFLAGS += -Werror=implicit-int
KBUILD_CFLAGS += -Werror=return-type
KBUILD_CFLAGS += -Werror=strict-prototypes
KBUILD_CFLAGS += -Wno-format-security
KBUILD_CFLAGS += -Wno-trigraphs
KBUILD_CFLAGS += $(call cc-disable-warning,frame-address,)
KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
KBUILD_CFLAGS += -Wmissing-declarations
KBUILD_CFLAGS += -Wmissing-prototypes
ifneq ($(CONFIG_FRAME_WARN),0)
KBUILD_CFLAGS += -Wframe-larger-than=$(CONFIG_FRAME_WARN)
endif
KBUILD_CPPFLAGS-$(CONFIG_WERROR) += -Werror
KBUILD_CPPFLAGS += $(KBUILD_CPPFLAGS-y)
KBUILD_CFLAGS-$(CONFIG_CC_NO_ARRAY_BOUNDS) += -Wno-array-bounds
ifdef CONFIG_CC_IS_CLANG
# The kernel builds with '-std=gnu11' so use of GNU extensions is acceptable.
KBUILD_CFLAGS += -Wno-gnu
# Clang checks for overflow/truncation with '%p', while GCC does not:
# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219
KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow-non-kprintf)
KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation-non-kprintf)
else
# gcc inanely warns about local variables called 'main'
KBUILD_CFLAGS += -Wno-main
endif
# These result in bogus false positives
KBUILD_CFLAGS += $(call cc-disable-warning, dangling-pointer)
# Variable Length Arrays (VLAs) should not be used anywhere in the kernel
KBUILD_CFLAGS += -Wvla
# disable pointer signed / unsigned warnings in gcc 4.0
KBUILD_CFLAGS += -Wno-pointer-sign
# In order to make sure new function cast mismatches are not introduced
# in the kernel (to avoid tripping CFI checking), the kernel should be
# globally built with -Wcast-function-type.
KBUILD_CFLAGS += $(call cc-option, -Wcast-function-type)
# The allocators already balk at large sizes, so silence the compiler
# warnings for bounds checks involving those possible values. While
# -Wno-alloc-size-larger-than would normally be used here, earlier versions
# of gcc (<9.1) weirdly don't handle the option correctly when _other_
# warnings are produced (?!). Using -Walloc-size-larger-than=SIZE_MAX
# doesn't work (as it is documented to), silently resolving to "0" prior to
# version 9.1 (and producing an error more recently). Numeric values larger
# than PTRDIFF_MAX also don't work prior to version 9.1, which are silently
# ignored, continuing to default to PTRDIFF_MAX. So, left with no other
# choice, we must perform a versioned check to disable this warning.
# https://lore.kernel.org/lkml/20210824115859.187f272f@canb.auug.org.au
KBUILD_CFLAGS-$(call gcc-min-version, 90100) += -Wno-alloc-size-larger-than
KBUILD_CFLAGS += $(KBUILD_CFLAGS-y) $(CONFIG_CC_IMPLICIT_FALLTHROUGH)
# Prohibit date/time macros, which would make the build non-deterministic
KBUILD_CFLAGS += -Werror=date-time
# enforce correct pointer usage
KBUILD_CFLAGS += $(call cc-option,-Werror=incompatible-pointer-types)
# Require designated initializers for all marked structures
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 += $(call cc-option,-fzero-init-padding-bits=all)
KBUILD_CFLAGS += -Wextra
KBUILD_CFLAGS += -Wunused
#
# W=1 - warnings which may be relevant and do not occur too often
#
ifneq ($(findstring 1, $(KBUILD_EXTRA_WARN)),)
KBUILD_CFLAGS += -Wmissing-format-attribute
KBUILD_CFLAGS += -Wmissing-include-dirs
KBUILD_CFLAGS += $(call cc-option, -Wunused-const-variable)
KBUILD_CPPFLAGS += -Wundef
KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN1
else
# Some diagnostics enabled by default are noisy.
# Suppress them by using -Wno... except for W=1.
KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
KBUILD_CFLAGS += $(call cc-disable-warning, packed-not-aligned)
KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow)
ifdef CONFIG_CC_IS_GCC
KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation)
endif
KBUILD_CFLAGS += $(call cc-disable-warning, stringop-truncation)
KBUILD_CFLAGS += -Wno-override-init # alias for -Wno-initializer-overrides in clang
ifdef CONFIG_CC_IS_CLANG
# Clang before clang-16 would warn on default argument promotions.
ifneq ($(call clang-min-version, 160000),y)
# Disable -Wformat
KBUILD_CFLAGS += -Wno-format
# Then re-enable flags that were part of the -Wformat group that aren't
# problematic.
KBUILD_CFLAGS += -Wformat-extra-args -Wformat-invalid-specifier
KBUILD_CFLAGS += -Wformat-zero-length -Wnonnull
# Requires clang-12+.
ifeq ($(call clang-min-version, 120000),y)
KBUILD_CFLAGS += -Wformat-insufficient-args
endif
endif
KBUILD_CFLAGS += $(call cc-disable-warning, pointer-to-enum-cast)
KBUILD_CFLAGS += -Wno-tautological-constant-out-of-range-compare
KBUILD_CFLAGS += $(call cc-disable-warning, unaligned-access)
KBUILD_CFLAGS += -Wno-enum-compare-conditional
endif
endif
#
# W=2 - warnings which occur quite often but may still be relevant
#
ifneq ($(findstring 2, $(KBUILD_EXTRA_WARN)),)
KBUILD_CFLAGS += -Wdisabled-optimization
KBUILD_CFLAGS += -Wshadow
KBUILD_CFLAGS += $(call cc-option, -Wlogical-op)
KBUILD_CFLAGS += $(call cc-option, -Wunused-macros)
KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN2
else
# The following turn off the warnings enabled by -Wextra
KBUILD_CFLAGS += -Wno-missing-field-initializers
KBUILD_CFLAGS += -Wno-type-limits
KBUILD_CFLAGS += -Wno-shift-negative-value
ifdef CONFIG_CC_IS_CLANG
KBUILD_CFLAGS += -Wno-enum-enum-conversion
endif
ifdef CONFIG_CC_IS_GCC
KBUILD_CFLAGS += -Wno-maybe-uninitialized
endif
endif
#
# W=3 - more obscure warnings, can most likely be ignored
#
ifneq ($(findstring 3, $(KBUILD_EXTRA_WARN)),)
KBUILD_CFLAGS += -Wbad-function-cast
KBUILD_CFLAGS += -Wcast-align
KBUILD_CFLAGS += -Wcast-qual
KBUILD_CFLAGS += -Wconversion
KBUILD_CFLAGS += -Wpacked
KBUILD_CFLAGS += -Wpadded
KBUILD_CFLAGS += -Wpointer-arith
KBUILD_CFLAGS += -Wredundant-decls
KBUILD_CFLAGS += -Wsign-compare
KBUILD_CFLAGS += -Wswitch-default
KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN3
else
# The following turn off the warnings enabled by -Wextra
KBUILD_CFLAGS += -Wno-sign-compare
KBUILD_CFLAGS += -Wno-unused-parameter
endif
#
# W=e - error out on warnings
#
ifneq ($(findstring e, $(KBUILD_EXTRA_WARN)),)
KBUILD_CFLAGS += -Werror
endif
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