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kbuild: Consolidate C dialect options

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Introduce CC_FLAGS_DIALECT to make it easier to update the various
places in the tree that rely on the GNU C standard and Microsoft
extensions flags atomically. All remaining uses of '-std=gnu11' and
'-fms-extensions' are in the tools directory (which has its own build
system) and other standalone Makefiles. This will allow the kernel to
use a narrower option to enable the Microsoft anonymous tagged structure
extension in a simpler manner. Place the CC_FLAGS_DIALECT block after
the configuration include (so that a future change can move the
selection of the flag to Kconfig) but before the
arch/$(SRCARCH)/Makefile include (so that CC_FLAGS_DIALECT is available
for use in those Makefiles).

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Helge Deller <deller@gmx.de>  # parisc
Link: https://patch.msgid.link/20260223-fms-anonymous-structs-v1-1-8ee406d3c36c@kernel.org
Signed-off-by: Nicolas Schier <nsc@kernel.org>
This commit is contained in:
Nathan Chancellor 2026-02-23 12:10:28 -07:00 committed by Nicolas Schier
parent febb5c81fa
commit ec4c28276c
No known key found for this signature in database
GPG Key ID: 07520A7016261269
11 changed files with 21 additions and 31 deletions
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16
Makefile
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@ -587,7 +587,6 @@ LINUXINCLUDE := \
KBUILD_AFLAGS := -D__ASSEMBLY__ -fno-PIE
KBUILD_CFLAGS :=
KBUILD_CFLAGS += -std=gnu11
KBUILD_CFLAGS += -fshort-wchar
KBUILD_CFLAGS += -funsigned-char
KBUILD_CFLAGS += -fno-common
@ -790,6 +789,18 @@ ifdef need-config
include $(objtree)/include/config/auto.conf
endif
CC_FLAGS_DIALECT := -std=gnu11
# Allow including a tagged struct or union anonymously in another struct/union.
CC_FLAGS_DIALECT += -fms-extensions
# Clang enables warnings about GNU and Microsoft extensions by default, disable
# them because this is expected with the above options.
ifdef CONFIG_CC_IS_CLANG
CC_FLAGS_DIALECT += -Wno-gnu
CC_FLAGS_DIALECT += -Wno-microsoft-anon-tag
endif
export CC_FLAGS_DIALECT
KBUILD_CFLAGS += $(CC_FLAGS_DIALECT)
ifeq ($(KBUILD_EXTMOD),)
# Objects we will link into vmlinux / subdirs we need to visit
core-y :=
@ -1093,9 +1104,6 @@ NOSTDINC_FLAGS += -nostdinc
# perform bounds checking.
KBUILD_CFLAGS += $(call cc-option, -fstrict-flex-arrays=3)
# Allow including a tagged struct or union anonymously in another struct/union.
KBUILD_CFLAGS += -fms-extensions
# disable invalid "can't wrap" optimizations for signed / pointers
KBUILD_CFLAGS += -fno-strict-overflow

3
arch/arm64/kernel/vdso32/Makefile
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@ -63,7 +63,7 @@ VDSO_CFLAGS += -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
$(filter -Werror,$(KBUILD_CPPFLAGS)) \
-Werror-implicit-function-declaration \
-Wno-format-security \
-std=gnu11 -fms-extensions
$(CC_FLAGS_DIALECT)
VDSO_CFLAGS += -O2
# Some useful compiler-dependent flags from top-level Makefile
VDSO_CFLAGS += $(call cc32-option,-Wno-pointer-sign)
@ -71,7 +71,6 @@ VDSO_CFLAGS += -fno-strict-overflow
VDSO_CFLAGS += $(call cc32-option,-Werror=strict-prototypes)
VDSO_CFLAGS += -Werror=date-time
VDSO_CFLAGS += $(call cc32-option,-Werror=incompatible-pointer-types)
VDSO_CFLAGS += $(if $(CONFIG_CC_IS_CLANG),-Wno-microsoft-anon-tag)
# Compile as THUMB2 or ARM. Unwinding via frame-pointers in THUMB2 is
# unreliable.

2
arch/loongarch/vdso/Makefile
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@ -24,7 +24,7 @@ endif
cflags-vdso := $(ccflags-vdso) \
-isystem $(shell $(CC) -print-file-name=include) \
$(filter -W%,$(filter-out -Wa$(comma)%,$(KBUILD_CFLAGS))) \
-std=gnu11 -fms-extensions -O2 -g -fno-strict-aliasing -fno-common -fno-builtin \
$(CC_FLAGS_DIALECT) -O2 -g -fno-strict-aliasing -fno-common -fno-builtin \
-fno-stack-protector -fno-jump-tables -DDISABLE_BRANCH_PROFILING \
$(call cc-option, -fno-asynchronous-unwind-tables) \
$(call cc-option, -fno-stack-protector)

2
arch/parisc/boot/compressed/Makefile
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@ -18,7 +18,7 @@ KBUILD_CFLAGS += -fno-PIE -mno-space-regs -mdisable-fpregs -Os
ifndef CONFIG_64BIT
KBUILD_CFLAGS += -mfast-indirect-calls
endif
KBUILD_CFLAGS += -std=gnu11 -fms-extensions
KBUILD_CFLAGS += $(CC_FLAGS_DIALECT)
LDFLAGS_vmlinux := -X -e startup --as-needed -T
$(obj)/vmlinux: $(obj)/vmlinux.lds $(addprefix $(obj)/, $(OBJECTS)) $(LIBGCC) FORCE

3
arch/powerpc/boot/Makefile
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@ -70,7 +70,7 @@ BOOTCPPFLAGS := -nostdinc $(LINUXINCLUDE)
BOOTCPPFLAGS += -isystem $(shell $(BOOTCC) -print-file-name=include)
BOOTCFLAGS := $(BOOTTARGETFLAGS) \
-std=gnu11 -fms-extensions \
$(CC_FLAGS_DIALECT) \
-Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
-fno-strict-aliasing -O2 \
-msoft-float -mno-altivec -mno-vsx \
@ -86,7 +86,6 @@ BOOTARFLAGS := -crD
ifdef CONFIG_CC_IS_CLANG
BOOTCFLAGS += $(CLANG_FLAGS)
BOOTCFLAGS += -Wno-microsoft-anon-tag
BOOTAFLAGS += $(CLANG_FLAGS)
endif

3
arch/s390/Makefile
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@ -22,7 +22,7 @@ KBUILD_AFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -D__ASSEMBLY__
ifndef CONFIG_AS_IS_LLVM
KBUILD_AFLAGS_DECOMPRESSOR += $(if $(CONFIG_DEBUG_INFO),$(aflags_dwarf))
endif
KBUILD_CFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -O2 -mpacked-stack -std=gnu11 -fms-extensions
KBUILD_CFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -O2 -mpacked-stack $(CC_FLAGS_DIALECT)
KBUILD_CFLAGS_DECOMPRESSOR += -DDISABLE_BRANCH_PROFILING -D__NO_FORTIFY
KBUILD_CFLAGS_DECOMPRESSOR += -D__DECOMPRESSOR
KBUILD_CFLAGS_DECOMPRESSOR += -Wno-pointer-sign
@ -35,7 +35,6 @@ KBUILD_CFLAGS_DECOMPRESSOR += $(call cc-disable-warning, address-of-packed-membe
KBUILD_CFLAGS_DECOMPRESSOR += $(if $(CONFIG_DEBUG_INFO),-g)
KBUILD_CFLAGS_DECOMPRESSOR += $(if $(CONFIG_DEBUG_INFO_DWARF4), $(call cc-option, -gdwarf-4,))
KBUILD_CFLAGS_DECOMPRESSOR += $(if $(CONFIG_CC_NO_ARRAY_BOUNDS),-Wno-array-bounds)
KBUILD_CFLAGS_DECOMPRESSOR += $(if $(CONFIG_CC_IS_CLANG),-Wno-microsoft-anon-tag)
UTS_MACHINE := s390x
STACK_SIZE := $(if $(CONFIG_KASAN),65536,$(if $(CONFIG_KMSAN),65536,16384))

3
arch/s390/purgatory/Makefile
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@ -13,7 +13,7 @@ CFLAGS_sha256.o := -D__NO_FORTIFY
$(obj)/mem.o: $(srctree)/arch/s390/lib/mem.S FORCE
$(call if_changed_rule,as_o_S)
KBUILD_CFLAGS := -std=gnu11 -fms-extensions -fno-strict-aliasing -Wall -Wstrict-prototypes
KBUILD_CFLAGS := $(CC_FLAGS_DIALECT) -fno-strict-aliasing -Wall -Wstrict-prototypes
KBUILD_CFLAGS += -Wno-pointer-sign -Wno-sign-compare
KBUILD_CFLAGS += -fno-zero-initialized-in-bss -fno-builtin -ffreestanding
KBUILD_CFLAGS += -Os -m64 -msoft-float -fno-common
@ -21,7 +21,6 @@ KBUILD_CFLAGS += -fno-stack-protector
KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
KBUILD_CFLAGS += -D__DISABLE_EXPORTS
KBUILD_CFLAGS += $(CLANG_FLAGS)
KBUILD_CFLAGS += $(if $(CONFIG_CC_IS_CLANG),-Wno-microsoft-anon-tag)
KBUILD_CFLAGS += $(call cc-option,-fno-PIE)
KBUILD_CFLAGS += $(call cc-option, -Wno-default-const-init-unsafe)
KBUILD_AFLAGS := $(filter-out -DCC_USING_EXPOLINE,$(KBUILD_AFLAGS))

6
arch/x86/Makefile
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@ -48,7 +48,7 @@ endif
# How to compile the 16-bit code. Note we always compile for -march=i386;
# that way we can complain to the user if the CPU is insufficient.
REALMODE_CFLAGS := -std=gnu11 -fms-extensions -m16 -g -Os \
REALMODE_CFLAGS := $(CC_FLAGS_DIALECT) -m16 -g -Os \
-DDISABLE_BRANCH_PROFILING -D__DISABLE_EXPORTS \
-Wall -Wstrict-prototypes -march=i386 -mregparm=3 \
-fno-strict-aliasing -fomit-frame-pointer -fno-pic \
@ -59,10 +59,6 @@ REALMODE_CFLAGS += -fno-stack-protector
REALMODE_CFLAGS += -Wno-address-of-packed-member
REALMODE_CFLAGS += $(cc_stack_align4)
REALMODE_CFLAGS += $(CLANG_FLAGS)
ifdef CONFIG_CC_IS_CLANG
REALMODE_CFLAGS += -Wno-gnu
REALMODE_CFLAGS += -Wno-microsoft-anon-tag
endif
export REALMODE_CFLAGS
# BITS is used as extension for files which are available in a 32 bit

6
arch/x86/boot/compressed/Makefile
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@ -25,7 +25,7 @@ targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \
# avoid errors with '-march=i386', and future flags may depend on the target to
# be valid.
KBUILD_CFLAGS := -m$(BITS) -O2 $(CLANG_FLAGS)
KBUILD_CFLAGS += -std=gnu11 -fms-extensions
KBUILD_CFLAGS += $(CC_FLAGS_DIALECT)
KBUILD_CFLAGS += -fno-strict-aliasing -fPIE
KBUILD_CFLAGS += -Wundef
KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
@ -36,10 +36,6 @@ KBUILD_CFLAGS += -mno-mmx -mno-sse
KBUILD_CFLAGS += -ffreestanding -fshort-wchar
KBUILD_CFLAGS += -fno-stack-protector
KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
ifdef CONFIG_CC_IS_CLANG
KBUILD_CFLAGS += -Wno-gnu
KBUILD_CFLAGS += -Wno-microsoft-anon-tag
endif
KBUILD_CFLAGS += -Wno-pointer-sign
KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
KBUILD_CFLAGS += -D__DISABLE_EXPORTS

3
drivers/firmware/efi/libstub/Makefile
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@ -11,12 +11,11 @@ cflags-y := $(KBUILD_CFLAGS)
cflags-$(CONFIG_X86_32) := -march=i386
cflags-$(CONFIG_X86_64) := -mcmodel=small
cflags-$(CONFIG_X86) += -m$(BITS) -D__KERNEL__ -std=gnu11 -fms-extensions \
cflags-$(CONFIG_X86) += -m$(BITS) -D__KERNEL__ $(CC_FLAGS_DIALECT) \
-fPIC -fno-strict-aliasing -mno-red-zone \
-mno-mmx -mno-sse -fshort-wchar \
-Wno-pointer-sign \
$(call cc-disable-warning, address-of-packed-member) \
$(if $(CONFIG_CC_IS_CLANG),-Wno-gnu -Wno-microsoft-anon-tag) \
-fno-asynchronous-unwind-tables \
$(CLANG_FLAGS)

5
scripts/Makefile.warn
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@ -28,11 +28,6 @@ endif
KBUILD_CFLAGS-$(CONFIG_CC_NO_ARRAY_BOUNDS) += -Wno-array-bounds
ifdef CONFIG_CC_IS_CLANG
# The kernel builds with '-std=gnu11' and '-fms-extensions' so use of GNU and
# Microsoft extensions is acceptable.
KBUILD_CFLAGS += -Wno-gnu
KBUILD_CFLAGS += -Wno-microsoft-anon-tag
# 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-option, -Wno-format-overflow-non-kprintf)