[PATCH 1/2] kbuild: Consolidate C dialect options

Nathan Chancellor
2026-02-23T19:11:05Z

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>
---
Makefile | 16 ++++++++++++----
arch/arm64/kernel/vdso32/Makefile | 3 +--
arch/loongarch/vdso/Makefile | 2 +-
arch/parisc/boot/compressed/Makefile | 2 +-
arch/powerpc/boot/Makefile | 3 +--
arch/s390/Makefile | 3 +--
arch/s390/purgatory/Makefile | 3 +--
arch/x86/Makefile | 6 +-----
arch/x86/boot/compressed/Makefile | 6 +-----
drivers/firmware/efi/libstub/Makefile | 3 +--
scripts/Makefile.warn | 5 -----
11 files changed, 21 insertions(+), 31 deletions(-)

diff --git a/Makefile b/Makefile
index e944c6e71e81..06ff3032a6bc 100644
--- a/Makefile
+++ b/Makefile
@@ -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

diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
index 9d0efed91414..bea3675fa668 100644
--- a/arch/arm64/kernel/vdso32/Makefile
+++ b/arch/arm64/kernel/vdso32/Makefile
@@ -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.
diff --git a/arch/loongarch/vdso/Makefile b/arch/loongarch/vdso/Makefile
index 520f1513f07d..904ef4a0d826 100644
--- a/arch/loongarch/vdso/Makefile
+++ b/arch/loongarch/vdso/Makefile
@@ -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)
diff --git a/arch/parisc/boot/compressed/Makefile b/arch/parisc/boot/compressed/Makefile
index f8481e4e9d21..14eefb5ed5d1 100644
--- a/arch/parisc/boot/compressed/Makefile
+++ b/arch/parisc/boot/compressed/Makefile
@@ -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
diff --git a/arch/powerpc/boot/Makefile b/arch/powerpc/boot/Makefile
index f1a4761ebd44..7ee0fa2d3371 100644
--- a/arch/powerpc/boot/Makefile
+++ b/arch/powerpc/boot/Makefile
@@ -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

diff --git a/arch/s390/Makefile b/arch/s390/Makefile
index d78ad6885ca2..c8d16aca1cdc 100644
--- a/arch/s390/Makefile
+++ b/arch/s390/Makefile
@@ -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))
diff --git a/arch/s390/purgatory/Makefile b/arch/s390/purgatory/Makefile
index 61d240a37633..95a8ac45b67e 100644
--- a/arch/s390/purgatory/Makefile
+++ b/arch/s390/purgatory/Makefile
@@ -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))
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5f881460a8b5..46fec0b08487 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -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
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 68f9d7a1683b..8924196927bf 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -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
diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index e386ffd009b7..fbb8d4e33256 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -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)

diff --git a/scripts/Makefile.warn b/scripts/Makefile.warn
index 5567da6c7dfe..e77ca875aea4 100644
--- a/scripts/Makefile.warn
+++ b/scripts/Makefile.warn
@@ -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)

--
2.53.0