linux-kernel - Re: fms extension (Was: [PATCH] fs/pipe: stop duplicating union pipe

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Message-ID: <20251030-zukunft-reduzieren-323e5f33dca6@brauner>
Date: Thu, 30 Oct 2025 21:16:02 +0100
From: Christian Brauner <brauner@...nel.org>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Ard Biesheuvel <ardb@...nel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, linux-efi@...r.kernel.org, 
	Rasmus Villemoes <linux@...musvillemoes.dk>, Alexander Viro <viro@...iv.linux.org.uk>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org, 
	David Sterba <dsterba@...e.com>
Subject: Re: fms extension (Was: [PATCH] fs/pipe: stop duplicating union
 pipe_index declaration)

On Thu, Oct 30, 2025 at 10:29:18AM -0700, Nathan Chancellor wrote:
> On Thu, Oct 30, 2025 at 02:38:50PM +0100, Ard Biesheuvel wrote:
> > On Thu, 30 Oct 2025 at 14:23, Christian Brauner <brauner@...nel.org> wrote:
> > >
> > > On Wed, Oct 29, 2025 at 04:30:57PM -0700, Nathan Chancellor wrote:
> > > > On Thu, Oct 30, 2025 at 12:13:11AM +0100, Christian Brauner wrote:
> > > > > I'm fine either way. @Nathan, if you just want to give Linus the patch
> > > > > if it's small enough or just want to give me a stable branch I can pull
> > > > > I'll be content. Thanks!
> > > >
> > > > I do not care either way but I created a shared branch/tag since it was
> > > > easy enough to do. If Linus wants to take these directly for -rc4, I am
> > > > fine with that as well.
> > > >
> > > > Cheers,
> > > > Nathan
> > > >
> > > > The following changes since commit 3a8660878839faadb4f1a6dd72c3179c1df56787:
> > > >
> > > >   Linux 6.18-rc1 (2025-10-12 13:42:36 -0700)
> > > >
> > > > are available in the Git repository at:
> > > >
> > > >   git://git.kernel.org/pub/scm/linux/kernel/git/kbuild/linux.git tags/kbuild-ms-extensions-6.19
> > >
> > > Thanks, I pulled this and placed it into a branch that I can base other
> > > branches on.
> > >
> > > _But_, I'm already running into problems. :)
> > >
> > ...
> > >
> > > Because struct cgroup_namespace embeddds struct ns_common and it
> > > proliferates via mm stuff into the efi code.
> > >
> > > So the EFI cod has it's own KBUILD_CFLAGS. It does:
> > >
> > > # non-x86 reuses KBUILD_CFLAGS, x86 does not
> > > cflags-y                        := $(KBUILD_CFLAGS)
> > >
> > > <snip>
> > >
> > > KBUILD_CFLAGS                   := $(subst $(CC_FLAGS_FTRACE),,$(cflags-y)) \
> > >                                    -Os -DDISABLE_BRANCH_PROFILING \
> > >                                    -include $(srctree)/include/linux/hidden.h \
> > >                                    -D__NO_FORTIFY \
> > >                                    -ffreestanding \
> > >                                    -fno-stack-protector \
> > >                                    $(call cc-option,-fno-addrsig) \
> > >                                    -D__DISABLE_EXPORTS
> > >
> > > which means x86 doesn't get -fms-extension breaking the build. If I
> > > manually insert:
> > >
> > > diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> > > index 94b05e4451dd..4ad2f8f42134 100644
> > > --- a/drivers/firmware/efi/libstub/Makefile
> > > +++ b/drivers/firmware/efi/libstub/Makefile
> > > @@ -42,6 +42,8 @@ KBUILD_CFLAGS                 := $(subst $(CC_FLAGS_FTRACE),,$(cflags-y)) \
> > >                                    -ffreestanding \
> > >                                    -fno-stack-protector \
> > >                                    $(call cc-option,-fno-addrsig) \
> > > +                                  -fms-extensions \
> > > +                                  -Wno-microsoft-anon-tag \
> > >                                    -D__DISABLE_EXPORTS
> > >
> > > The build works...
> > >
> > > I think we need to decide how to fix this now because as soon as someone
> > > makes use of the extension that is indirectly included by that libstub
> > > thing we're fscked.
> > 
> > Unless anyone is feeling brave and wants to untangle the x86 command
> > line delta between the stub and core kernel, I suggest we just add
> > these flags just like you proposed (assuming all supported compilers
> > tolerate their presence)
> 
> There are several other places in the kernel that blow away
> KBUILD_CFLAGS like this that will need the same fix (I went off of
> searching for -std=gnu11, as that was needed in many places to fix GCC
> 15). It is possible that we might want to take the opportunity to unify
> these flags into something like KBUILD_DIALECT_CFLAGS but for now, I
> just bothered with adding the flags in the existing places.

That should hopefully do it. Can you update the shared branch with that
and then tell me when I can repull?

> 
> diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
> index ffa3536581f6..9d0efed91414 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
> +               -std=gnu11 -fms-extensions
>  VDSO_CFLAGS  += -O2
>  # Some useful compiler-dependent flags from top-level Makefile
>  VDSO_CFLAGS += $(call cc32-option,-Wno-pointer-sign)
> @@ -71,6 +71,7 @@ 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 d8316f993482..c0cc3ca5da9f 100644
> --- a/arch/loongarch/vdso/Makefile
> +++ b/arch/loongarch/vdso/Makefile
> @@ -19,7 +19,7 @@ ccflags-vdso := \
>  cflags-vdso := $(ccflags-vdso) \
>  	-isystem $(shell $(CC) -print-file-name=include) \
>  	$(filter -W%,$(filter-out -Wa$(comma)%,$(KBUILD_CFLAGS))) \
> -	-std=gnu11 -O2 -g -fno-strict-aliasing -fno-common -fno-builtin \
> +	-std=gnu11 -fms-extensions -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 17c42d718eb3..f8481e4e9d21 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
> +KBUILD_CFLAGS += -std=gnu11 -fms-extensions
>  
>  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 c47b78c1d3e7..f1a4761ebd44 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 \
> +		   -std=gnu11 -fms-extensions \
>  		   -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
>  		   -fno-strict-aliasing -O2 \
>  		   -msoft-float -mno-altivec -mno-vsx \
> @@ -86,6 +86,7 @@ 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 b4769241332b..8578361133a4 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
> +KBUILD_CFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -O2 -mpacked-stack -std=gnu11 -fms-extensions
>  KBUILD_CFLAGS_DECOMPRESSOR += -DDISABLE_BRANCH_PROFILING -D__NO_FORTIFY
>  KBUILD_CFLAGS_DECOMPRESSOR += -D__DECOMPRESSOR
>  KBUILD_CFLAGS_DECOMPRESSOR += -Wno-pointer-sign
> @@ -35,6 +35,7 @@ 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 bd39b36e7bd6..0c196a5b194a 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 -fno-strict-aliasing -Wall -Wstrict-prototypes
> +KBUILD_CFLAGS := -std=gnu11 -fms-extensions -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
> @@ -21,6 +21,7 @@ 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_AFLAGS := $(filter-out -DCC_USING_EXPOLINE,$(KBUILD_AFLAGS))
>  KBUILD_AFLAGS += -D__DISABLE_EXPORTS
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 4db7e4bf69f5..e20e25b8b16c 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -48,7 +48,8 @@ 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 -m16 -g -Os -DDISABLE_BRANCH_PROFILING -D__DISABLE_EXPORTS \
> +REALMODE_CFLAGS	:= -std=gnu11 -fms-extensions -m16 -g -Os \
> +		   -DDISABLE_BRANCH_PROFILING -D__DISABLE_EXPORTS \
>  		   -Wall -Wstrict-prototypes -march=i386 -mregparm=3 \
>  		   -fno-strict-aliasing -fomit-frame-pointer -fno-pic \
>  		   -mno-mmx -mno-sse $(call cc-option,-fcf-protection=none)
> @@ -60,6 +61,7 @@ REALMODE_CFLAGS += $(cc_stack_align4)
>  REALMODE_CFLAGS += $(CLANG_FLAGS)
>  ifdef CONFIG_CC_IS_CLANG
>  REALMODE_CFLAGS += -Wno-gnu
> +REALMODE_CFLAGS += -Wno-microsoft-anon-tag
>  endif
>  export REALMODE_CFLAGS
>  
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 74657589264d..68f9d7a1683b 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -25,7 +25,7 @@ targets := vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2 vmlinux.bin.lzma \
>  # avoid errors with '-march=i386', and future flags may depend on the target to
>  # be valid.
>  KBUILD_CFLAGS := -m$(BITS) -O2 $(CLANG_FLAGS)
> -KBUILD_CFLAGS += -std=gnu11
> +KBUILD_CFLAGS += -std=gnu11 -fms-extensions
>  KBUILD_CFLAGS += -fno-strict-aliasing -fPIE
>  KBUILD_CFLAGS += -Wundef
>  KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
> @@ -36,7 +36,10 @@ 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)
> -KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
> +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 94b05e4451dd..7d15a85d579f 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -11,12 +11,12 @@ cflags-y			:= $(KBUILD_CFLAGS)
>  
>  cflags-$(CONFIG_X86_32)		:= -march=i386
>  cflags-$(CONFIG_X86_64)		:= -mcmodel=small
> -cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ -std=gnu11 \
> +cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ -std=gnu11 -fms-extensions \
>  				   -fPIC -fno-strict-aliasing -mno-red-zone \
>  				   -mno-mmx -mno-sse -fshort-wchar \
>  				   -Wno-pointer-sign \
>  				   $(call cc-disable-warning, address-of-packed-member) \
> -				   $(call cc-disable-warning, gnu) \
> +				   $(if $(CONFIG_CC_IS_CLANG),-Wno-gnu -Wno-microsoft-anon-tag) \
>  				   -fno-asynchronous-unwind-tables \
>  				   $(CLANG_FLAGS)
>