linux-kernel - Re: Build failure with GCC 15 (-std=gnu23)

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Message-ID: <20241119041550.GA573925@thelio-3990X>
Date: Mon, 18 Nov 2024 21:15:50 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Sam James <sam@...too.org>
Cc: Kostadin Shishmanov <kostadinshishmanov@...tonmail.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	arnd@...db.de, linux-kbuild@...r.kernel.org
Subject: Re: Build failure with GCC 15 (-std=gnu23)

On Tue, Nov 19, 2024 at 02:57:28AM +0000, Sam James wrote:
> Nathan Chancellor <nathan@...nel.org> writes:
> 
> > Hi Kostadin,
> >
> > Just a quick FYI off the bat, you only directed this to LKML, which is
> > basically like sending it into the void because very few people actually
> > read every message on LKML. I only caught it because I have a filter set
> > up for mentions of Clang and LLVM. I'd suggest adding at least the
> > Kbuild mailing list, which I have done now. I have also added Arnd
> > because I seem to recall him looking into how hard it would be to build
> > the kernel with C23.
> 
> FWIW, scripts/get_maintainers.pl for stddef.h and types.h doesn't
> include kbuild -- maybe we should add that in.

Yeah, it would be good to have someone own these files. Not sure it
makes sense for Kbuild to be it though, I merely suggested that since
the actual root cause of the error is more in Kbuild's realm.

> I can reproduce it with `make defconfig` at
> 158f238aa69d91ad74e535c73f552bd4b025109c in Linus' tree with just `make
> V=1 -j$(nproc) -l$(nproc)` (i.e. no CFLAGS manipulation at all).
> 
> ```
> # CC      drivers/firmware/efi/libstub/x86-5lvl.o
>   gcc -Wp,-MMD,drivers/firmware/efi/libstub/.x86-5lvl.o.d -nostdinc -I./arch/x86/include -I./arch/x86/include/generated  -I./include -I./arch/x86/include/uapi -I./arch/x86/include/genera
> ted/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/compiler-version.h -include ./include/linux/kconfig.h -include ./include/linux/compiler_types.h -D__KERNEL__
>  -fmacro-prefix-map=./= -mcmodel=small -m64 -D__KERNEL__ -fPIC -fno-strict-aliasing -mno-red-zone -mno-mmx -mno-sse -fshort-wchar -Wno-pointer-sign -Wno-address-of-packed-member -fno-asy
> nchronous-unwind-tables -Os -DDISABLE_BRANCH_PROFILING -include ./include/linux/hidden.h -D__NO_FORTIFY -ffreestanding -fno-stack-protector -D__DISABLE_EXPORTS    -DKBUILD_MODFILE='"driv
> ers/firmware/efi/libstub/x86-5lvl"' -DKBUILD_BASENAME='"x86_5lvl"' -DKBUILD_MODNAME='"x86_5lvl"' -D__KBUILD_MODNAME=kmod_x86_5lvl -c -o drivers/firmware/efi/libstub/x86-5lvl.o drivers/fi
> rmware/efi/libstub/x86-5lvl.c
> In file included from ./include/uapi/linux/posix_types.h:5,
>                  from ./include/uapi/linux/types.h:14,
>                  from ./include/linux/types.h:6,
>                  from ./include/linux/kasan-checks.h:5,
>                  from ./include/asm-generic/rwonce.h:26,
>                  from ./arch/x86/include/generated/asm/rwonce.h:1,
>                  from ./include/linux/compiler.h:317,
>                  from ./include/linux/build_bug.h:5,
>                  from ./include/linux/init.h:5,
>                  from ./include/linux/efi.h:15,
>                  from drivers/firmware/efi/libstub/file.c:10:
> ./include/linux/stddef.h:11:9: error: expected identifier before ‘false’
>    11 |         false   = 0,
>       |         ^~~~~
> ```
> 
> -std=gnu11 certainly isn't there.

Ugh, this is because drivers/firmware/efi/libstub does not use
KBUILD_CFLAGS from the rest of the kernel when targeting x86:

$ sed -n '9,21p' drivers/firmware/efi/libstub/Makefile
# non-x86 reuses KBUILD_CFLAGS, x86 does not
cflags-y                        := $(KBUILD_CFLAGS)

cflags-$(CONFIG_X86_32)         := -march=i386
cflags-$(CONFIG_X86_64)         := -mcmodel=small
cflags-$(CONFIG_X86)            += -m$(BITS) -D__KERNEL__ \
                                   -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) \
                                   -fno-asynchronous-unwind-tables \
                                   $(CLANG_FLAGS)

This isn't the first time this peculiarity has bitten us :/ sticking
'-std=gnu11' in there should resolve that issue.

arch/x86/boot/compressed/Makefile might need the same treatment. It
might make sense to introduce something like 'CSTD_FLAG := -std=gnu11'
then use that in the various places within the kernel that need it so it
can be consistently updated in the future whenever needed. I see that
flag in Makefile, arch/arm64/kernel/vdso32/Makefile, and
arch/x86/Makefile.

Cheers,
Nathan