Re: [PATCH] kbuild: add $(CLANG_CFLAGS) to KBUILD_CPPFLAGS

[Nathan Chancellor <nathan@kernel.org>]

Hi Masahiro,

On Sun, Apr 09, 2023 at 11:53:57PM +0900, Masahiro Yamada wrote:
> When preprocessing arch/*/kernel/vmlinux.lds.S, the target triple is
> not passed to $(CPP) because we add it only to KBUILD_{C,A}FLAGS.
> 
> As a result, the linker script is preprocessed with predefined macros
> for the build host instead of the target.
> 
> Assuming you use an x86 build machine, compare the following:
> 
>  $ clang -dM -E -x c /dev/null
>  $ clang -dM -E -x c /dev/null -target aarch64-linux-gnu
> 
> There is no actual problem presumably because our linker scripts do not
> rely on such predefined macros, but it is better to define correct ones.
> 
> Move $(CFLAGS_CFLAGS) to KBUILD_CPPFLAGS, so that all *.c, *.S, *.lds.S
> will be processed with the proper target triple.
> 
> Reported-by: Tom Rini <trini@konsulko.com>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> ---
> 
>  scripts/Makefile.clang | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
> index 70b354fa1cb4..93ca059cc3b8 100644
> --- a/scripts/Makefile.clang
> +++ b/scripts/Makefile.clang
> @@ -38,6 +38,5 @@ CLANG_FLAGS	+= -Werror=unknown-warning-option
>  CLANG_FLAGS	+= -Werror=ignored-optimization-argument
>  CLANG_FLAGS	+= -Werror=option-ignored
>  CLANG_FLAGS	+= -Werror=unused-command-line-argument
> -KBUILD_CFLAGS	+= $(CLANG_FLAGS)
> -KBUILD_AFLAGS	+= $(CLANG_FLAGS)
> +KBUILD_CPPFLAGS	+= $(CLANG_FLAGS)
>  export CLANG_FLAGS
> -- 
> 2.37.2
> 

I am doubling back to this change, as the lack of '--target' in
KBUILD_CPPFLAGS is now an active bug with clang-17 due to a new change
that rejects '-mbig-endian' and '-mlittle-endian' when not supported by
the target, which breaks the arm64 vDSO build when preprocessing its
linker script:

  # Turn on CONFIG_CPU_BIG_ENDIAN in menuconfig
  $ make -skj"$(nproc)" ARCH=arm64 LLVM=1 O=build mrproper virtconfig menuconfig arch/arm64/kernel/vdso/
  ...
  clang: error: unsupported option '-mbig-endian' for target 'x86_64-pc-linux-gnu'
  make[3]: *** [.../scripts/Makefile.build:387: arch/arm64/kernel/vdso/vdso.lds] Error 1
  ...

  https://github.com/llvm/llvm-project/commit/d81ce04587c006b6731198956c522c93d0df1050
  https://github.com/ClangBuiltLinux/linux/issues/1859

This change resolves that issue. I was able to figure out why those new
warnings appeared for ARCH=mips, it is the shell invocation for
CHECKFLAGS. The following diff resolves it for me:

diff --git a/arch/mips/Makefile b/arch/mips/Makefile
index a7a4ee66a9d3..ef7b05ae92ce 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -346,7 +346,7 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
 KBUILD_LDFLAGS		+= -m $(ld-emul)
 
 ifdef CONFIG_MIPS
-CHECKFLAGS += $(shell $(CC) $(KBUILD_CFLAGS) -dM -E -x c /dev/null | \
+CHECKFLAGS += $(shell $(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -dM -E -x c /dev/null | \
 	grep -E -vw '__GNUC_(MINOR_|PATCHLEVEL_)?_' | \
 	sed -e "s/^\#define /-D'/" -e "s/ /'='/" -e "s/$$/'/" -e 's/\$$/&&/g')
 endif

I will run this change plus that diff through my build matrix to see if
any other issues pop up. If not, I will respond with some tags and
perhaps this could be taken as a fix for 6.4 so that it could
potentially be backported?

Cheers,
Nathan