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

From: Masahiro Yamada <masahiroy@kernel.org>
To: Nathan Chancellor <nathan@kernel.org>
Cc: linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
	 Tom Rini <trini@konsulko.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	 Nicolas Schier <nicolas@fjasle.eu>, Tom Rix <trix@redhat.com>,
	llvm@lists.linux.dev
Subject: Re: [PATCH] kbuild: add $(CLANG_CFLAGS) to KBUILD_CPPFLAGS
Date: Sun, 4 Jun 2023 01:23:08 +0900	[thread overview]
Message-ID: <CAK7LNASeNAx5a1P9fkf+8+Fj4HFujPN=57fWtAoc7XL97LdkxA@mail.gmail.com> (raw)
In-Reply-To: <20230531213319.GA2201875@dev-arch.thelio-3990X>

On Thu, Jun 1, 2023 at 6:33 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> 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.

Ah. Now we have a good reason to apply this patch.

> 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

Agree. This is the right fix because this $(shell ...) retrieves
the predefined macros of the compiler.
So, KBUILD_CPPFLAGS is correct.

> 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

-- 
Best Regards
Masahiro Yamada