linux-kernel - Re: [PATCH] kbuild: make Clang build userprogs for target architecture

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Date:   Mon, 29 Jun 2020 10:38:56 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Sam Ravnborg <sam@...nborg.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] kbuild: make Clang build userprogs for target architecture

On Mon, Jun 29, 2020 at 1:59 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> Programs added 'userprogs' should be compiled for the target
> architecture i.e. the same architecture as the kernel.
>
> GCC does this correctly since the target architecture is implied
> by the toolchain prefix.
>
> Clang builds standalone programs always for the host architecture
> because the target triple is currently missing.
>
> Fix this.
>
> Fixes: 7f3a59db274c ("kbuild: add infrastructure to build userspace programs")

This is a neat feature I didn't know about; looks relatively new.
What's the test case command line invocation to test this with Clang?

> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> ---
>
>  Makefile | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 73948798ce3f..cac29cc2ec25 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -970,8 +970,8 @@ LDFLAGS_vmlinux     += --pack-dyn-relocs=relr
>  endif
>
>  # Align the bit size of userspace programs with the kernel
> -KBUILD_USERCFLAGS  += $(filter -m32 -m64, $(KBUILD_CFLAGS))
> -KBUILD_USERLDFLAGS += $(filter -m32 -m64, $(KBUILD_CFLAGS))
> +KBUILD_USERCFLAGS  += $(filter -m32 -m64 --target=%, $(KBUILD_CFLAGS))
> +KBUILD_USERLDFLAGS += $(filter -m32 -m64 --target=%, $(KBUILD_CFLAGS))

That should be fine.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
https://www.gnu.org/software/make/manual/html_node/Text-Functions.html

>
>  # make the checker run with the right architecture
>  CHECKFLAGS += --arch=$(ARCH)
> --


-- 
Thanks,
~Nick Desaulniers