linux-kernel - Re: [PATCH v3] Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang cross compilation

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Date:   Wed, 22 Jul 2020 07:14:39 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Jian Cai <jiancai@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        stable <stable@...r.kernel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v3] Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang cross compilation

On Wed, Jul 22, 2020 at 2:31 AM 'Fangrui Song' via Clang Built Linux
<clang-built-linux@...glegroups.com> wrote:
>
> When CROSS_COMPILE is set (e.g. aarch64-linux-gnu-), if
> $(CROSS_COMPILE)elfedit is found at /usr/bin/aarch64-linux-gnu-elfedit,
> GCC_TOOLCHAIN_DIR will be set to /usr/bin/.  --prefix= will be set to
> /usr/bin/ and Clang as of 11 will search for both
> $(prefix)aarch64-linux-gnu-$needle and $(prefix)$needle.
>
> GCC searchs for $(prefix)aarch64-linux-gnu/$version/$needle,
> $(prefix)aarch64-linux-gnu/$needle and $(prefix)$needle. In practice,
> $(prefix)aarch64-linux-gnu/$needle rarely contains executables.
>
> To better model how GCC's -B/--prefix takes in effect in practice, newer
> Clang (since
> https://github.com/llvm/llvm-project/commit/3452a0d8c17f7166f479706b293caf6ac76ffd90)
> only searches for $(prefix)$needle. Currently it will find /usr/bin/as
> instead of /usr/bin/aarch64-linux-gnu-as.
>
> Set --prefix= to $(GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE)
> (/usr/bin/aarch64-linux-gnu-) so that newer Clang can find the
> appropriate cross compiling GNU as (when -no-integrated-as is in
> effect).
>
> Cc: stable@...r.kernel.org
> Reported-by: Nathan Chancellor <natechancellor@...il.com>
> Signed-off-by: Fangrui Song <maskray@...gle.com>
> Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
> Tested-by: Nathan Chancellor <natechancellor@...il.com>
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1099
> ---
> Changes in v2:
> * Updated description to add tags and the llvm-project commit link.
> * Fixed a typo.
>
> Changes in v3:
> * Add Cc: stable@...r.kernel.org
> ---
>  Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index 0b5f8538bde5..3ac83e375b61 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -567,7 +567,7 @@ ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),)
>  ifneq ($(CROSS_COMPILE),)
>  CLANG_FLAGS    += --target=$(notdir $(CROSS_COMPILE:%-=%))
>  GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
> -CLANG_FLAGS    += --prefix=$(GCC_TOOLCHAIN_DIR)
> +CLANG_FLAGS    += --prefix=$(GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE)



CROSS_COMPILE may contain the directory path
to the cross toolchains.


For example, I use aarch64-linux-gnu-*
installed in
/home/masahiro/tools/aarch64-linaro-7.5/bin



Basically, there are two ways to use it.

[1]
PATH=$PATH:/home/masahiro/tools/aarch64-linaro-7.5/bin
CROSS_COMPILE=aarch64-linux-gnu-


[2]
Without setting PATH,
CROSS_COMPILE=~/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu-



I usually do [2] (and so does intel's 0day bot).



This patch works for the use-case [1]
but if I do [2], --prefix is set to a strange path:

--prefix=/home/masahiro/tools/aarch64-linaro-7.5/bin//home/masahiro/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu-



Interestingly, the build is still successful.
Presumably Clang searches for more paths
when $(prefix)$needle is not found ?



I applied your patch and added -v option
to see which assembler was internally invoked:

 "/home/masahiro/tools/aarch64-linaro-7.5/lib/gcc/aarch64-linux-gnu/7.5.0/../../../../aarch64-linux-gnu/bin/as"
-EL -I ./arch/arm64/include -I ./arch/arm64/include/generated -I
./include -I ./arch/arm64/include/uapi -I
./arch/arm64/include/generated/uapi -I ./include/uapi -I
./include/generated/uapi -o kernel/smp.o /tmp/smp-2ec2c7.s


Ok, it looks like Clang found an alternative path
to the correct 'as'.




But, to keep the original behavior for both [1] and [2],
how about this?

CLANG_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))



Then, I can get this:

 "/home/masahiro/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu-as"
-EL -I ./arch/arm64/include -I ./arch/arm64/include/generated -I
./include -I ./arch/arm64/include/uapi -I
./arch/arm64/include/generated/uapi -I ./include/uapi -I
./include/generated/uapi -o kernel/smp.o /tmp/smp-16d76f.s





>  GCC_TOOLCHAIN  := $(realpath $(GCC_TOOLCHAIN_DIR)/..)
>  endif
>  ifneq ($(GCC_TOOLCHAIN),)
> --
> 2.28.0.rc0.105.gf9edc3c819-goog
>
> --
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--
Best Regards
Masahiro Yamada