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

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Date:   Mon, 20 Jul 2020 11:16:46 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang cross
 compilation

On Mon, Jul 20, 2020 at 11:12:22AM -0700, Fangrui Song wrote:
> When CROSS_COMPILE is set (e.g. aarch64-linux-gnu-), if
> $(CROSS_COMPILE)elfedit is found at /usr/bin/aarch64-linux-gnu-,
> 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 only searches for $(prefix)$needle and for example 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).
> 
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> Signed-off-by: Fangrui Song <maskray@...gle.com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1099

Sorry that I did not pay attention before but this needs

Cc: stable@...r.kernel.org

in the body so that it gets automatically backported into all of our
stable branches. I am not sure if Masahiro is okay with adding that
after the fact or if he will want a v2.

I am fine with having my signed-off-by on the patch but I did not really
do much :) I am fine with having that downgraded to

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Tested-by: Nathan Chancellor <natechancellor@...il.com>

if people find it odd.

Thanks for sending this along!

Cheers,
Nathan

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