linux-kernel - Re: [PATCH] Makefile: add -fuse-ld=lld to KBUILD

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Date:   Sat, 22 Aug 2020 14:13:38 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     stable <stable@...r.kernel.org>,
        Matthias Maennich <maennich@...gle.com>,
        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>
Subject: Re: [PATCH] Makefile: add -fuse-ld=lld to KBUILD_HOSTLDFLAGS when LLVM=1

On Fri, Aug 21, 2020 at 7:10 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@...glegroups.com> wrote:
>
> While moving Android kernels over to use LLVM=1, we observe the failure
> when building in a hermetic docker image:
>   HOSTCC  scripts/basic/fixdep
> clang: error: unable to execute command: Executable "ld" doesn't exist!
>
> The is because the build of the host utility fixdep builds the fixdep
> executable in one step by invoking the compiler as the driver, rather
> than individual compile then link steps.
>
> Clang when configured from source defaults to use the system's linker,
> and not LLVM's own LLD, unless the CMake config
> -DCLANG_DEFAULT_LINKER='lld' is set when configuring a build of clang
> itself.
>
> Don't rely on the compiler's implicit default linker; be explicit.


I do not understand this patch.

The host compiler should be able to link executables
without any additional settings.

So, can you link a hello world program
in your docker?

masahiro@zoe:~$ cat test.c
#include <stdio.h>
int main(void)
{
        printf("helloworld\n");
        return 0;
}
masahiro@zoe:~$ clang test.c


If this fails, your environment is broken.

Just do  -DCLANG_DEFAULT_LINKER='lld'
if you know GNU ld is missing in your docker environment.








> Cc: stable@...r.kernel.org
> Fixes: commit a0d1c951ef08 ("kbuild: support LLVM=1 to switch the default tools to Clang/LLVM")
> Reported-by: Matthias Maennich <maennich@...gle.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
>  Makefile | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/Makefile b/Makefile
> index def590b743a9..b4e93b228a26 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -436,6 +436,7 @@ OBJDUMP             = llvm-objdump
>  READELF                = llvm-readelf
>  OBJSIZE                = llvm-size
>  STRIP          = llvm-strip
> +KBUILD_HOSTLDFLAGS     += -fuse-ld=lld
>  else
>  CC             = $(CROSS_COMPILE)gcc
>  LD             = $(CROSS_COMPILE)ld
> --
> 2.28.0.297.g1956fa8f8d-goog
>
> --
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-- 
Best Regards
Masahiro Yamada