linux-kernel - Re: [PATCH v2] kbuild: userprogs: use correct lld when linking through clang

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Message-ID: <20250217185456.GA435791@ax162>
Date: Mon, 17 Feb 2025 10:54:56 -0800
From: Nathan Chancellor <nathan@...nel.org>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: Masahiro Yamada <masahiroy@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Sam Ravnborg <sam@...nborg.org>, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
	stable@...r.kernel.org
Subject: Re: [PATCH v2] kbuild: userprogs: use correct lld when linking
 through clang

On Mon, Feb 17, 2025 at 08:27:54AM +0100, Thomas Weißschuh wrote:
> The userprog infrastructure links objects files through $(CC).
> Either explicitly by manually calling $(CC) on multiple object files or
> implicitly by directly compiling a source file to an executable.
> The documentation at Documentation/kbuild/llvm.rst indicates that ld.lld
> would be used for linking if LLVM=1 is specified.
> However clang instead will use either a globally installed cross linker
> from $PATH called ${target}-ld or fall back to the system linker, which
> probably does not support crosslinking.
> For the normal kernel build this is not an issue because the linker is
> always executed directly, without the compiler being involved.
> 
> Explicitly pass --ld-path to clang so $(LD) is respected.
> As clang 13.0.1 is required to build the kernel, this option is available.
> 
> Fixes: 7f3a59db274c ("kbuild: add infrastructure to build userspace programs")
> Cc: stable@...r.kernel.org # needs wrapping in $(cc-option) for < 6.9
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
> Reproducer, using nolibc to avoid libc requirements for cross building:
> 
> $ tail -2 init/Makefile
> userprogs-always-y += test-llvm
> test-llvm-userccflags += -nostdlib -nolibc -static -isystem usr/ -include $(srctree)/tools/include/nolibc/nolibc.h
> 
> $ cat init/test-llvm.c
> int main(void)
> {
> 	return 0;
> }
> 
> $ make ARCH=arm64 LLVM=1 allnoconfig headers_install init/
> 
> Validate that init/test-llvm builds and has the correct binary format.
> ---
> Changes in v2:
> - Use --ld-path instead of -fuse-ld
> - Drop already applied patch
> - Link to v1: https://lore.kernel.org/r/20250213-kbuild-userprog-fixes-v1-0-f255fb477d98@linutronix.de
> ---
>  Makefile | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index 96407c1d6be167b04ed5883e455686918c7a75ee..b41c164533d781d010ff8b2522e523164dc375d0 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1123,6 +1123,11 @@ endif
>  KBUILD_USERCFLAGS  += $(filter -m32 -m64 --target=%, $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS))
>  KBUILD_USERLDFLAGS += $(filter -m32 -m64 --target=%, $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS))
>  
> +# userspace programs are linked via the compiler, use the correct linker
> +ifeq ($(CONFIG_CC_IS_CLANG)$(CONFIG_LD_IS_LLD),yy)
> +KBUILD_USERLDFLAGS += --ld-path=$(LD)
> +endif
> +
>  # make the checker run with the right architecture
>  CHECKFLAGS += --arch=$(ARCH)
>  
> 
> ---
> base-commit: 0ad2507d5d93f39619fc42372c347d6006b64319
> change-id: 20250213-kbuild-userprog-fixes-4f07b62ae818
> 
> Best regards,
> -- 
> Thomas Weißschuh <thomas.weissschuh@...utronix.de>
> 
>