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

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Message-Id: <20250217-kbuild-userprog-fixes-v2-1-4da179778be0@linutronix.de>
Date: Mon, 17 Feb 2025 08:27:54 +0100
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Masahiro Yamada <masahiroy@...nel.org>, 
 Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>, 
 Nick Desaulniers <ndesaulniers@...gle.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 Sam Ravnborg <sam@...nborg.org>
Cc: linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
 llvm@...ts.linux.dev, stable@...r.kernel.org, 
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: [PATCH v2] kbuild: userprogs: use correct lld when linking through
 clang

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>
---
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>