linux-kernel - Re: [PATCH] kbuild: userprogs: use correct linker when mixing clang and GNU ld

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Message-ID: <20250730004717.GA2984390@ax162>
Date: Tue, 29 Jul 2025 17:47:17 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: Masahiro Yamada <masahiroy@...nel.org>,
	Nicolas Schier <nicolas.schier@...ux.dev>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
	stable@...r.kernel.org
Subject: Re: [PATCH] kbuild: userprogs: use correct linker when mixing clang
 and GNU ld

On Fri, Jul 25, 2025 at 12:36:34PM +0200, Thomas Weißschuh wrote:
> FWIW some architectures use GNU ld implicitly with clang because they also link
> through $(CC) but do not use --ld-path. One example is UML, where the vDSO and
> vmlinux are linked this way. But linking vmlinux of UML with ld.lld will
> require changes to at least the linker script. Something for the ClangBuiltLinux
> TODO? There were more examples, but I don't remember them right now.

Yes, I believe this is the issue we had for tracking using ld.lld with
UML: https://github.com/ClangBuiltLinux/linux/issues/1715

I had not considered that it could be a linker script handling
difference. I will have to look into that soon.

> Longterm --ld-path should probably be added to the global KBUILD_CFLAGS, too.

'--ld-path' is only relevant when the linking phase is run by the
compiler, which is not really normal for the primary kernel build, as
calling the linker directly with $(LD) is preferred. Doing that would
break the build because of -Werror=unused-command-line-argument, which
is needed to make cc-option work correctly.

  $ echo 'int main(void) { return 0; }' | /usr/bin/clang --ld-path=/usr/bin/ld.lld -c -o /dev/null -x c -
  clang: warning: argument unused during compilation: '--ld-path=/usr/bin/ld.lld' [-Wunused-command-line-argument]

  $ echo 'int main(void) { return 0; }' | /usr/bin/clang --ld-path=/usr/bin/ld.lld -o /dev/null -x c -

> No, it isn't respected. On the other hand I didn't yet run into any issues.
> Do we want to fix it proactively?

No, I think it is fine to just leave it as is and fix it if it comes up
in the future, as I believe getting LLVM_IAS=1 working for sparc64 is
the next major focus of the whole LLVM sparc endeavour.

Cheers,
Nathan