linux-kernel - Re: [PATCH] kbuild: Reject unexpected values for LLVM=

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Message-ID: <20260112231624.GA2272167@ax162>
Date: Mon, 12 Jan 2026 16:16:24 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: Nicolas Schier <nsc@...nel.org>,
	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
Subject: Re: [PATCH] kbuild: Reject unexpected values for LLVM=

On Mon, Jan 12, 2026 at 07:43:52AM +0100, Thomas Weißschuh wrote:
> The LLVM argument is documented to accept one of three forms:
> * a literal '1' to use the default 'clang',
> * a toolchain prefix path, ending in a trailing '/',
> * a version suffix.
> 
> All other values are silently treated as '1'. If for example
> the user accidentally forgets the trailing '/' of a toolchain prefix,
> kbuild will unexpectedly and silently fall back to the system toolchain.

Yeah, I do not think falling back to the system toolchain is great here.
This would also catch misuse of LLVM=0, since that is treated as LLVM=1,
rather than reverting to GCC + binutils.

> Instead report an error if the user specified an invalid value for LLVM.
> 
> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> ---
>  Makefile | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index e404e4767944..2a085a1b6875 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -447,6 +447,8 @@ ifneq ($(filter %/,$(LLVM)),)
>  LLVM_PREFIX := $(LLVM)
>  else ifneq ($(filter -%,$(LLVM)),)
>  LLVM_SUFFIX := $(LLVM)
> +else ifneq ($(LLVM),1)
> +$(error Invalid value for LLVM=)

If having a hard error is untenable (I am not sure that it is), we could
just warn. Pointing to Documentation/kbuild/llvm.rst might be worthwhile
in the message as well.

>  endif
>  
>  HOSTCC	= $(LLVM_PREFIX)clang$(LLVM_SUFFIX)
> 
> ---
> base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
> change-id: 20260111-kbuild-llvm-arg-c7346a4f2b3a
> 
> Best regards,
> -- 
> Thomas Weißschuh <linux@...ssschuh.net>
> 
>