[PATCH] kbuild: Reject unexpected values for LLVM=

[PATCH] kbuild: Reject unexpected values for LLVM=

[Thomas Weißschuh]

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.

Instead report an error if the user specified an invalid value for LLVM.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.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=)
 endif
 
 HOSTCC	= $(LLVM_PREFIX)clang$(LLVM_SUFFIX)

---
base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
change-id: 20260111-kbuild-llvm-arg-c7346a4f2b3a

Best regards,
-- 
Thomas Weißschuh <linux@weissschuh.net>

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

[WangYuli]

Hi Thomas Weißschuh,

On 2026/1/12 14:43, 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.
>
> Instead report an error if the user specified an invalid value for LLVM.
>
"Falling back to the system toolchain" may be an acceptable behavior; I 
think reporting a warning here rather than an error would be more 
appropriate.

Thanks,

--

WangYuli

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

[Thomas Weißschuh]

On 2026-01-12 14:58:08+0800, WangYuli wrote:
> Hi Thomas Weißschuh,
> 
> On 2026/1/12 14:43, 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.
> > 
> > Instead report an error if the user specified an invalid value for LLVM.
> > 
> "Falling back to the system toolchain" may be an acceptable behavior; I
> think reporting a warning here rather than an error would be more
> appropriate.

For which value of 'LLVM' would a silent fallback be the thing to do?


Thomas

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

[Nathan Chancellor]

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@weissschuh.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@weissschuh.net>
> 
> 

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

[Nicolas Schier]

On Mon, Jan 12, 2026 at 04:16:24PM -0700, Nathan Chancellor wrote:
> 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@weissschuh.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.

I second to have a hard error here with the documentation pointer to
prevent building with an unexpected toolchain, and as there is also no
fallback for gcc toolchains.

Kind regards,
Nicolas