On Thu, Feb 13, 2025 at 10:54:37AM -0700, Nathan Chancellor wrote:
> Hi Thomas,
>
> On Thu, Feb 13, 2025 at 03:55:18PM +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.
> >
> > Fix this by passing -fuse-lld and let clang find its matching lld.
> >
> > Fixes: 7f3a59db274c ("kbuild: add infrastructure to build userspace programs")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>
>
> First of all, thank you for catching and noticing this!
>
> > ---
> > Makefile | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/Makefile b/Makefile
> > index bb5737ce7f9e79f4023c9c1f578a49a951d1e239..b4c208ae4041c1f4e32c2a158322422ce7353d06 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -510,6 +510,7 @@ OBJCOPY = $(LLVM_PREFIX)llvm-objcopy$(LLVM_SUFFIX)
> > OBJDUMP = $(LLVM_PREFIX)llvm-objdump$(LLVM_SUFFIX)
> > READELF = $(LLVM_PREFIX)llvm-readelf$(LLVM_SUFFIX)
> > STRIP = $(LLVM_PREFIX)llvm-strip$(LLVM_SUFFIX)
> > +KBUILD_USERLDFLAGS += -fuse-ld=lld
>
> Now that our minimum supported version upstream is 13.0.1, I think we
> can make this
>
> KBUILD_USERLDFLAGS += --ld-path=$(LD)
>
> as it should respect the user's choice of linker a little bit more, such
> as if they specific LLVM=<prefix>/bin/ or LLVM=-20. That reminds me that
> I can clean up what I did in commit 4406b12214f6 ("powerpc/vdso: Link
> with ld.lld when requested").
That is much better, thanks!
I looked for something like this but could't find it.
> Additionally, this would not fix someone using CC=clang and LD=ld.lld
> (it is uncommon but still techincally supported) so could we use a check
> like
>
> ifeq ($(CONFIG_CC_IS_CLANG)$(CONFIG_LD_IS_LLD),yy)
> KBUILD_USERLDFLAGS += --ld-path=$(LD)
> endif
>
> further down in Makefile to make it more robust?
Ack.
> The stable backport may want to use cc-option like I did for the powerpc
> vdso since there is a lower minimum supported version of LLVM there.
Ack.
FYI, in case you want to have a look:
The kunit config for powerpc is currently broken on LLVM=1.
$ ./tools/testing/kunit/kunit.py run --arch powerpc --make_options LLVM=1
arch/powerpc/boot/wrapper manually inspects with CROSS_COMPILE, which does not
exist when LLVM=1. Instead it falls back to the system objcopy, etc.
> > else
> > CC = $(CROSS_COMPILE)gcc
> > LD = $(CROSS_COMPILE)ld
