Re: [PATCH 2/2] powerpc/vdso: Link with ld.lld when requested

[Nathan Chancellor <nathan@kernel.org>]

On Tue, May 10, 2022 at 04:22:12PM +1000, Alexey Kardashevskiy wrote:
> 
> 
> On 5/10/22 06:46, Nathan Chancellor wrote:
> > The PowerPC vDSO is linked with $(CC) instead of $(LD), which means the
> > default linker of the compiler is used instead of the linker requested
> > by the builder.
> > 
> >    $ make ARCH=powerpc LLVM=1 mrproper defconfig arch/powerpc/kernel/vdso/
> >    ...
> > 
> >    $ llvm-readelf -p .comment arch/powerpc/kernel/vdso/vdso{32,64}.so.dbg
> > 
> >    File: arch/powerpc/kernel/vdso/vdso32.so.dbg
> >    String dump of section '.comment':
> >    [     0] clang version 14.0.0 (Fedora 14.0.0-1.fc37)
> > 
> >    File: arch/powerpc/kernel/vdso/vdso64.so.dbg
> >    String dump of section '.comment':
> >    [     0] clang version 14.0.0 (Fedora 14.0.0-1.fc37)
> > 
> > The compiler option '-fuse-ld' tells the compiler which linker to use
> > when it is invoked as both the compiler and linker. Use '-fuse-ld=lld'
> > when LD=ld.lld has been specified (CONFIG_LD_IS_LLD) so that the vDSO is
> > linked with the same linker as the rest of the kernel.
> > 
> >    $ llvm-readelf -p .comment arch/powerpc/kernel/vdso/vdso{32,64}.so.dbg
> > 
> >    File: arch/powerpc/kernel/vdso/vdso32.so.dbg
> >    String dump of section '.comment':
> >    [     0] Linker: LLD 14.0.0
> >    [    14] clang version 14.0.0 (Fedora 14.0.0-1.fc37)
> > 
> >    File: arch/powerpc/kernel/vdso/vdso64.so.dbg
> >    String dump of section '.comment':
> >    [     0] Linker: LLD 14.0.0
> >    [    14] clang version 14.0.0 (Fedora 14.0.0-1.fc37)
> > 
> > LD can be a full path to ld.lld, which will not be handled properly by
> > '-fuse-ld=lld' if the full path to ld.lld is outside of the compiler's
> > search path. '-fuse-ld' can take a path to the linker but it is
> > deprecated in clang 12.0.0; '--ld-path' is preferred for this scenario.
> > 
> > Use '--ld-path' if it is supported, as it will handle a full path or
> > just 'ld.lld' properly. See the LLVM commit below for the full details
> > of '--ld-path'.
> > 
> > Link: https://github.com/ClangBuiltLinux/linux/issues/774
> > Link: https://github.com/llvm/llvm-project/commit/1bc5c84710a8c73ef21295e63c19d10a8c71f2f5
> > Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> > ---
> >   arch/powerpc/kernel/vdso/Makefile | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/powerpc/kernel/vdso/Makefile b/arch/powerpc/kernel/vdso/Makefile
> > index 954974287ee7..096b0bf1335f 100644
> > --- a/arch/powerpc/kernel/vdso/Makefile
> > +++ b/arch/powerpc/kernel/vdso/Makefile
> > @@ -48,6 +48,7 @@ UBSAN_SANITIZE := n
> >   KASAN_SANITIZE := n
> >   ccflags-y := -shared -fno-common -fno-builtin -nostdlib -Wl,--hash-style=both
> > +ccflags-$(CONFIG_LD_IS_LLD) += $(call cc-option,--ld-path=$(LD),-fuse-ld=lld)
> 
> 
> Out of curiosity - how does this work exactly? I can see --ld-path= in the
> output so it works but there is no -fuse-ld=lld, is the second argument of
> cc-option only picked when the first one is not supported?

Correct. See Documentation/kbuild/makefiles.rst for more information
about those macros if you are curious.

> Anyway,
> 
> Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>

Thanks a lot!

Cheers,
Nathan