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

[Nick Desaulniers]

On Mon, May 9, 2022 at 1:47 PM Nathan Chancellor <nathan at kernel.org> 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'.

Perhaps worth adding some additional background from the cover letter
to the commit message that will actually go into the kernel,
particularly:
1. Kbuild mostly invokes the compiler and linker distinctly; the ppc
vdso code uses the compiler as the linker driver though.
2. When doing so, depending on how the compiler was configured, the
implicit default linker the compiler invokes might not match $LD.
3. This is a problem for LTO since clang may try to invoke ld.gold,
which is not supported as of
commit 75959d44f9dc ("kbuild: Fail if gold linker is detected")
4. Using the linker as the driver can cause ld.bfd 2.26 to crash.
https://lore.kernel.org/all/b2066ccd-2b81-6032-08e3-41105b400f75@csgroup.eu/
(Though, I wonder if that's because I was trying to add
--orphan-handling=warn, which we're not yet doing for the ppc vdso
AFAICT).

Reviewed-by: Nick Desaulniers <ndesaulniers at google.com>

>
> Link: https://github.com/ClangBuiltLinux/linux/issues/774
> Link: https://github.com/llvm/llvm-project/commit/1bc5c84710a8c73ef21295e63c19d10a8c71f2f5
> Signed-off-by: Nathan Chancellor <nathan at 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)
>
>  CC32FLAGS := -Wl,-soname=linux-vdso32.so.1 -m32
>  AS32FLAGS := -D__VDSO32__ -s
> --
> 2.36.1
>


-- 
Thanks,
~Nick Desaulniers