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

[Alexey Kardashevskiy]

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 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)


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?

Anyway,

Tested-by: Alexey Kardashevskiy <aik at ozlabs.ru>
Reviewed-by: Alexey Kardashevskiy <aik at ozlabs.ru>



>   
>   CC32FLAGS := -Wl,-soname=linux-vdso32.so.1 -m32
>   AS32FLAGS := -D__VDSO32__ -s