Re: [PATCH] MIPS: vdso: Wrap -mexplicit-relocs in cc-option

[Nathan Chancellor]

On Tue, Feb 18, 2020 at 10:28:15AM -0800, Nick Desaulniers wrote:
> On Mon, Feb 17, 2020 at 1:11 PM Nathan Chancellor
> <natechancellor@xxxxxxxxx> wrote:
> >
> > Clang does not support this option and errors out:
> >
> > clang-11: error: unknown argument: '-mexplicit-relocs'
> >
> > Clang does not appear to need this flag like GCC does because the jalr
> > check that was added in commit 976c23af3ee5 ("mips: vdso: add build
> > time check that no 'jalr t9' calls left") passes just fine with
> >
> > $ make ARCH=mips CC=clang CROSS_COMPILE=mipsel-linux-gnu- malta_defconfig arch/mips/vdso/
> >
> > even before commit d3f703c4359f ("mips: vdso: fix 'jalr t9' crash in
> > vdso code").
> >
> > -mrelax-pic-calls has been supported since clang 9, which is the
> > earliest version that could build a working MIPS kernel, and it is the
> > default for clang so just leave it be.
> >
> > Fixes: d3f703c4359f ("mips: vdso: fix 'jalr t9' crash in vdso code")
> > Link: https://github.com/ClangBuiltLinux/linux/issues/890
> > Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> 
> Thanks for the patch, and detailed context.
> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> 
> With this patch applied:
> $ make ARCH=mips CC=clang CROSS_COMPILE=mipsel-linux-gnu-
> malta_defconfig arch/mips/vdso/
> $ lvm-objdump --disassemble arch/mips/vdso/vdso.so.dbg.raw | egrep -h "jarl.*t9"
> $ llvm-objdump --disassemble arch/mips/vdso/vdso.so.dbg.raw | grep jarl
> So jarl instructions aren't emitted.
> 
> Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

Thank you for double checking and the review!

Cheers,
Nathan