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

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Date:   Tue, 18 Feb 2020 10:28:15 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paulburton@...nel.org>,
        linux-mips@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] MIPS: vdso: Wrap -mexplicit-relocs in cc-option

On Mon, Feb 17, 2020 at 1:11 PM Nathan Chancellor
<natechancellor@...il.com> 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@...il.com>

Thanks for the patch, and detailed context.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

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@...gle.com>

> ---
>  arch/mips/vdso/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/mips/vdso/Makefile b/arch/mips/vdso/Makefile
> index 77374c1f0c77..d7fe8408603e 100644
> --- a/arch/mips/vdso/Makefile
> +++ b/arch/mips/vdso/Makefile
> @@ -33,7 +33,7 @@ endif
>  cflags-vdso := $(ccflags-vdso) \
>         $(filter -W%,$(filter-out -Wa$(comma)%,$(KBUILD_CFLAGS))) \
>         -O3 -g -fPIC -fno-strict-aliasing -fno-common -fno-builtin -G 0 \
> -       -mrelax-pic-calls -mexplicit-relocs \
> +       -mrelax-pic-calls $(call cc-option, -mexplicit-relocs) \
>         -fno-stack-protector -fno-jump-tables -DDISABLE_BRANCH_PROFILING \
>         $(call cc-option, -fno-asynchronous-unwind-tables) \
>         $(call cc-option, -fno-stack-protector)
> --

-- 
Thanks,
~Nick Desaulniers