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 <[email protected]>
---
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)
--
2.25.1
On Mon, Feb 17, 2020 at 1:11 PM Nathan Chancellor
<[email protected]> 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 <[email protected]>
Thanks for the patch, and detailed context.
Reviewed-by: Nick Desaulniers <[email protected]>
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 <[email protected]>
> ---
> 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
On Tue, Feb 18, 2020 at 10:28:15AM -0800, Nick Desaulniers wrote:
> On Mon, Feb 17, 2020 at 1:11 PM Nathan Chancellor
> <[email protected]> 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 <[email protected]>
>
> Thanks for the patch, and detailed context.
> Reviewed-by: Nick Desaulniers <[email protected]>
>
> 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 <[email protected]>
Thank you for double checking and the review!
Cheers,
Nathan
Hello,
Nathan Chancellor 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.
Applied to mips-fixes.
> commit 72cf3b3df423
> https://git.kernel.org/mips/c/72cf3b3df423
>
> Fixes: d3f703c4359f ("mips: vdso: fix 'jalr t9' crash in vdso code")
> Link: https://github.com/ClangBuiltLinux/linux/issues/890
> Signed-off-by: Nathan Chancellor <[email protected]>
> Reviewed-by: Nick Desaulniers <[email protected]>
> Tested-by: Nick Desaulniers <[email protected]>
> Signed-off-by: Paul Burton <[email protected]>
Thanks,
Paul
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