MIPS: vdso: Wrap -mexplicit-relocs in cc-option

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(-)

arch/mips/vdso/Makefile +1 -2

index 77374c1f0c77..d7fe8408603e 100644
@@ -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)
--

Quoted 14 lines · Nathan Chancellor

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").

Quoted 7 lines

-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]>

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.

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]>