Re: [PATCH v8 12/12] efi/libstub: disable SCS

Ard Biesheuvel
2020-02-19T07:41:10Z

On Wed, 19 Feb 2020 at 01:09, Sami Tolvanen <samitolvanen@google.com> wrote:
>
> Disable SCS for the EFI stub and allow x18 to be used.
>
> Suggested-by: James Morse <james.morse@arm.com>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
> drivers/firmware/efi/libstub/Makefile | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index 98a81576213d..dff9fa5a3f1c 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -30,6 +30,9 @@ KBUILD_CFLAGS := $(cflags-y) -DDISABLE_BRANCH_PROFILING \
> $(call cc-option,-fno-stack-protector) \
> -D__DISABLE_EXPORTS
>
> +# remove SCS flags from all objects in this directory
> +KBUILD_CFLAGS := $(filter-out -ffixed-x18 $(CC_FLAGS_SCS), $(KBUILD_CFLAGS))
> +

I don't see why you'd need to remove -ffixed-x18 again here. Not using
x18 anywhere in the kernel is a much more maintainable approach.

In fact, now that I think of it, the EFI AArch64 platform binding
forbids the use of x18, so it would be better to add the -ffixed-x18
unconditionally for arm64 (even though the reason it forbids it is to
ensure compatibility with an OS using it as a platform register, and
so nothing is actually broken atm).

> GCOV_PROFILE := n
> KASAN_SANITIZE := n
> UBSAN_SANITIZE := n
> --
> 2.25.0.265.gbab2e86ba0-goog
>