Re: [RFC PATCH v9 05/27] x86/cet/shstk: Add Kconfig option for user-mode Shadow Stack protection

Dave Hansen
2020-02-26T18:05:54Z

On 2/5/20 10:19 AM, Yu-cheng Yu wrote:
> +# Check assembler Shadow Stack suppot

^ support

> +ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
> + ifeq ($(call as-instr, saveprevssp, y),)
> + $(error CONFIG_X86_INTEL_SHADOW_STACK_USER not supported by the assembler)
> + endif
> +endif

Is this *just* looking for instruction support in the assembler?

We usually just .byte them, like this for pkeys:

asm volatile(".byte 0x0f,0x01,0xee\n\t"
: "=a" (pkru), "=d" (edx)
: "c" (ecx));

That way everybody with old toolchains can still build the kernel (and
run/test code with your config option on, btw...).