Re: [PATCH 18/18] arm64: implement Shadow Call Stack

[Sami Tolvanen]

On Fri, Oct 18, 2019 at 10:23 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> I think scs_save() would better live in assembly in cpu_switch_to(),
> where we switch the stack and current. It shouldn't matter whether
> scs_load() is inlined or not, since the x18 value _should_ be invariant
> from the PoV of the task.

Note that there's also a call to scs_save in cpu_die, because the
current task's shadow stack pointer is only stored in x18 and we don't
want to lose it.

> We just need to add a TSK_TI_SCS to asm-offsets.c, and then insert a
> single LDR at the end:
>
>         mov     sp, x9
>         msr     sp_el0, x1
> #ifdef CONFIG_SHADOW_CALL_STACK
>         ldr     x18, [x1, TSK_TI_SCS]
> #endif
>         ret

TSK_TI_SCS is already defined, so yes, we could move this to
cpu_switch_to. I would still prefer to have the overflow check that's
in scs_thread_switch though.

Sami