arm64: kprobes: WARN if attempting to step with PSTATE.D=1

Stepping with PSTATE.D=1 is bad news. The step won't generate a debug
exception and we'll likely walk off into random data structures. This
should never happen, but when it does, it's a PITA to debug. Add a
WARN_ON to shout if we realise this is about to take place.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/probes/kprobes.c

@@ -254,6 +254,8 @@ static void __kprobes setup_singlestep(struct kprobe *p,
 
 		if (kcb->kprobe_status == KPROBE_REENTER)
 			spsr_set_debug_flag(regs, 0);
+		else
+			WARN_ON(regs->pstate & PSR_D_BIT);
 
 		/* IRQs and single stepping do not mix well. */
 		kprobes_save_local_irqflag(kcb, regs);