arm64: add on_accessible_stack()

Both unwind_frame() and dump_backtrace() try to check whether a stack
address is sane to access, with very similar logic. Both will need
updating in order to handle overflow stacks.

Factor out this logic into a helper, so that we can avoid further
duplication when we add overflow stacks.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>

arch/arm64/include/asm/stacktrace.h

@@ -57,4 +57,20 @@ static inline bool on_task_stack(struct task_struct *tsk, unsigned long sp)
 	return (low <= sp && sp < high);
 }
 
+/*
+ * We can only safely access per-cpu stacks from current in a non-preemptible
+ * context.
+ */
+static inline bool on_accessible_stack(struct task_struct *tsk, unsigned long sp)
+{
+	if (on_task_stack(tsk, sp))
+		return true;
+	if (tsk != current || preemptible())
+		return false;
+	if (on_irq_stack(sp))
+		return true;
+
+	return false;
+}
+
 #endif	/* __ASM_STACKTRACE_H */