arm64: make irq_stack_ptr more robust

Switching between stacks is only valid if we are tracing ourselves while on the
irq_stack, so it is only valid when in current and non-preemptible context,
otherwise is is just zeroed off.

Fixes: 132cd887b5c5 ("arm64: Modify stack trace and dump for use with irq_stack")
Acked-by: James Morse <[e-mail]>
Tested-by: James Morse <[e-mail]>
Signed-off-by: Yang Shi <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/kernel/stacktrace.c

···
 	unsigned long irq_stack_ptr;
 
 	/*
-	 * Use raw_smp_processor_id() to avoid false-positives from
-	 * CONFIG_DEBUG_PREEMPT. get_wchan() calls unwind_frame() on sleeping
-	 * task stacks, we can be pre-empted in this case, so
-	 * {raw_,}smp_processor_id() may give us the wrong value. Sleeping
-	 * tasks can't ever be on an interrupt stack, so regardless of cpu,
-	 * the checks will always fail.
+	 * Switching between stacks is valid when tracing current and in
+	 * non-preemptible context.
 	 */
-	irq_stack_ptr = IRQ_STACK_PTR(raw_smp_processor_id());
+	if (tsk == current && !preemptible())
+		irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
+	else
+		irq_stack_ptr = 0;
 
 	low  = frame->sp;
 	/* irq stacks are not THREAD_SIZE aligned */

arch/arm64/kernel/traps.c

···
 static void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
 {
 	struct stackframe frame;
-	unsigned long irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
+	unsigned long irq_stack_ptr;
 	int skip;
+
+	/*
+	 * Switching between stacks is valid when tracing current and in
+	 * non-preemptible context.
+	 */
+	if (tsk == current && !preemptible())
+		irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
+	else
+		irq_stack_ptr = 0;
 
 	pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);