arm64: reduce stack use in irq_handler

The code for switching to irq_stack stores three pieces of information on
the stack, fp+lr, as a fake stack frame (that lets us walk back onto the
interrupted tasks stack frame), and the address of the struct pt_regs that
contains the register values from kernel entry. (which dump_backtrace()
will print in any stack trace).

To reduce this, we store fp, and the pointer to the struct pt_regs.
unwind_frame() can recognise this as the irq_stack dummy frame, (as it only
appears at the top of the irq_stack), and use the struct pt_regs values
to find the missing interrupted link-register.

Suggested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 16 insertions, 3 deletions

diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index d916d5b6aef6..b9fd3a8abfc1 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -70,17 +70,30 @@ int notrace unwind_frame(struct stackframe *frame)
 	 * Check whether we are going to walk through from interrupt stack
 	 * to task stack.
 	 * If we reach the end of the stack - and its an interrupt stack,
-	 * read the original task stack pointer from the dummy frame.
+	 * unpack the dummy frame to find the original elr.
 	 *
 	 * Check the frame->fp we read from the bottom of the irq_stack,
 	 * and the original task stack pointer are both in current->stack.
 	 */
 	if (frame->sp == irq_stack_ptr) {
+		struct pt_regs *irq_args;
 		unsigned long orig_sp = IRQ_STACK_TO_TASK_STACK(irq_stack_ptr);
 
-		if(object_is_on_stack((void *)orig_sp) &&
-		   object_is_on_stack((void *)frame->fp))
+		if (object_is_on_stack((void *)orig_sp) &&
+		   object_is_on_stack((void *)frame->fp)) {
 			frame->sp = orig_sp;
+
+			/* orig_sp is the saved pt_regs, find the elr */
+			irq_args = (struct pt_regs *)orig_sp;
+			frame->pc = irq_args->pc;
+		} else {
+			/*
+			 * This frame has a non-standard format, and we
+			 * didn't fix it, because the data looked wrong.
+			 * Refuse to output this frame.
+			 */
+			return -EINVAL;
+		}
 	}
 
 	return 0;