arm64: stacktrace: fix the usage of ftrace_graph_ret_addr()

ftrace_graph_ret_addr() takes an 'idx' integer pointer that is used to
optimize the stack unwinding process. arm64 currently passes `NULL` for
this parameter which stops it from utilizing these optimizations.

Further, the current code for ftrace_graph_ret_addr() will just return
the passed in return address if it is NULL which will break this usage.

Pass a valid integer pointer to ftrace_graph_ret_addr() similar to
x86_64's stack unwinder.

Signed-off-by: Puranjay Mohan <[email protected]>
Fixes: 29c1c24 ("function_graph: Fix up ftrace_graph_ret_addr()")
Acked-by: Steven Rostedt (Google) <[email protected]>
Acked-by: Will Deacon <[email protected]>
Reviewed-by: Mark Rutland <[email protected]>
Tested-by: Mark Rutland <[email protected]>
Reviewed-by: Steven Rostedt (Google) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

Author: puranjaymohan

arch/arm64/kernel/stacktrace.c

@@ -25,13 +25,15 @@
  *
  * @common:      Common unwind state.
  * @task:        The task being unwound.
+ * @graph_idx:   Used by ftrace_graph_ret_addr() for optimized stack unwinding.
  * @kr_cur:      When KRETPROBES is selected, holds the kretprobe instance
  *               associated with the most recently encountered replacement lr
  *               value.
  */
 struct kunwind_state {
 	struct unwind_state common;
 	struct task_struct *task;
+	int graph_idx;
 #ifdef CONFIG_KRETPROBES
 	struct llist_node *kr_cur;
 #endif
@@ -106,7 +108,7 @@ kunwind_recover_return_address(struct kunwind_state *state)
 	if (state->task->ret_stack &&
 	    (state->common.pc == (unsigned long)return_to_handler)) {
 		unsigned long orig_pc;
-		orig_pc = ftrace_graph_ret_addr(state->task, NULL,
+		orig_pc = ftrace_graph_ret_addr(state->task, &state->graph_idx,
 						state->common.pc,
 						(void *)state->common.fp);
 		if (WARN_ON_ONCE(state->common.pc == orig_pc))