ftrace: Check for null ret_stack on profile function graph entry function

There's a small race when function graph shutsdown and the calling of the
registered function graph entry callback. The callback must not reference
the task's ret_stack without first checking that it is not NULL. Note, when
a ret_stack is allocated for a task, it stays allocated until the task exits.
The problem here, is that function_graph is shutdown, and a new task was
created, which doesn't have its ret_stack allocated. But since some of the
functions are still being traced, the callbacks can still be called.

The normal function_graph code handles this, but starting with commit
8861dd30 ("ftrace: Access ret_stack->subtime only in the function
profiler") the profiler code references the ret_stack on function entry, but
doesn't check if it is NULL first.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=196611

Cc: stable@vger.kernel.org
Fixes: 8861dd30 ("ftrace: Access ret_stack->subtime only in the function profiler")
Reported-by: lilydjwg@gmail.com
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -889,6 +889,10 @@ static int profile_graph_entry(struct ftrace_graph_ent *trace)
 
 	function_profile_call(trace->func, 0, NULL, NULL);
 
+	/* If function graph is shutting down, ret_stack can be NULL */
+	if (!current->ret_stack)
+		return 0;
+
 	if (index >= 0 && index < FTRACE_RETFUNC_DEPTH)
 		current->ret_stack[index].subtime = 0;