ftrace: Fix regression with module command in stack_trace_filter

commit 45af52e upstream.

When executing the following command:

    # echo "write*:mod:ext3" > /sys/kernel/tracing/stack_trace_filter

The current mod command causes a null pointer dereference. While commit
0f17976 ("ftrace: Fix regression with module command in stack_trace_filter")
has addressed part of the issue, it left a corner case unhandled, which still
results in a kernel crash.

Cc: [email protected]
Cc: Masami Hiramatsu <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Link: https://lore.kernel.org/[email protected]
Fixes: 04ec7bb ("tracing: Have the trace_array hold the list of registered func probes");
Signed-off-by: guoweikang <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: guoweikang

kernel/trace/ftrace.c

@@ -4437,6 +4437,9 @@ ftrace_mod_callback(struct trace_array *tr, struct ftrace_hash *hash,
 	char *func;
 	int ret;
 
+	if (!tr)
+		return -ENODEV;
+
 	/* match_records() modifies func, and we need the original */
 	func = kstrdup(func_orig, GFP_KERNEL);
 	if (!func)