[PATCH] tracing: fix invalid function_graph entry

[Lai Jiangshan <laijs@cn.fujitsu.com>]

print_graph_entry() consumes current event, if this event
is the last event in the page, the ring_buffer may reuse
the page. It will become invalid.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index abf7c4a..02102a3 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -835,9 +835,16 @@ print_graph_function(struct trace_iterator *iter)
 
 	switch (entry->type) {
 	case TRACE_GRAPH_ENT: {
-		struct ftrace_graph_ent_entry *field;
+		/*
+		 * print_graph_entry() may consume the current event,
+		 * thus @field may become invalid, so we need to save it.
+		 * sizeof(struct ftrace_graph_ent_entry) is very small,
+		 * it is safely saved at the stack.
+		 */
+		struct ftrace_graph_ent_entry *field, saved;
 		trace_assign_type(field, entry);
-		return print_graph_entry(field, s, iter);
+		saved = *field;
+		return print_graph_entry(&saved, s, iter);
 	}
 	case TRACE_GRAPH_RET: {
 		struct ftrace_graph_ret_entry *field;