[RFC]ftrace: fix a unallocated memory access in function_graph

[Shaohua Li <shaohua.li@intel.com>]

With CONFIG_DEBUG_PAGEALLOC, I observed a unallocated memory access in
function_graph trace. It appears we find a small size entry in ring buffer, but
we access it as a big size entry. The access overflows a page size and touch
a unallocated page.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>

diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index 79f4bac..33b379d 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -507,7 +507,10 @@ get_return_for_leaf(struct trace_iterator *iter,
 			 * if the output fails.
 			 */
 			data->ent = *curr;
-			data->ret = *next;
+			if (next->ent.type == TRACE_GRAPH_RET)
+				data->ret = *next;
+			else
+				data->ret.ent.type = next->ent.type;
 		}
 	}