tracing: Fix invalid function_graph entry

When print_graph_entry() computes a function call entry event, it needs
to also check the next entry to guess if it matches the return event of
the current function entry.
In order to look at this next event, it needs to consume the current
entry before going ahead in the ring buffer.

However, if the current event that gets consumed is the last one in the
ring buffer head page, the ring_buffer may reuse the page for writers.
The consumed entry will then become invalid because of possible
racy overwriting.

Me must then handle this entry by making a copy of it.

The fix also applies on 2.6.30

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: stable@kernel.org
LKML-Reference: <4A6EEAEC.3050508@cn.fujitsu.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

kernel/trace/trace_functions_graph.c

@@ -843,9 +843,16 @@ 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 can be 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;