function-graph: always initialize task ret_stack

On creating a new task while running the function graph tracer, if
we fail to allocate the ret_stack, and then fail the fork, the
code will free the parent ret_stack. This is because the child
duplicated the parent and currently points to the parent's ret_stack.

This patch always initializes the task's ret_stack to NULL.

[ Impact: prevent crash of parent on low memory during fork ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 1664d3f33d38..bb081f37cac7 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -2738,6 +2738,9 @@ void unregister_ftrace_graph(void)
 /* Allocate a return stack for newly created task */
 void ftrace_graph_init_task(struct task_struct *t)
 {
+        /* Make sure we do not use the parent ret_stack */
+        t->ret_stack = NULL;
+
         if (atomic_read(&ftrace_graph_active)) {
                 struct ftrace_ret_stack *ret_stack;
 
@@ -2753,8 +2756,7 @@ void ftrace_graph_init_task(struct task_struct *t)
                 /* make curr_ret_stack visable before we add the ret_stack */
                 smp_wmb();
                 t->ret_stack = ret_stack;
-        } else
-                t->ret_stack = NULL;
+        }
 }
 
 void ftrace_graph_exit_task(struct task_struct *t)