ftrace: fix race in function graph during fork

Impact: graph tracer race/crash fix

There is a nasy race in startup of a new process running the
function graph tracer. In fork.c:

	total_forks++;
	spin_unlock(&current->sighand->siglock);
	write_unlock_irq(&tasklist_lock);
	ftrace_graph_init_task(p);
	proc_fork_connector(p);
	cgroup_post_fork(p);
	return p;

The new task is free to run as soon as the tasklist_lock is released.
This is before the ftrace_graph_init_task. If the task does run
it will be using the same ret_stack and curr_ret_stack as the parent.
This will cause crashes that are difficult to debug.

This patch moves the ftrace_graph_init_task to just after the alloc_pid
code. This fixes the above race.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/fork.c

@@ -1137,6 +1137,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 		}
 	}
 
+	ftrace_graph_init_task(p);
+
 	p->pid = pid_nr(pid);
 	p->tgid = p->pid;
 	if (clone_flags & CLONE_THREAD)
@@ -1145,7 +1147,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	if (current->nsproxy != p->nsproxy) {
 		retval = ns_cgroup_clone(p, pid);
 		if (retval)
-			goto bad_fork_free_pid;
+			goto bad_fork_free_graph;
 	}
 
 	p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
@@ -1238,7 +1240,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 		spin_unlock(&current->sighand->siglock);
 		write_unlock_irq(&tasklist_lock);
 		retval = -ERESTARTNOINTR;
-		goto bad_fork_free_pid;
+		goto bad_fork_free_graph;
 	}
 
 	if (clone_flags & CLONE_THREAD) {
@@ -1271,11 +1273,12 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	total_forks++;
 	spin_unlock(&current->sighand->siglock);
 	write_unlock_irq(&tasklist_lock);
-	ftrace_graph_init_task(p);
 	proc_fork_connector(p);
 	cgroup_post_fork(p);
 	return p;
 
+bad_fork_free_graph:
+	ftrace_graph_exit_task(p);
 bad_fork_free_pid:
 	if (pid != &init_struct_pid)
 		free_pid(pid);