function-graph: move initialization of new tasks up in fork

When the function graph tracer is enabled, all new tasks must allocate
a ret_stack to place the return address of functions. This is because
the function graph tracer will replace the real return address with a
call to the tracing of the exit function.

This initialization happens in fork, but it happens too late. If fork
fails, then it will call free_task and that calls the freeing of this
ret_stack. But before initialization happens, the new (failed) task
points to its parents ret_stack. If a fork failure happens during
the function trace, it would be catastrophic for the parent.

Also, there's no need to call ftrace_graph_exit_task from fork, since
it is called by free_task which fork calls on failure.

[ Impact: prevent crash during failed fork running function graph tracer ]

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

1 files changed, 4 insertions, 6 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index b9e2edd00726..c4b1e35c430b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -982,6 +982,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
         if (!p)
                 goto fork_out;
 
+        ftrace_graph_init_task(p);
+
         rt_mutex_init_task(p);
 
 #ifdef CONFIG_PROVE_LOCKING
@@ -1131,8 +1133,6 @@ 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)
@@ -1141,7 +1141,7 @@ 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_graph;
+                        goto bad_fork_free_pid;
         }
 
         p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
@@ -1233,7 +1233,7 @@ 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_graph;
+                goto bad_fork_free_pid;
         }
 
         if (clone_flags & CLONE_THREAD) {
@@ -1268,8 +1268,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
         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);