[PATCH 1/2] exit: call disassociate_ctty() before exit_task_namespaces()

[Oleg Nesterov <oleg@redhat.com>]

8aac62706ada "move exit_task_namespaces() outside of exit_notify()"
breaks pppd and the exiting service crashes the kernel:

    BUG: unable to handle kernel NULL pointer dereference at 0000000000000028
    IP: [<ffffffffa06985a3>] ppp_register_channel+0x13/0x20 [ppp_generic]
    ...
    Call Trace:
     [<ffffffffa06ab2ab>] ? ppp_asynctty_open+0x12b/0x170 [ppp_async]
     [<ffffffff812ec907>] ? tty_ldisc_open.isra.2+0x27/0x60
     [<ffffffff812ed1d3>] ? tty_ldisc_hangup+0x1e3/0x220
     [<ffffffff812e4614>] ? __tty_hangup+0x2c4/0x440
     [<ffffffff812e56a1>] ? disassociate_ctty+0x61/0x270
     [<ffffffff810483c2>] ? do_exit+0x7f2/0xa50

ppp_register_channel() needs ->net_ns and current->nsproxy == NULL.

Move disassociate_ctty() before exit_task_namespaces(), it doesn't
make sense to delay it after perf_event_exit_task() or cgroup_exit().

This also allows to use task_work_add() inside the (nontrivial) code
paths in disassociate_ctty().

Cc: stable@vger.kernel.org # v3.10+
Reported-by: Sree Harsha Totakura <sreeharsha@totakura.in>
Investigated-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/exit.c |    6 ++----
 1 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 790b73c..5d5b472 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -784,6 +784,8 @@ void do_exit(long code)
 	exit_shm(tsk);
 	exit_files(tsk);
 	exit_fs(tsk);
+	if (group_dead)
+		disassociate_ctty(1);
 	exit_task_namespaces(tsk);
 	exit_task_work(tsk);
 	check_stack_usage();
@@ -799,13 +801,9 @@ void do_exit(long code)
 
 	cgroup_exit(tsk, 1);
 
-	if (group_dead)
-		disassociate_ctty(1);
-
 	module_put(task_thread_info(tsk)->exec_domain->module);
 
 	proc_exit_connector(tsk);
-
 	/*
 	 * FIXME: do that only when needed, using sched_exit tracepoint
 	 */
-- 
1.5.5.1