[PATCH 1/3] pidns: Use task_active_pid_ns in do_notify_parent.

[ebiederm@xmission.com (Eric W. Biederman)]

Using task_active_pid_ns is more robust because it works even after we
have called exit_namespaces.  This change allows us to have parent
processes that are zombies.  Normally a zombie parent processes is crazy
and the last thing you would want to have but in the case of not letting
the init process of a pid namespace be reaped until all of it's children
are dead and reaped a zombie parent process is exactly what we want.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
---
 kernel/signal.c |   11 +++++------
 1 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 17afcaf..0e4ef99 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1665,19 +1665,18 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
 	info.si_signo = sig;
 	info.si_errno = 0;
 	/*
-	 * we are under tasklist_lock here so our parent is tied to
-	 * us and cannot exit and release its namespace.
+	 * We are under tasklist_lock here so our parent is tied to
+	 * us and cannot change.
 	 *
-	 * the only it can is to switch its nsproxy with sys_unshare,
-	 * bu uncharing pid namespaces is not allowed, so we'll always
-	 * see relevant namespace
+	 * task_active_pid_ns will always return the same pid namespace
+	 * until a task passes through release_task.
 	 *
 	 * write_lock() currently calls preempt_disable() which is the
 	 * same as rcu_read_lock(), but according to Oleg, this is not
 	 * correct to rely on this
 	 */
 	rcu_read_lock();
-	info.si_pid = task_pid_nr_ns(tsk, tsk->parent->nsproxy->pid_ns);
+	info.si_pid = task_pid_nr_ns(tsk, task_active_pid_ns(tsk->parent));
 	info.si_uid = map_cred_ns(__task_cred(tsk),
 			task_cred_xxx(tsk->parent, user_ns));
 	rcu_read_unlock();
-- 
1.7.5.4