Re: [PATCH 4/5] nsfs: add ioctl to get a parent namespace

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 3529a03..a63adfb 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c

@@ -388,12 +388,38 @@ static int pidns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 	return 0;
 }
 
+static struct ns_common *pidns_get_parent(struct ns_common *ns)
+{
+	struct pid_namespace *active = task_active_pid_ns(current);
+	struct pid_namespace *pid_ns, *p;
+
+	pid_ns = to_pid_ns(ns);
+	if (pid_ns == &init_pid_ns) {
+		if (capable(CAP_SYS_ADMIN))
+			return ERR_PTR(-ENOENT);
+		return ERR_PTR(-EPERM);
+	}
+
+	pid_ns = p = pid_ns->parent;
+
+	for (;;) {
+		if (p == active)
+			break;
+		if (p == &init_pid_ns)
+			return ERR_PTR(-EPERM);
+		p = p->parent;
+	}

+
+	return &get_pid_ns(pid_ns)->ns;
+}
+
 const struct proc_ns_operations pidns_operations = {
 	.name		= "pid",
 	.type		= CLONE_NEWPID,
 	.get		= pidns_get,
 	.put		= pidns_put,
 	.install	= pidns_install,
+	.get_parent	= pidns_get_parent,
 };