netns: add register_pernet_gen_subsys/unregister_pernet_gen_subsys

netns ops which are registered with register_pernet_gen_device() are
shutdown strictly before those which are registered with
register_pernet_subsys(). Sometimes this leads to opposite (read: buggy)
shutdown ordering between two modules.

Add register_pernet_gen_subsys()/unregister_pernet_gen_subsys() for modules
which aren't elite enough for entry in struct net, and which can't use
register_pernet_gen_device(). PPTP conntracking module is such one.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -214,6 +214,8 @@ struct pernet_operations {
 
 extern int register_pernet_subsys(struct pernet_operations *);
 extern void unregister_pernet_subsys(struct pernet_operations *);
+extern int register_pernet_gen_subsys(int *id, struct pernet_operations *);
+extern void unregister_pernet_gen_subsys(int id, struct pernet_operations *);
 extern int register_pernet_device(struct pernet_operations *);
 extern void unregister_pernet_device(struct pernet_operations *);
 extern int register_pernet_gen_device(int *id, struct pernet_operations *);

net/core/net_namespace.c

@@ -325,6 +325,38 @@ void unregister_pernet_subsys(struct pernet_operations *module)
 }
 EXPORT_SYMBOL_GPL(unregister_pernet_subsys);
 
+int register_pernet_gen_subsys(int *id, struct pernet_operations *ops)
+{
+	int rv;
+
+	mutex_lock(&net_mutex);
+again:
+	rv = ida_get_new_above(&net_generic_ids, 1, id);
+	if (rv < 0) {
+		if (rv == -EAGAIN) {
+			ida_pre_get(&net_generic_ids, GFP_KERNEL);
+			goto again;
+		}
+		goto out;
+	}
+	rv = register_pernet_operations(first_device, ops);
+	if (rv < 0)
+		ida_remove(&net_generic_ids, *id);
+	mutex_unlock(&net_mutex);
+out:
+	return rv;
+}
+EXPORT_SYMBOL_GPL(register_pernet_gen_subsys);
+
+void unregister_pernet_gen_subsys(int id, struct pernet_operations *ops)
+{
+	mutex_lock(&net_mutex);
+	unregister_pernet_operations(ops);
+	ida_remove(&net_generic_ids, id);
+	mutex_unlock(&net_mutex);
+}
+EXPORT_SYMBOL_GPL(unregister_pernet_gen_subsys);
+
 /**
  *      register_pernet_device - register a network namespace device
  *	@ops:  pernet operations structure for the subsystem