[PATCH v2] bonding: ban stacked bonding support

[Alexey Dobriyan <adobriyan@gmail.com>]

If you add bonding master as a slave, and then release it,
it will no longer be an IFF_BONDING creating problems like described at
https://bugzilla.kernel.org/show_bug.cgi?id=89541

	echo +bond1 >/sys/class/net/bonding_masters
	echo 1 >/sys/class/net/bond1/bonding/mode
	echo +bond2 >/sys/class/net/bonding_masters
	echo +bond2 >/sys/class/net/bond1/bonding/slaves
	echo -bond2 >/sys/class/net/bond1/bonding/slaves
	echo -bond2 >/sys/class/net/bonding_masters

	cat /proc/net/bonding/bond2	# should not exist
		[oops]

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 drivers/net/bonding/bond_main.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -1248,6 +1248,11 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev)
 			    slave_dev->name);
 	}
 
+	if (netif_is_bond_master(slave_dev)) {
+		netdev_err(bond_dev, "device is bond master\n");
+		return -EBUSY;
+	}
+
 	/* already enslaved */
 	if (slave_dev->flags & IFF_SLAVE) {
 		netdev_dbg(bond_dev, "Error: Device was already enslaved\n");