Re: [net-next] bonding: don't allow the master to become its slave

quoted

How about other devices who do not use "->master" like vlan, macvlan?

And they shouldn't use master, because they allow multiple upper devices
may be stacked on a single lower device.  Instead they use iflink, but
that's an ifindex and not a net_device pointer.

So I think we can catch simple loops with:

--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -4445,8 +4445,22 @@ int netdev_set_master(struct net_device *slave, struct net_device *master)

	ASSERT_RTNL();

	if (master) {
+		struct net_device *bottom, *top;
+
		if (old)
			return -EBUSY;
+
+		/* Prevent loops */
+		bottom = slave;
+		while (bottom->iflink != bottom->ifindex)
+			bottom = __dev_get_by_index(dev_net(bottom),
+						    bottom->iflink);
+		top = master;
+		while (top->master)
+			top = top->master;
+		if (top == bottom)
+			return -EBUSY;
+
		dev_hold(master);
	}

--- END ---

But then there can be quite silly device relationships like:

              +-------+
              | bond0 |
              ++-----++
              /       \
+-------+ +---+---+ +---+---+ +-------+
| vlan0 | | vlan1 | | vlan2 | | vlan3 |
+---+---+ +---+---+ +---+---+ +---+---+
    \       /           \       /
    ++-----++           ++--+--++
    | bond1 |           | bond2 |
    +-------+           +-------+
     :     :             :     :

Suppose the user tries to make bond0 a slave of bond1; we need to go to
somewhat more effort to detect the loop.