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

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Date:	Thu, 09 Aug 2012 14:27:08 -0700
From:	Jay Vosburgh <fubar@...ibm.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
cc:	Jiri Pirko <jpirko@...hat.com>, Flavio Leitner <fbl@...hat.com>,
	netdev <netdev@...r.kernel.org>,
	Andy Gospodarek <andy@...yhouse.net>,
	Leonardo Chiquitto <lchiquitto@...e.com>
Subject: Re: [net-next] bonding: don't allow the master to become its slave

Ben Hutchings <bhutchings@...arflare.com> wrote:
>On Thu, 2012-08-09 at 21:55 +0200, Jiri Pirko wrote:
[...]
>> 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.

	If that's hard to do (and it might be; I'm not aware of a
standard way to run up and down those stacks of interfaces, which might
not always be vlans in the middle), there's still the priv_flags &
IFF_BONDING test that bonding could (and probably should) do itself as
well.  The team driver could presumably have a similar test, although I
seem to recall that team was allowed to nest.

	FWIW, I've seen both the top and bottom halves of that picture
in use (i.e., bonds consisting of vlans as slaves or bonds with vlans
configured above them), but not combined as in your diagram.

	-J

---
	-Jay Vosburgh, IBM Linux Technology Center, fubar@...ibm.com

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