[PATCH 3.16 393/410] ip_tunnel: Emit events for post-register MTU changes

[Ben Hutchings]

3.16.57-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Petr Machata <petrm@xxxxxxxxxxxx>

commit f6cc9c054e77b9a28d4594bcc201697edb21dfd2 upstream.

For tunnels created with IFLA_MTU, MTU of the netdevice is set by
rtnl_create_link() (called from rtnl_newlink()) before the device is
registered. However without IFLA_MTU that's not done.

rtnl_newlink() proceeds by calling struct rtnl_link_ops.newlink, which
via ip_tunnel_newlink() calls register_netdevice(), and that emits
NETDEV_REGISTER. Thus any listeners that inspect the netdevice get the
MTU of 0.

After ip_tunnel_newlink() corrects the MTU after registering the
netdevice, but since there's no event, the listeners don't get to know
about the MTU until something else happens--such as a NETDEV_UP event.
That's not ideal.

So instead of setting the MTU directly, go through dev_set_mtu(), which
takes care of distributing the necessary NETDEV_PRECHANGEMTU and
NETDEV_CHANGEMTU events.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Petr Machata <petrm@xxxxxxxxxxxx>
Signed-off-by: Ido Schimmel <idosch@xxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
[bwh: Backported to 3.16: Resolve conflict with commit 24fc79798b8d
 "ip_tunnel: Clamp MTU to bounds on new link", referring to commit
 5568cdc368c3 "ip_tunnel: Resolve ipsec merge conflict properly."]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -405,17 +405,26 @@ static struct ip_tunnel *ip_tunnel_creat
 {
 	struct ip_tunnel *nt;
 	struct net_device *dev;
+	int mtu;
+	int err;
 
 	BUG_ON(!itn->fb_tunnel_dev);
 	dev = __ip_tunnel_create(net, itn->fb_tunnel_dev->rtnl_link_ops, parms);
 	if (IS_ERR(dev))
 		return ERR_CAST(dev);
 
-	dev->mtu = ip_tunnel_bind_dev(dev);
+	mtu = ip_tunnel_bind_dev(dev);
+	err = dev_set_mtu(dev, mtu);
+	if (err)
+		goto err_dev_set_mtu;
 
 	nt = netdev_priv(dev);
 	ip_tunnel_add(itn, nt);
 	return nt;
+
+err_dev_set_mtu:
+	unregister_netdevice(dev);
+	return ERR_PTR(err);
 }
 
 int ip_tunnel_rcv(struct ip_tunnel *tunnel, struct sk_buff *skb,
@@ -962,7 +971,7 @@ int ip_tunnel_newlink(struct net_device
 	nt->parms = *p;
 	err = register_netdevice(dev);
 	if (err)
-		goto out;
+		goto err_register_netdevice;
 
 	if (dev->type == ARPHRD_ETHER && !tb[IFLA_ADDRESS])
 		eth_hw_addr_random(dev);
@@ -971,15 +980,20 @@ int ip_tunnel_newlink(struct net_device
 	if (tb[IFLA_MTU]) {
 		unsigned int max = 0xfff8 - dev->hard_header_len - nt->hlen;
 
-		dev->mtu = clamp(dev->mtu, (unsigned int)ETH_MIN_MTU,
-				 (unsigned int)(max - sizeof(struct iphdr)));
-	} else {
-		dev->mtu = mtu;
+		mtu = clamp(dev->mtu, (unsigned int)ETH_MIN_MTU,
+			    (unsigned int)(max - sizeof(struct iphdr)));
 	}
 
+	err = dev_set_mtu(dev, mtu);
+	if (err)
+		goto err_dev_set_mtu;
+
 	ip_tunnel_add(itn, nt);
+	return 0;
 
-out:
+err_dev_set_mtu:
+	unregister_netdevice(dev);
+err_register_netdevice:
 	return err;
 }
 EXPORT_SYMBOL_GPL(ip_tunnel_newlink);