Commit 82612de1 authored May 31, 2018 by Nicolas Dichtel Committed by David S. Miller Jun 01, 2018

ip_tunnel: restore binding to ifaces with a large mtu

After commit f6cc9c05, the following conf is broken (note that the
default loopback mtu is 65536, ie IP_MAX_MTU + 1):

$ ip tunnel add gre1 mode gre local 10.125.0.1 remote 10.125.0.2 dev lo
add tunnel "gre0" failed: Invalid argument
$ ip l a type dummy
$ ip l s dummy1 up
$ ip l s dummy1 mtu 65535
$ ip tunnel add gre1 mode gre local 10.125.0.1 remote 10.125.0.2 dev dummy1
add tunnel "gre0" failed: Invalid argument

dev_set_mtu() doesn't allow to set a mtu which is too large.
First, let's cap the mtu returned by ip_tunnel_bind_dev(). Second, remove
the magic value 0xFFF8 and use IP_MAX_MTU instead.
0xFFF8 seems to be there for ages, I don't know why this value was used.

With a recent kernel, it's also possible to set a mtu > IP_MAX_MTU:
$ ip l s dummy1 mtu 66000
After that patch, it's also possible to bind an ip tunnel on that kind of
interface.

CC: Petr Machata <petrm@mellanox.com>
CC: Ido Schimmel <idosch@mellanox.com>
Link: https://git.kernel.org/pub/scm/linux/kernel/git/davem/netdev-vger-cvs.git/commit/?id=e5afd356a411a


Fixes: f6cc9c05 ("ip_tunnel: Emit events for post-register MTU changes")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent ccfde6e2

net/ipv4/ip_tunnel.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -328,7 +328,7 @@ static int ip_tunnel_bind_dev(struct net_device *dev)

		if (tdev) {
		hlen = tdev->hard_header_len + tdev->needed_headroom;
		mtu = tdev->mtu;
		mtu = min(tdev->mtu, IP_MAX_MTU);
		}

		dev->needed_headroom = t_hlen + hlen;
		@@ -362,7 +362,7 @@ static struct ip_tunnel ip_tunnel_create(struct net net,
		nt = netdev_priv(dev);
		t_hlen = nt->hlen + sizeof(struct iphdr);
		dev->min_mtu = ETH_MIN_MTU;
		dev->max_mtu = 0xFFF8 - dev->hard_header_len - t_hlen;
		dev->max_mtu = IP_MAX_MTU - dev->hard_header_len - t_hlen;
		ip_tunnel_add(itn, nt);
		return nt;

		@@ -930,7 +930,7 @@ int __ip_tunnel_change_mtu(struct net_device *dev, int new_mtu, bool strict)
		{
		struct ip_tunnel *tunnel = netdev_priv(dev);
		int t_hlen = tunnel->hlen + sizeof(struct iphdr);
		int max_mtu = 0xFFF8 - dev->hard_header_len - t_hlen;
		int max_mtu = IP_MAX_MTU - dev->hard_header_len - t_hlen;

		if (new_mtu < ETH_MIN_MTU)
		return -EINVAL;
		@@ -1107,7 +1107,7 @@ int ip_tunnel_newlink(struct net_device dev, struct nlattr tb[],

		mtu = ip_tunnel_bind_dev(dev);
		if (tb[IFLA_MTU]) {
		unsigned int max = 0xfff8 - dev->hard_header_len - nt->hlen;
		unsigned int max = IP_MAX_MTU - dev->hard_header_len - nt->hlen;

		mtu = clamp(dev->mtu, (unsigned int)ETH_MIN_MTU,
		(unsigned int)(max - sizeof(struct iphdr)));

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