ip6_gre: init dev->mtu and dev->hard_header_len correctly

Commit b05229f4 ("gre6: Cleanup GREv6 transmit path,
call common GRE functions") moved dev->mtu initialization
from ip6gre_tunnel_setup() to ip6gre_tunnel_init(), as a
result, the previously set values, before ndo_init(), are
reset in the following cases:

* rtnl_create_link() can update dev->mtu from IFLA_MTU
  parameter.

* ip6gre_tnl_link_config() is invoked before ndo_init() in
  netlink and ioctl setup, so ndo_init() can reset MTU
  adjustments with the lower device MTU as well, dev->mtu
  and dev->hard_header_len.

  Not applicable for ip6gretap because it has one more call
  to ip6gre_tnl_link_config(tunnel, 1) in ip6gre_tap_init().

Fix the first case by updating dev->mtu with 'tb[IFLA_MTU]'
parameter if a user sets it manually on a device creation,
and fix the second one by moving ip6gre_tnl_link_config()
call after register_netdevice().

Fixes: b05229f4 ("gre6: Cleanup GREv6 transmit path, call common GRE functions")
Fixes: db2ec95d ("ip6_gre: Fix MTU setting")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -337,11 +337,12 @@ static struct ip6_tnl *ip6gre_tunnel_locate(struct net *net,
 
 	nt->dev = dev;
 	nt->net = dev_net(dev);
-	ip6gre_tnl_link_config(nt, 1);
 
 	if (register_netdevice(dev) < 0)
 		goto failed_free;
 
+	ip6gre_tnl_link_config(nt, 1);
+
 	/* Can use a lockless transmit, unless we generate output sequences */
 	if (!(nt->parms.o_flags & TUNNEL_SEQ))
 		dev->features |= NETIF_F_LLTX;
@@ -1303,7 +1304,6 @@ static void ip6gre_netlink_parms(struct nlattr *data[],
 
 static int ip6gre_tap_init(struct net_device *dev)
 {
-	struct ip6_tnl *tunnel;
 	int ret;
 
 	ret = ip6gre_tunnel_init_common(dev);
@@ -1312,10 +1312,6 @@ static int ip6gre_tap_init(struct net_device *dev)
 
 	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE;
 
-	tunnel = netdev_priv(dev);
-
-	ip6gre_tnl_link_config(tunnel, 1);
-
 	return 0;
 }
 
@@ -1408,12 +1404,16 @@ static int ip6gre_newlink(struct net *src_net, struct net_device *dev,
 
 	nt->dev = dev;
 	nt->net = dev_net(dev);
-	ip6gre_tnl_link_config(nt, !tb[IFLA_MTU]);
 
 	err = register_netdevice(dev);
 	if (err)
 		goto out;
 
+	ip6gre_tnl_link_config(nt, !tb[IFLA_MTU]);
+
+	if (tb[IFLA_MTU])
+		ip6_tnl_change_mtu(dev, nla_get_u32(tb[IFLA_MTU]));
+
 	dev_hold(dev);
 	ip6gre_tunnel_link(ign, nt);