[PATCH net] ip6_gre: init dev->mtu and dev->hard_header_len correctly

[Alexey Kodanev <alexey.kodanev@oracle.com>]

Commit b05229f44228 ("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().

Since, initially net_device allocated with kvzalloc, make sure
that dev->mtu is zero, i.e. not changed, before setting default
MTU inside ndo_init(), and invoke ip6gre_tnl_link_config after
setting default values.

For ip6gretap, reset dev->mtu to zero in ip6gre_tap_setup()
after ether_setup(), in order for it to work with the new check
in ip6gre_tunnel_init_common().

Fixes: b05229f44228 ("gre6: Cleanup GREv6 transmit path, call common GRE functions")
Fixes: db2ec95d1ba4 ("ip6_gre: Fix MTU setting")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
---
 net/ipv6/ip6_gre.c |   22 ++++++++++------------
 1 files changed, 10 insertions(+), 12 deletions(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 7726959..edf65d0 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -337,7 +337,6 @@ static void ip6gre_tunnel_unlink(struct ip6gre_net *ign, struct ip6_tnl *t)
 
 	nt->dev = dev;
 	nt->net = dev_net(dev);
-	ip6gre_tnl_link_config(nt, 1);
 
 	if (register_netdevice(dev) < 0)
 		goto failed_free;
@@ -1047,6 +1046,7 @@ static void ip6gre_tnl_init_features(struct net_device *dev)
 static int ip6gre_tunnel_init_common(struct net_device *dev)
 {
 	struct ip6_tnl *tunnel;
+	int set_mtu = !dev->mtu;
 	int ret;
 	int t_hlen;
 
@@ -1072,13 +1072,16 @@ static int ip6gre_tunnel_init_common(struct net_device *dev)
 	t_hlen = tunnel->hlen + sizeof(struct ipv6hdr);
 
 	dev->hard_header_len = LL_MAX_HEADER + t_hlen;
-	dev->mtu = ETH_DATA_LEN - t_hlen;
-	if (dev->type == ARPHRD_ETHER)
-		dev->mtu -= ETH_HLEN;
-	if (!(tunnel->parms.flags & IP6_TNL_F_IGN_ENCAP_LIMIT))
-		dev->mtu -= 8;
+	if (set_mtu) {
+		dev->mtu = ETH_DATA_LEN - t_hlen;
+		if (dev->type == ARPHRD_ETHER)
+			dev->mtu -= ETH_HLEN;
+		if (!(tunnel->parms.flags & IP6_TNL_F_IGN_ENCAP_LIMIT))
+			dev->mtu -= 8;
+	}
 
 	ip6gre_tnl_init_features(dev);
+	ip6gre_tnl_link_config(tunnel, set_mtu);
 
 	return 0;
 }
@@ -1303,7 +1306,6 @@ 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 +1314,6 @@ 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;
 }
 
@@ -1335,6 +1333,7 @@ static void ip6gre_tap_setup(struct net_device *dev)
 
 	ether_setup(dev);
 
+	dev->mtu = 0;
 	dev->max_mtu = 0;
 	dev->netdev_ops = &ip6gre_tap_netdev_ops;
 	dev->needs_free_netdev = true;
@@ -1408,7 +1407,6 @@ 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)
-- 
1.7.1