ip6_gre: process toobig in a better way

Now ip6gre processes toobig icmp packet by setting gre dev's mtu in
ip6gre_err, which would cause few things not good:

  - It couldn't set mtu with dev_set_mtu due to it's not in user context,
    which causes route cache and idev->cnf.mtu6 not to be updated.

  - It has to update sk dst pmtu in tx path according to gredev->mtu for
    ip6gre, while it updates pmtu again according to lower dst pmtu in
    ip6_tnl_xmit.

  - To change dev->mtu by toobig icmp packet is not a good idea, it should
    only work on pmtu.

This patch is to process toobig by updating the lower dst's pmtu, as later
sk dst pmtu will be updated in ip6_tnl_xmit, the same way as in ip4gre.

Note that gre dev's mtu will not be updated any more, it doesn't make any
sense to change dev's mtu after receiving a toobig packet.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -403,9 +403,8 @@ static void ip6gre_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		return;
 
 	switch (type) {
-		__u32 teli;
 		struct ipv6_tlv_tnl_enc_lim *tel;
-		__u32 mtu;
+		__u32 teli;
 	case ICMPV6_DEST_UNREACH:
 		net_dbg_ratelimited("%s: Path to destination invalid or inactive!\n",
 				    t->parms.name);
@@ -436,12 +435,7 @@ static void ip6gre_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 		}
 		return;
 	case ICMPV6_PKT_TOOBIG:
-		mtu = be32_to_cpu(info) - offset - t->tun_hlen;
-		if (t->dev->type == ARPHRD_ETHER)
-			mtu -= ETH_HLEN;
-		if (mtu < IPV6_MIN_MTU)
-			mtu = IPV6_MIN_MTU;
-		t->dev->mtu = mtu;
+		ip6_update_pmtu(skb, net, info, 0, 0, sock_net_uid(net, NULL));
 		return;
 	case NDISC_REDIRECT:
 		ip6_redirect(skb, net, skb->dev->ifindex, 0,
@@ -508,7 +502,6 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
 			       __u32 *pmtu, __be16 proto)
 {
 	struct ip6_tnl *tunnel = netdev_priv(dev);
-	struct dst_entry *dst = skb_dst(skb);
 	__be16 protocol;
 
 	if (dev->type == ARPHRD_ETHER)
@@ -527,10 +520,6 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
 	gre_build_header(skb, tunnel->tun_hlen, tunnel->parms.o_flags,
 			 protocol, tunnel->parms.o_key, htonl(tunnel->o_seqno));
 
-	/* TooBig packet may have updated dst->dev's mtu */
-	if (dst && dst_mtu(dst) > dst->dev->mtu)
-		dst->ops->update_pmtu(dst, NULL, skb, dst->dev->mtu);
-
 	return ip6_tnl_xmit(skb, dev, dsfield, fl6, encap_limit, pmtu,
 			    NEXTHDR_GRE);
 }