From: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Date: Thu, 31 May 2018 10:59:33 +0200
Subject: ip6_tunnel: remove magic mtu value 0xFFF8
Git-commit: f7ff1fde9441b4fcc8ffb6e66e6e5a00d008937e
Patch-mainline: v4.17
References: networking-stable-18_06_08

I don't know where this value comes from (probably a copy and paste and
paste and paste ...).
Let's use standard values which are a bit greater.

Link: https://git.kernel.org/pub/scm/linux/kernel/git/davem/netdev-vger-cvs.git/commit/?id=e5afd356a411a
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 net/ipv6/ip6_tunnel.c |   11 ++++++++---
 net/ipv6/sit.c        |    2 +-
 2 files changed, 9 insertions(+), 4 deletions(-)

--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1688,8 +1688,13 @@ int ip6_tnl_change_mtu(struct net_device
 		if (new_mtu < IPV6_MIN_MTU)
 			return -EINVAL;
 	}
-	if (new_mtu > 0xFFF8 - dev->hard_header_len)
-		return -EINVAL;
+	if (tnl->parms.proto == IPPROTO_IPV6 || tnl->parms.proto == 0) {
+		if (new_mtu > IP6_MAX_MTU - dev->hard_header_len)
+			return -EINVAL;
+	} else {
+		if (new_mtu > IP_MAX_MTU - dev->hard_header_len)
+			return -EINVAL;
+	}
 	dev->mtu = new_mtu;
 	return 0;
 }
@@ -1837,7 +1842,7 @@ ip6_tnl_dev_init_gen(struct net_device *
 	if (!(t->parms.flags & IP6_TNL_F_IGN_ENCAP_LIMIT))
 		dev->mtu -= 8;
 	dev->min_mtu = ETH_MIN_MTU;
-	dev->max_mtu = 0xFFF8 - dev->hard_header_len;
+	dev->max_mtu = IP6_MAX_MTU - dev->hard_header_len;
 
 	return 0;
 
--- a/net/ipv6/sit.c
+++ b/net/ipv6/sit.c
@@ -1360,7 +1360,7 @@ static void ipip6_tunnel_setup(struct ne
 	dev->hard_header_len	= LL_MAX_HEADER + t_hlen;
 	dev->mtu		= ETH_DATA_LEN - t_hlen;
 	dev->min_mtu		= IPV6_MIN_MTU;
-	dev->max_mtu		= 0xFFF8 - t_hlen;
+	dev->max_mtu		= IP6_MAX_MTU - t_hlen;
 	dev->flags		= IFF_NOARP;
 	netif_keep_dst(dev);
 	dev->addr_len		= 4;