compat: Backport ip_skb_dst_mtu().

>From upstream f87c10a8aa1e ("ipv4: introduce ip_dst_mtu_maybe_forward
and protect forwarding path against pmtu spoofing")

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>

Author: joestringer

acinclude.m4

@@ -348,6 +348,7 @@ AC_DEFUN([OVS_CHECK_LINUX_COMPAT], [
   OVS_GREP_IFELSE([$KSRC/include/net/ip.h], [inet_get_local_port_range.*net],
                   [OVS_DEFINE([HAVE_INET_GET_LOCAL_PORT_RANGE_USING_NET])])
   OVS_GREP_IFELSE([$KSRC/include/net/ip.h], [ip_is_fragment])
+  OVS_GREP_IFELSE([$KSRC/include/net/ip.h], [ip_skb_dst_mtu])
   OVS_GREP_IFELSE([$KSRC/include/net/dst_metadata.h], [metadata_dst])
 
   OVS_GREP_IFELSE([$KSRC/include/linux/net.h], [sock_create_kern.*net],

datapath/linux/compat/include/net/ip.h

@@ -22,4 +22,43 @@ static inline void rpl_inet_get_local_port_range(struct net *net, int *low,
 
 #endif
 
+/* IPv4 datagram length is stored into 16bit field (tot_len) */
+#ifndef IP_MAX_MTU
+#define IP_MAX_MTU	0xFFFFU
+#endif
+
+#ifndef HAVE_IP_SKB_DST_MTU
+static inline bool rpl_ip_sk_use_pmtu(const struct sock *sk)
+{
+	return inet_sk(sk)->pmtudisc < IP_PMTUDISC_PROBE;
+}
+#define ip_sk_use_pmtu rpl_ip_sk_use_pmtu
+
+static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
+						    bool forwarding)
+{
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(3,14,0)
+	struct net *net = dev_net(dst->dev);
+
+	if (net->ipv4.sysctl_ip_fwd_use_pmtu ||
+	    dst_metric_locked(dst, RTAX_MTU) ||
+	    !forwarding)
+		return dst_mtu(dst);
+#endif
+
+	return min(dst->dev->mtu, IP_MAX_MTU);
+}
+
+static inline unsigned int rpl_ip_skb_dst_mtu(const struct sk_buff *skb)
+{
+	if (!skb->sk || ip_sk_use_pmtu(skb->sk)) {
+		bool forwarding = IPCB(skb)->flags & IPSKB_FORWARDED;
+		return ip_dst_mtu_maybe_forward(skb_dst(skb), forwarding);
+	} else {
+		return min(skb_dst(skb)->dev->mtu, IP_MAX_MTU);
+	}
+}
+#define ip_skb_dst_mtu rpl_ip_skb_dst_mtu
+#endif /* HAVE_IP_SKB_DST_MTU */
+
 #endif