ipv6: Use rt6i_idev index for echo replies to a local address

Tariq repored local pings to linklocal address is failing:
$ ifconfig ens8
ens8: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 11.141.16.6  netmask 255.255.0.0  broadcast 11.141.255.255
        inet6 fe80::7efe:90ff:fecb:7502  prefixlen 64  scopeid 0x20<link>
        ether 7c:fe:90:cb:75:02  txqueuelen 1000  (Ethernet)
        RX packets 12  bytes 1164 (1.1 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 30  bytes 2484 (2.4 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

$  /bin/ping6 -c 3 fe80::7efe:90ff:fecb:7502%ens8
PING fe80::7efe:90ff:fecb:7502%ens8(fe80::7efe:90ff:fecb:7502) 56 data bytes
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/ip6_route.h

@@ -164,6 +164,16 @@ void rt6_mtu_change(struct net_device *dev, unsigned int mtu);
 void rt6_remove_prefsrc(struct inet6_ifaddr *ifp);
 void rt6_clean_tohost(struct net *net, struct in6_addr *gateway);
 
+static inline const struct rt6_info *skb_rt6_info(const struct sk_buff *skb)
+{
+	const struct dst_entry *dst = skb_dst(skb);
+	const struct rt6_info *rt6 = NULL;
+
+	if (dst)
+		rt6 = container_of(dst, struct rt6_info, dst);
+
+	return rt6;
+}
 
 /*
  *	Store a destination cache entry in a socket

net/ipv6/icmp.c

@@ -399,6 +399,24 @@ static struct dst_entry *icmpv6_route_lookup(struct net *net,
 	return ERR_PTR(err);
 }
 
+static int icmp6_iif(const struct sk_buff *skb)
+{
+	int iif = skb->dev->ifindex;
+
+	/* for local traffic to local address, skb dev is the loopback
+	 * device. Check if there is a dst attached to the skb and if so
+	 * get the real device index.
+	 */
+	if (unlikely(iif == LOOPBACK_IFINDEX)) {
+		const struct rt6_info *rt6 = skb_rt6_info(skb);
+
+		if (rt6)
+			iif = rt6->rt6i_idev->dev->ifindex;
+	}
+
+	return iif;
+}
+
 /*
  *	Send an ICMP message in response to a packet in error
  */
@@ -460,18 +478,7 @@ static void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info,
 	 */
 
 	if (__ipv6_addr_needs_scope_id(addr_type)) {
-		iif = skb->dev->ifindex;
-
-		/* for local packets, get the real device index */
-		if (iif == LOOPBACK_IFINDEX) {
-			dst = skb_dst(skb);
-			if (dst) {
-				struct rt6_info *rt;
-
-				rt = container_of(dst, struct rt6_info, dst);
-				iif = rt->rt6i_idev->dev->ifindex;
-			}
-		}
+		iif = icmp6_iif(skb);
 	} else {
 		dst = skb_dst(skb);
 		iif = l3mdev_master_ifindex(dst ? dst->dev : skb->dev);
@@ -694,7 +701,7 @@ static void icmpv6_echo_reply(struct sk_buff *skb)
 	fl6.daddr = ipv6_hdr(skb)->saddr;
 	if (saddr)
 		fl6.saddr = *saddr;
-	fl6.flowi6_oif = skb->dev->ifindex;
+	fl6.flowi6_oif = icmp6_iif(skb);
 	fl6.fl6_icmp_type = ICMPV6_ECHO_REPLY;
 	fl6.flowi6_mark = mark;
 	fl6.flowi6_uid = sock_net_uid(net, NULL);