net: original ingress device index in PKTINFO

Applications such as OSPF and BFD need the original ingress device not
the VRF device; the latter can be derived from the former. To that end
add the skb_iif to inet_skb_parm and set it in ipv4 code after clearing
the skb control buffer similar to IPv6. From there the pktinfo can just
pull it from cb with the PKTINFO_SKB_CB cast.

The previous patch moving the skb->dev change to L3 means nothing else
is needed for IPv6; it just works.
Signed-off-by: NDavid Ahern <dsa@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/ip.h

@@ -36,6 +36,7 @@
 struct sock;
 
 struct inet_skb_parm {
+	int			iif;
 	struct ip_options	opt;		/* Compiled IP options		*/
 	unsigned char		flags;
 

net/ipv4/ip_input.c

@@ -478,6 +478,7 @@ int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt,
 
 	/* Remove any debris in the socket control block */
 	memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));
+	IPCB(skb)->iif = skb->skb_iif;
 
 	/* Must drop socket now because of tproxy. */
 	skb_orphan(skb);

net/ipv4/ip_sockglue.c

@@ -1193,7 +1193,12 @@ void ipv4_pktinfo_prepare(const struct sock *sk, struct sk_buff *skb)
 		       ipv6_sk_rxinfo(sk);
 
 	if (prepare && skb_rtable(skb)) {
-		pktinfo->ipi_ifindex = inet_iif(skb);
+		/* skb->cb is overloaded: prior to this point it is IP{6}CB
+		 * which has interface index (iif) as the first member of the
+		 * underlying inet{6}_skb_parm struct. This code then overlays
+		 * PKTINFO_SKB_CB and in_pktinfo also has iif as the first
+		 * element so the iif is picked up from the prior IPCB
+		 */
 		pktinfo->ipi_spec_dst.s_addr = fib_compute_spec_dst(skb);
 	} else {
 		pktinfo->ipi_ifindex = 0;