tcp: Add SNMP counters for backlog and min_ttl drops

Commit 6b03a53a (tcp: use limited socket backlog) added the possibility
of dropping frames when backlog queue is full.

Commit d218d111 (tcp: Generalized TTL Security Mechanism) added the
possibility of dropping frames when TTL is under a given limit.

This patch adds new SNMP MIB entries, named TCPBacklogDrop and
TCPMinTTLDrop, published in /proc/net/netstat in TcpExt: line

netstat -s | egrep "TCPBacklogDrop|TCPMinTTLDrop"
    TCPBacklogDrop: 0
    TCPMinTTLDrop: 0
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/snmp.h

@@ -225,6 +225,8 @@ enum
 	LINUX_MIB_SACKSHIFTED,
 	LINUX_MIB_SACKMERGED,
 	LINUX_MIB_SACKSHIFTFALLBACK,
+	LINUX_MIB_TCPBACKLOGDROP,
+	LINUX_MIB_TCPMINTTLDROP, /* RFC 5082 */
 	__LINUX_MIB_MAX
 };
 

net/ipv4/proc.c

@@ -249,6 +249,8 @@ static const struct snmp_mib snmp4_net_list[] = {
 	SNMP_MIB_ITEM("TCPSackShifted", LINUX_MIB_SACKSHIFTED),
 	SNMP_MIB_ITEM("TCPSackMerged", LINUX_MIB_SACKMERGED),
 	SNMP_MIB_ITEM("TCPSackShiftFallback", LINUX_MIB_SACKSHIFTFALLBACK),
+	SNMP_MIB_ITEM("TCPBacklogDrop", LINUX_MIB_TCPBACKLOGDROP),
+	SNMP_MIB_ITEM("TCPMinTTLDrop", LINUX_MIB_TCPMINTTLDROP),
 	SNMP_MIB_SENTINEL
 };
 

net/ipv4/tcp_ipv4.c

@@ -1651,8 +1651,10 @@ int tcp_v4_rcv(struct sk_buff *skb)
 	if (!sk)
 		goto no_tcp_socket;
 
-	if (iph->ttl < inet_sk(sk)->min_ttl)
+	if (unlikely(iph->ttl < inet_sk(sk)->min_ttl)) {
+		NET_INC_STATS_BH(net, LINUX_MIB_TCPMINTTLDROP);
 		goto discard_and_relse;
+	}
 
 process:
 	if (sk->sk_state == TCP_TIME_WAIT)
@@ -1682,8 +1684,9 @@ process:
 			if (!tcp_prequeue(sk, skb))
 				ret = tcp_v4_do_rcv(sk, skb);
 		}
-	} else if (sk_add_backlog(sk, skb)) {
+	} else if (unlikely(sk_add_backlog(sk, skb))) {
 		bh_unlock_sock(sk);
+		NET_INC_STATS_BH(net, LINUX_MIB_TCPBACKLOGDROP);
 		goto discard_and_relse;
 	}
 	bh_unlock_sock(sk);

net/ipv6/tcp_ipv6.c

@@ -1740,8 +1740,9 @@ process:
 			if (!tcp_prequeue(sk, skb))
 				ret = tcp_v6_do_rcv(sk, skb);
 		}
-	} else if (sk_add_backlog(sk, skb)) {
+	} else if (unlikely(sk_add_backlog(sk, skb))) {
 		bh_unlock_sock(sk);
+		NET_INC_STATS_BH(net, LINUX_MIB_TCPBACKLOGDROP);
 		goto discard_and_relse;
 	}
 	bh_unlock_sock(sk);