tcp: add TCP_OLD_SEQUENCE drop reason

tcp_sequence() uses two conditions to decide to drop a packet,
and we currently report generic TCP_INVALID_SEQUENCE drop reason.

Duplicates are common, we need to distinguish them from
the other case.

I chose to not reuse TCP_OLD_DATA, and instead added
TCP_OLD_SEQUENCE drop reason.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230719064754.2794106-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: edumazet

include/net/dropreason-core.h

@@ -30,6 +30,7 @@
 	FN(TCP_OVERWINDOW)		\
 	FN(TCP_OFOMERGE)		\
 	FN(TCP_RFC7323_PAWS)		\
+	FN(TCP_OLD_SEQUENCE)		\
 	FN(TCP_INVALID_SEQUENCE)	\
 	FN(TCP_RESET)			\
 	FN(TCP_INVALID_SYN)		\
@@ -188,6 +189,8 @@ enum skb_drop_reason {
 	 * LINUX_MIB_PAWSESTABREJECTED
 	 */
 	SKB_DROP_REASON_TCP_RFC7323_PAWS,
+	/** @SKB_DROP_REASON_TCP_OLD_SEQUENCE: Old SEQ field (duplicate packet) */
+	SKB_DROP_REASON_TCP_OLD_SEQUENCE,
 	/** @SKB_DROP_REASON_TCP_INVALID_SEQUENCE: Not acceptable SEQ field */
 	SKB_DROP_REASON_TCP_INVALID_SEQUENCE,
 	/** @SKB_DROP_REASON_TCP_RESET: Invalid RST packet */

net/ipv4/tcp_input.c

@@ -4312,10 +4312,16 @@ static inline bool tcp_paws_discard(const struct sock *sk,
  * (borrowed from freebsd)
  */
 
-static inline bool tcp_sequence(const struct tcp_sock *tp, u32 seq, u32 end_seq)
+static enum skb_drop_reason tcp_sequence(const struct tcp_sock *tp,
+					 u32 seq, u32 end_seq)
 {
-	return	!before(end_seq, tp->rcv_wup) &&
-		!after(seq, tp->rcv_nxt + tcp_receive_window(tp));
+	if (before(end_seq, tp->rcv_wup))
+		return SKB_DROP_REASON_TCP_OLD_SEQUENCE;
+
+	if (after(seq, tp->rcv_nxt + tcp_receive_window(tp)))
+		return SKB_DROP_REASON_TCP_INVALID_SEQUENCE;
+
+	return SKB_NOT_DROPPED_YET;
 }
 
 /* When we get a reset we do this. */
@@ -5738,7 +5744,8 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 	}
 
 	/* Step 1: check sequence number */
-	if (!tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq)) {
+	reason = tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);
+	if (reason) {
 		/* RFC793, page 37: "In all states except SYN-SENT, all reset
 		 * (RST) segments are validated by checking their SEQ-fields."
 		 * And page 69: "If an incoming segment is not acceptable,
@@ -5755,7 +5762,6 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 		} else if (tcp_reset_check(sk, skb)) {
 			goto reset;
 		}
-		SKB_DR_SET(reason, TCP_INVALID_SEQUENCE);
 		goto discard;
 	}