[PATCH v2] net:ipv4: send an ack when seg.ack > snd.nxt

[zhouzhouyi@gmail.com]

From: Zhouyi Zhou <zhouzhouyi@gmail.com>

In RFC 793, page 72: "If the ACK acks something not yet sent
(SEG.ACK > SND.NXT) then send an ACK, drop the segment,
and return."

Fix Linux's behavior according to RFC 793.

Reported-by: Wei Xu <xuweihf@ustc.edu.cn>
Signed-off-by: Wei Xu <xuweihf@ustc.edu.cn>
Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
---
Thank Florian Westphal for pointing out
the potential duplicated ack bug in patch version 1.
--
 net/ipv4/tcp_input.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index bfe4112e000c..4bbf85d7ea8c 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3771,11 +3771,13 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 		goto old_ack;
 	}
 
-	/* If the ack includes data we haven't sent yet, discard
-	 * this segment (RFC793 Section 3.9).
+	/* If the ack includes data we haven't sent yet, then send
+	 * an ack, drop this segment, and return (RFC793 Section 3.9 page 72).
 	 */
-	if (after(ack, tp->snd_nxt))
-		return -1;
+	if (after(ack, tp->snd_nxt)) {
+		tcp_send_ack(sk);
+		return -2;
+	}
 
 	if (after(ack, prior_snd_una)) {
 		flag |= FLAG_SND_UNA_ADVANCED;
@@ -6385,6 +6387,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 	struct request_sock *req;
 	int queued = 0;
 	bool acceptable;
+	int ret;
 
 	switch (sk->sk_state) {
 	case TCP_CLOSE:
@@ -6451,14 +6454,16 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 		return 0;
 
 	/* step 5: check the ACK field */
-	acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH |
-				      FLAG_UPDATE_TS_RECENT |
-				      FLAG_NO_CHALLENGE_ACK) > 0;
+	ret = tcp_ack(sk, skb, FLAG_SLOWPATH |
+				FLAG_UPDATE_TS_RECENT |
+				FLAG_NO_CHALLENGE_ACK);
+	acceptable = ret > 0;
 
 	if (!acceptable) {
 		if (sk->sk_state == TCP_SYN_RECV)
 			return 1;	/* send one RST */
-		tcp_send_challenge_ack(sk);
+		if (ret > -2)
+			tcp_send_challenge_ack(sk);
 		goto discard;
 	}
 	switch (sk->sk_state) {
-- 
2.25.1