[PATCH net-next] tcp: remove dead code in tcp_rcv_state_process()

Since commit 3d501dd326fb ("tcp: do not accept ACK of bytes we never
sent"), tcp_ack() bounds the acceptable old ACK window by
min(tp->max_window, tp->bytes_acked).

When sk->sk_state == TCP_SYN_RECV, tp->bytes_acked is always 0, so
any segment with before(ack, prior_snd_una) immediately returns
-SKB_DROP_REASON_TCP_TOO_OLD_ACK and never reaches the old_ack label
(which returns 0).

Therefore, tcp_ack() can only return 0 in closing states (where old
ACKs are accepted), and can never return 0 in TCP_SYN_RECV.

Simplify the tcp_ack() return value check in tcp_rcv_state_process()
to only check for negative return values and remove the unreachable
!reason branch.

Signed-off-by: Eric Dumazet <redacted>
---
 net/ipv4/tcp_input.c | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 92bc60716f33d81e9ce90d9de2e5d989ba71c8a3..892ff256e235272a8483a11949cc98b319dd7cc9 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c

@@ -7233,19 +7233,14 @@ tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 				  FLAG_UPDATE_TS_RECENT |
 				  FLAG_NO_CHALLENGE_ACK);
 
-	if ((int)reason <= 0) {
-		if (sk->sk_state == TCP_SYN_RECV) {
+	/* accept old ack (reason == 0) during closing */
+	if ((int)reason < 0) {
+		reason = -reason;
+		if (sk->sk_state == TCP_SYN_RECV)
 			/* send one RST */
-			if (!reason)
-				return SKB_DROP_REASON_TCP_OLD_ACK;
-			return -reason;
-		}
-		/* accept old ack during closing */
-		if ((int)reason < 0) {
-			tcp_send_challenge_ack(sk, false);
-			reason = -reason;
-			goto discard;
-		}
+			return reason;
+		tcp_send_challenge_ack(sk, false);
+		goto discard;
 	}
 	SKB_DR_SET(reason, NOT_SPECIFIED);
 	switch (sk->sk_state) {

-- 
2.55.0.1082.g2b9226bbc0-goog