tcp: fix tcp_ofo_queue() to avoid including too much DUP SACK range

If the new coming segment covers more than one skbs in the ofo queue,
and which seq is equal to rcv_nxt, then the sequence range
that is duplicated will be sent as DUP SACK, the detail as below,
in step6, the {501,2001} range is clearly including too much
DUP SACK range, in violation of RFC 2883 rules.

1. client > server: Flags [.], seq 501:1001, ack 1325288529, win 20000, length 500
2. server > client: Flags [.], ack 1, [nop,nop,sack 1 {501:1001}], length 0
3. client > server: Flags [.], seq 1501:2001, ack 1325288529, win 20000, length 500
4. server > client: Flags [.], ack 1, [nop,nop,sack 2 {1501:2001} {501:1001}], length 0
5. client > server: Flags [.], seq 1:2001, ack 1325288529, win 20000, length 2000
6. server > client: Flags [.], ack 2001, [nop,nop,sack 1 {501:2001}], length 0

After this fix, the final ACK is as below:

6. server > client: Flags [.], ack 2001, options [nop,nop,sack 1 {501:1001}], length 0

[edumazet] added a new packetdrill test in the following patch.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: xin.guo <guoxin0309@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250626123420.1933835-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 19a1542883dfba7211e08dcb44c82b4564d76f04..79e3bfb0108fd4e21de2295e879fbd521daa62f9 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4845,8 +4845,9 @@ static void tcp_ofo_queue(struct sock *sk)
 
                if (before(TCP_SKB_CB(skb)->seq, dsack_high)) {
                        __u32 dsack = dsack_high;
+
                        if (before(TCP_SKB_CB(skb)->end_seq, dsack_high))
-                               dsack_high = TCP_SKB_CB(skb)->end_seq;
+                               dsack = TCP_SKB_CB(skb)->end_seq;
                        tcp_dsack_extend(sk, TCP_SKB_CB(skb)->seq, dsack);
                }
                p = rb_next(p);