When a duplicate ack from the tap side triggers a fast re-transmit, we set both conn->seq_ack_from_tap and conn->seq_to_tap to the sequence number of the duplicate ack. Setting seq_to_tap is correct: this is what triggers the retransmit from this point onwards. Setting seq_ack_from_tap is not correct, though. In most cases setting seq_ack_from_tap will be redundant but harmless: it will have already been updated to the same value by tcp_update_seqack_from_tap() a few lines above. However that call can be skipped if tcp_sock_consume() fails, which is rare but possible. In that case this update will cause problems. We use seq_ack_from_tap to track two logically distinct things: how much of the stream has been acked by the guest, and how much of the stream from the socket has been read and discarded (as opposed to MSG_PEEKed). We attempt to keep those values the same, because we discard data exactly when it is acked by the guest. However tcp_sock_consume() failing means we weren't able to disard the acked data. To handle that case, we skip the usual update of seq_ack_from_tap, effectively ignoring the ack assuming we'll get one which supersedes it soon enough. Setting seq_ack_from_tap in the fast retransmit path, however, means we now really will have the read/discard point in the stream out of sync with seq_ack_from_tap. Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au> --- tcp.c | 1 - 1 file changed, 1 deletion(-) diff --git a/tcp.c b/tcp.c index 905d26f6..2ab443d5 100644 --- a/tcp.c +++ b/tcp.c @@ -2365,7 +2365,6 @@ static int tcp_data_from_tap(struct ctx *c, struct tcp_tap_conn *conn, flow_trace(conn, "fast re-transmit, ACK: %u, previous sequence: %u", max_ack_seq, conn->seq_to_tap); - conn->seq_ack_from_tap = max_ack_seq; conn->seq_to_tap = max_ack_seq; tcp_data_from_sock(c, conn); } -- 2.43.0