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