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authorDavid Gibson <david@gibson.dropbear.id.au>2024-02-07 23:57:21 +1100
committerStefano Brivio <sbrivio@redhat.com>2024-02-11 18:30:01 +0100
commit78901eeae3b37c0e15f34512e5ea0c1b7acc4642 (patch)
tree709bc3d7fcb94d675bb633904e2d54274a94029c
parent6c7623d07bbd44d81beb4aa60b0a1de12b622630 (diff)
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tcp: Fix subtle bug in fast re-transmit path
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@gibson.dropbear.id.au> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
-rw-r--r--tcp.c1
1 files changed, 0 insertions, 1 deletions
diff --git a/tcp.c b/tcp.c
index 905d26f..2ab443d 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);
}