[PATCH 2/4] tcp: Completely ignore data segment in CLOSE-WAIT state, log a message

[Stefano Brivio <sbrivio@redhat.com>]

According to RFC 9293 we should ignore data (note: not data segments)
in CLOSE-WAIT state (indicated by TAP_FIN_RCVD), see 3.10.7.4
"Other states":

  [...]

  Seventh, process the segment text:

  [...]

  CLOSE-WAIT STATE

  This should not occur since a FIN has been received from the remote
  side. Ignore the segment text.

and we almost do that, except that we would look at the data length
to decide whether it's a request for fast re-transmission, so fix
that, and while at it, log a message, so that cases such as the
following one are more apparent in debug logs:

28692   0.009758 88.198.0.164 → 93.235.151.95 54 TCP 55414 → 47080 [FIN, ACK] Seq=121441 Ack=141 Win=65536 Len=0

we should ignore this FIN flag, because we didn't accept data up
to this sequence (see next segment), but we don't do it, so, here:

28693   0.000036 93.235.151.95 → 88.198.0.164 54 TCP 47080 → 55414 [ACK] Seq=141 Ack=90722 Win=32128 Len=0
28694   0.034597 93.235.151.95 → 88.198.0.164 54 TCP 47080 → 55414 [FIN, ACK] Seq=141 Ack=90722 Win=121216 Len=0
28695   0.000019 88.198.0.164 → 93.235.151.95 54 TCP 55414 → 47080 [ACK] Seq=121442 Ack=142 Win=65536 Len=0
28696   0.162968 88.198.0.164 → 93.235.151.95 30773 TCP [TCP Retransmission] 55414 → 47080 [FIN, PSH, ACK] Seq=90722 Ack=142 Win=65536 Len=30719 [TCP segment of a reassembled PDU]

we are erroneously in CLOSE-WAIT (TAP_FIN_RCVD) state, and this
segment would look pretty strange there.

This specific case is fixed by the next patch, so it should never
happen again.

Link: https://archives.passt.top/passt-dev/20250910115726.432bbb8d@elisabeth/
Link: https://bugs.passt.top/show_bug.cgi?id=126
Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 tcp.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tcp.c b/tcp.c
index 48b1ef2..3f7dc82 100644
--- a/tcp.c
+++ b/tcp.c
@@ -2130,9 +2130,15 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
 
 	/* Established connections not accepting data from tap */
 	if (conn->events & TAP_FIN_RCVD) {
+		size_t dlen;
 		bool retr;
 
-		retr = th->ack && !tcp_packet_data_len(th, l4len) && !th->fin &&
+		if ((dlen = tcp_packet_data_len(th, l4len))) {
+			flow_dbg(conn, "data segment in CLOSE-WAIT (%zu B)",
+				 dlen);
+		}
+
+		retr = th->ack && !th->fin &&
 		       ntohl(th->ack_seq) == conn->seq_ack_from_tap &&
 		       ntohs(th->window) == conn->wnd_from_tap;
 
-- 
2.43.0