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

On Thu, Oct 02, 2025 at 02:06:44AM +0200, Stefano Brivio wrote:

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 Signed-off-by: Stefano Brivio

Reviewed-by: David Gibson

--- 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

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson