Re: [PATCH v4 7/8] tcp: Fast re-transmit if half-closed, make TAP_FIN_RCVD path consistent

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 10 Sep 2025 12:27:12 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Tue, Sep 09, 2025 at 08:16:54PM +0200, Stefano Brivio wrote:
> > We currently have a number of discrepancies in the tcp_tap_handler()
> > path between the half-closed connection path and the regular one, and
> > they are mostly a result of code duplication, which comes in turn from
> > the fact that tcp_data_from_tap() deals with data transfers as well as
> > general connection bookkeeping, so we can't use it for half-closed
> > connections.
> > 
> > This suggests that we should probably rework it into two or more
> > functions, in the long term, but for the moment being I'm just fixing
> > one obvious issue, which is the lack of fast retransmissions in the
> > TAP_FIN_RCVD path, and a potential one, which is the fact we don't
> > handle socket flush failures.
> > 
> > Add fast re-transmit for half-closed connections, and handle the case
> > of socket flush (tcp_sock_consume()) flush failure in the same way as
> > tcp_data_from_tap() handles it.
> > 
> > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>  
> 
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> 
> > ---
> >  tcp.c | 42 +++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 39 insertions(+), 3 deletions(-)
> > 
> > diff --git a/tcp.c b/tcp.c
> > index 9c70a25..5163dbf 100644
> > --- a/tcp.c
> > +++ b/tcp.c
> > @@ -1652,6 +1652,23 @@ static int tcp_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
> >  	return tcp_buf_data_from_sock(c, conn);
> >  }
> >  
> > +/**
> > + * tcp_packet_data_len() - Get data (TCP payload) length for a TCP packet
> > + * @th:		Pointer to TCP header
> > + * @l4len:	TCP packet length, including TCP header
> > + *
> > + * Return: data length of TCP packet, -1 on invalid value of Data Offset field
> > + */
> > +static ssize_t tcp_packet_data_len(const struct tcphdr *th, size_t l4len)
> > +{
> > +	size_t off = th->doff * 4UL;
> > +
> > +	if (off < sizeof(*th) || off > l4len)
> > +		return -1;
> > +
> > +	return l4len - off;
> > +}
> > +
> >  /**
> >   * tcp_data_from_tap() - tap/guest data for established connection
> >   * @c:		Execution context
> > @@ -2113,9 +2130,28 @@ 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) {
> > -		tcp_sock_consume(conn, ntohl(th->ack_seq));
> > -		tcp_update_seqack_from_tap(c, conn, ntohl(th->ack_seq));
> > -		if (tcp_tap_window_update(c, conn, ntohs(th->window)))
> > +		bool retr;
> > +
> > +		retr = th->ack && !tcp_packet_data_len(th, l4len) && !th->fin &&  
> 
> Not really in scope here, but I wonder if we should log an error
> and/or RST if we get a non-zero data length in this situation.

According to RFC 9293 we should ignore data (note: not data segments)
in this case, 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.

  https://www.rfc-editor.org/rfc/rfc9293.html#section-3.10.7.4-2.7.2.7.1

We could add a debug() message perhaps (in a further patch), but I don't
think we are allowed to reset the connection.

> > +		       ntohl(th->ack_seq) == conn->seq_ack_from_tap &&
> > +		       ntohs(th->window) == conn->wnd_from_tap;
> > +
> > +		/* On socket flush failure, pretend there was no ACK, try again
> > +		 * later
> > +		 */
> > +		if (th->ack && !tcp_sock_consume(conn, ntohl(th->ack_seq)))
> > +			tcp_update_seqack_from_tap(c, conn, ntohl(th->ack_seq));
> > +
> > +		if (retr) {
> > +			flow_trace(conn,
> > +				   "fast re-transmit, ACK: %u, previous sequence: %u",
> > +				   ntohl(th->ack_seq), conn->seq_to_tap);
> > +
> > +			if (tcp_rewind_seq(c, conn))
> > +				return -1;
> > +		}
> > +
> > +		if (tcp_tap_window_update(c, conn, ntohs(th->window)) || retr)
> >  			tcp_data_from_sock(c, conn);
> >  
> >  		if (conn->seq_ack_from_tap == conn->seq_to_tap) {
> > -- 
> > 2.43.0

-- 
Stefano