Re: [PATCH 6/6] tcp: Fast re-transmit if half-closed, make TAP_FIN_RCVD path consistent

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Aug 15, 2025 at 06:10:42PM +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,

Agreed.

> 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.

Fair enough for the time being.

> Add fast re-transmit for half-closed connections, and extract the
> logic to determine the TCP payload length from tcp_data_from_tap()
> into the new tcp_packet_data_len() helper to decrease a bit the amount
> of resulting code duplication.
> 
> 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 | 79 ++++++++++++++++++++++++++++++++++++++++++-----------------
>  1 file changed, 56 insertions(+), 23 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index 624e7f4..163f820 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -1640,6 +1640,22 @@ 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
> @@ -1671,27 +1687,21 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
>  	for (i = idx, iov_i = 0; i < (int)p->count; i++) {
>  		uint32_t seq, seq_offset, ack_seq;
>  		const struct tcphdr *th;
> +		ssize_t dlen;
>  		char *data;
> -		size_t off;
>  
>  		th = packet_get(p, i, 0, sizeof(*th), &len);
>  		if (!th)
>  			return -1;
>  		len += sizeof(*th);
>  
> -		off = th->doff * 4UL;
> -		if (off < sizeof(*th) || off > len)
> -			return -1;
> -
>  		if (th->rst) {
>  			conn_event(c, conn, CLOSED);
>  			return 1;
>  		}
>  
> -		len -= off;
> -		data = packet_get(p, i, off, len, NULL);
> -		if (!data)
> -			continue;
> +		if ((dlen = tcp_packet_data_len(th, len)) < 0)
> +			return -1;
>  
>  		seq = ntohl(th->seq);
>  		if (SEQ_LT(seq, conn->seq_from_tap) && len <= 1) {
> @@ -1719,7 +1729,7 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
>  			if (SEQ_GE(ack_seq, conn->seq_ack_from_tap) &&
>  			    SEQ_GE(ack_seq, max_ack_seq)) {
>  				/* Fast re-transmit */
> -				retr = !len && !th->fin &&
> +				retr = !dlen && !th->fin &&
>  				       ack_seq == max_ack_seq &&
>  				       ntohs(th->window) == max_ack_seq_wnd;
>  
> @@ -1731,33 +1741,37 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
>  		if (th->fin)
>  			fin = 1;
>  
> -		if (!len)
> +		if (!dlen)
> +			continue;
> +
> +		data = packet_get(p, i, th->doff * 4UL, dlen, NULL);
> +		if (!data)
>  			continue;
>  
>  		seq_offset = seq_from_tap - seq;
>  		/* Use data from this buffer only in these two cases:
>  		 *
> -		 *      , seq_from_tap           , seq_from_tap
> -		 * |--------| <-- len            |--------| <-- len
> +		 *      , seq_from_tap            , seq_from_tap
> +		 * |--------| <-- dlen           |--------| <-- dlen
>  		 * '----' <-- offset             ' <-- offset
>  		 * ^ seq                         ^ seq
> -		 *    (offset >= 0, seq + len > seq_from_tap)
> +		 *    (offset >= 0, seq + dlen > seq_from_tap)
>  		 *
>  		 * discard in these two cases:
> -		 *          , seq_from_tap                , seq_from_tap
> -		 * |--------| <-- len            |--------| <-- len
> +		 *          , seq_from_tap                 , seq_from_tap
> +		 * |--------| <-- dlen           |--------| <-- dlen
>  		 * '--------' <-- offset            '-----| <- offset
>  		 * ^ seq                            ^ seq
> -		 *    (offset >= 0, seq + len <= seq_from_tap)
> +		 *    (offset >= 0, seq + dlen <= seq_from_tap)
>  		 *
>  		 * keep, look for another buffer, then go back, in this case:
>  		 *      , seq_from_tap
> -		 *          |--------| <-- len
> +		 *          |--------| <-- dlen
>  		 *      '===' <-- offset
>  		 *          ^ seq
>  		 *    (offset < 0)
>  		 */
> -		if (SEQ_GE(seq_offset, 0) && SEQ_LE(seq + len, seq_from_tap))
> +		if (SEQ_GE(seq_offset, 0) && SEQ_LE(seq + dlen, seq_from_tap))
>  			continue;
>  
>  		if (SEQ_LT(seq_offset, 0)) {
> @@ -1767,7 +1781,7 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
>  		}
>  
>  		tcp_iov[iov_i].iov_base = data + seq_offset;
> -		tcp_iov[iov_i].iov_len = len - seq_offset;
> +		tcp_iov[iov_i].iov_len = dlen - seq_offset;
>  		seq_from_tap += tcp_iov[iov_i].iov_len;
>  		iov_i++;
>  
> @@ -2078,9 +2092,28 @@ 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, len) && !th->fin &&
> +		       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) {

-- 
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

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