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
Reviewed-by: David Gibson
--- 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.
+ 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
-- 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