[PATCH 4/4] tcp: Unconditionally force ACK for all !SYN, !RST packets

Currently we set ACK on flags packets only when the acknowledged byte pointer has advanced, or we hadn't previously set a window. This means in particular that we can send a window update with no ACK flag, which doesn't appear to be correct. RFC 9293 requires a receiver to ignore such a packet [0], and indeed it appears that every non-SYN, non-RST packet should have the ACK flag. The reason for the existing logic, rather than always forcing an ACK seems to be to avoid having the packet mistaken as a duplicate ACK which might trigger a fast retransmit. However, earlier tests in the function mean we won't reach here if we don't have either an advance in the ack pointer - which will already set the ACK flag, or a window update - which shouldn't trigger a fast retransmit. [0] https://www.ietf.org/rfc/rfc9293.html#section-3.10.7.4-2.5.2.1 Link: https://github.com/containers/podman/issues/22146 Link: https://bugs.passt.top/show_bug.cgi?id=84 Signed-off-by: David Gibson --- tcp.c | 6 +----- 1 file changed, 1 insertion(+), 5 deletions(-) diff --git a/tcp.c b/tcp.c index b65ddeb5..28562b7f 100644 --- a/tcp.c +++ b/tcp.c @@ -1593,8 +1593,6 @@ static void tcp_update_seqack_from_tap(const struct ctx *c, */ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags) { - uint32_t prev_ack_to_tap = conn->seq_ack_to_tap; - uint32_t prev_wnd_to_tap = conn->wnd_to_tap; struct tcp4_l2_flags_buf_t *b4 = NULL; struct tcp6_l2_flags_buf_t *b6 = NULL; struct tcp_info tinfo = { 0 }; @@ -1675,9 +1673,7 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags) *data++ = OPT_WS_LEN; *data++ = conn->ws_to_tap; } else if (!(flags & RST)) { - if (conn->seq_ack_to_tap != prev_ack_to_tap || - !prev_wnd_to_tap) - flags |= ACK; + flags |= ACK; } th->doff = (sizeof(*th) + optlen) / 4; -- 2.44.0