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 <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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 };
@@ -1674,16 +1672,13 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
 		*data++ = OPT_WS;
 		*data++ = OPT_WS_LEN;
 		*data++ = conn->ws_to_tap;
-
+	} else if (!(flags & RST)) {
-		th->ack = !!(flags & ACK);
+		flags |= ACK;
-	} else {
-		th->ack = !!(flags & (ACK | DUP_ACK)) ||
-			  conn->seq_ack_to_tap != prev_ack_to_tap ||
-			  !prev_wnd_to_tap;
 	}
 
 	th->doff = (sizeof(*th) + optlen) / 4;
 
+	th->ack = !!(flags & ACK);
 	th->rst = !!(flags & RST);
 	th->syn = !!(flags & SYN);
 	th->fin = !!(flags & FIN);
@@ -2503,7 +2498,7 @@ out:
 		 */
 		if (conn->seq_dup_ack_approx != (conn->seq_from_tap & 0xff)) {
 			conn->seq_dup_ack_approx = conn->seq_from_tap & 0xff;
-			tcp_send_flag(c, conn, DUP_ACK);
+			tcp_send_flag(c, conn, ACK | DUP_ACK);
 		}
 		return p->count - idx;
 	}