tcp: Don't use TCP_WINDOW_CLAMP

On the L2 tap side, we see TCP headers and know the TCP window that the
ultimate receiver is advertising.  In order to avoid unnecessary buffering
within passt/pasta (or by the kernel on passt/pasta's behalf) we attempt
to advertise that window back to the original sock-side sender using
TCP_WINDOW_CLAMP.

However, TCP_WINDOW_CLAMP just doesn't work like this.  Prior to kernel
commit 3aa7857fe1d7 ("tcp: enable mid stream window clamp"), it simply
had no effect on established sockets.  After that commit, it does affect
established sockets but doesn't behave the way we need:
  * It appears to be designed only to shrink the window, not to allow it to
    re-expand.
  * More importantly, that commit has a serious bug where if the
    setsockopt() is made when the existing kernel advertised window for the
    socket happens to be zero, it will now become locked at zero, stopping
    any further data from being received on the socket.

Since this has never worked as intended, simply remove it.  It might be
possible to re-implement the intended behaviour by manipulating SO_RCVBUF,
so we leave a comment to that effect.

This kernel bug is the underlying cause of both the linked passt bug and
the linked podman bug.  We attempted to fix this before with passt commit
d3192f67 ("tcp: Force TCP_WINDOW_CLAMP before resetting STALLED flag").
However while that commit masked the bug for some cases, it didn't really
address the problem.

Fixes: d3192f67c4 ("tcp: Force TCP_WINDOW_CLAMP before resetting STALLED flag")
Link: https://github.com/containers/podman/issues/20170
Link: https://bugs.passt.top/show_bug.cgi?id=74
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

tcp.c

@@ -46,8 +46,8 @@
  *   - avoiding port and address translations whenever possible
  *   - mirroring TCP dynamics by observation of socket parameters (TCP_INFO
  *     socket option) and TCP headers of packets coming from the tap interface,
- *     reapplying those parameters in both flow directions (including TCP_MSS,
+ *     reapplying those parameters in both flow directions (including TCP_MSS
- *     TCP_WINDOW_CLAMP socket options)
+ *     socket option)
  * - simplicity: only a small subset of TCP logic is implemented here and
  *   delegated as much as possible to the TCP implementations of guest and host
  *   kernel. This is achieved by:
@@ -236,12 +236,10 @@
  *     - update @seq_ack_from_tap from ack_seq in header
  *     - on two duplicated ACKs, reset @seq_to_tap to @seq_ack_from_tap, and
  *       resend with steps listed above
- *     - set TCP_WINDOW_CLAMP from TCP header from tap
  *
  * - from tap/guest to socket:
  *   - on packet from tap/guest:
  *     - set @ts_tap_act
- *     - set TCP_WINDOW_CLAMP from TCP header from tap
  *     - check seq from header against @seq_from_tap, if data is missing, send
  *       two ACKs with number @seq_ack_to_tap, discard packet
  *     - otherwise queue data to socket, set @seq_from_tap to seq from header
@@ -420,7 +418,7 @@ static const char *tcp_state_str[] __attribute((__unused__)) = {
 };
 
 static const char *tcp_flag_str[] __attribute((__unused__)) = {
-	"STALLED", "LOCAL", "WND_CLAMPED", "ACTIVE_CLOSE", "ACK_TO_TAP_DUE",
+	"STALLED", "LOCAL", "ACTIVE_CLOSE", "ACK_TO_TAP_DUE",
 	"ACK_FROM_TAP_DUE",
 };
 
@@ -1790,58 +1788,16 @@ static void tcp_get_tap_ws(struct tcp_tap_conn *conn,
 
 /**
  * tcp_tap_window_update() - Process an updated window from tap side
- * @c:		Execution context
  * @conn:	Connection pointer
  * @window:	Window value, host order, unscaled
  */
-static void tcp_tap_window_update(const struct ctx *c,
+static void tcp_tap_window_update(struct tcp_tap_conn *conn, unsigned wnd)
-				  struct tcp_tap_conn *conn, unsigned wnd)
 {
-	uint32_t prev_scaled = conn->wnd_from_tap << conn->ws_from_tap;
-	int s = conn->sock;
-
 	wnd = MIN(MAX_WINDOW, wnd << conn->ws_from_tap);
 	conn->wnd_from_tap = MIN(wnd >> conn->ws_from_tap, USHRT_MAX);
 
-	/* TODO: With (at least) Linux kernel versions 6.1 to 6.5, if we end up
+	/* FIXME: reflect the tap-side receiver's window back to the sock-side
-	 * with a zero-sized window on a TCP socket, dropping data (once
+	 * sender by adjusting SO_RCVBUF? */
-	 * acknowledged by the guest) with recv() and MSG_TRUNC doesn't appear
-	 * to be enough to make the kernel advertise a non-zero window to the
-	 * sender. Forcing a TCP_WINDOW_CLAMP setting, even with the existing
-	 * value, fixes this.
-	 *
-	 * The STALLED flag on a connection is a sufficient indication that we
-	 * might have a zero-sized window on the socket, because it's set if we
-	 * exhausted the tap-side window, or if everything we receive from a
-	 * socket is already in flight to the guest.
-	 *
-	 * So, if STALLED is set, and we received a window value from the tap,
-	 * force a TCP_WINDOW_CLAMP setsockopt(). This should be investigated
-	 * further and fixed in the kernel instead, if confirmed.
-	 */
-	if (!(conn->flags & STALLED) && conn->flags & WND_CLAMPED) {
-		if (prev_scaled == wnd)
-			return;
-
-		/* Discard +/- 1% updates to spare some syscalls. */
-		/* TODO: cppcheck, starting from commit b4d455df487c ("Fix
-		 * 11349: FP negativeIndex for clamped array index (#4627)"),
-		 * reports wnd > prev_scaled as always being true, see also:
-		 *
-		 *	https://github.com/danmar/cppcheck/pull/4627
-		 *
-		 * drop this suppression once that's resolved.
-		 */
-		/* cppcheck-suppress [knownConditionTrueFalse, unmatchedSuppression] */
-		if ((wnd > prev_scaled && wnd * 99 / 100 < prev_scaled) ||
-		    (wnd < prev_scaled && wnd * 101 / 100 > prev_scaled))
-			return;
-	}
-
-	if (setsockopt(s, SOL_TCP, TCP_WINDOW_CLAMP, &wnd, sizeof(wnd)))
-		trace("TCP: failed to set TCP_WINDOW_CLAMP on socket %i", s);
-
-	conn_flag(c, conn, WND_CLAMPED);
 }
 
 /**
@@ -2452,7 +2408,7 @@ static int tcp_data_from_tap(struct ctx *c, struct tcp_tap_conn *conn,
 	if (ack && !tcp_sock_consume(conn, max_ack_seq))
 		tcp_update_seqack_from_tap(c, conn, max_ack_seq);
 
-	tcp_tap_window_update(c, conn, max_ack_seq_wnd);
+	tcp_tap_window_update(conn, max_ack_seq_wnd);
 
 	if (retr) {
 		trace("TCP: fast re-transmit, ACK: %u, previous sequence: %u",
@@ -2537,7 +2493,7 @@ static void tcp_conn_from_sock_finish(struct ctx *c, struct tcp_tap_conn *conn,
 				      const struct tcphdr *th,
 				      const char *opts, size_t optlen)
 {
-	tcp_tap_window_update(c, conn, ntohs(th->window));
+	tcp_tap_window_update(conn, ntohs(th->window));
 	tcp_get_tap_ws(conn, opts, optlen);
 
 	/* First value is not scaled */
@@ -2645,7 +2601,7 @@ int tcp_tap_handler(struct ctx *c, uint8_t pif, int af,
 		if (!th->ack)
 			goto reset;
 
-		tcp_tap_window_update(c, conn, ntohs(th->window));
+		tcp_tap_window_update(conn, ntohs(th->window));
 
 		tcp_data_from_sock(c, conn);
 
@@ -2669,9 +2625,6 @@ int tcp_tap_handler(struct ctx *c, uint8_t pif, int af,
 	if (count == -1)
 		goto reset;
 
-	/* Note: STALLED matters for tcp_tap_window_update(): unset it only
-	 * after processing data (and window) from the tap side
-	 */
 	conn_flag(c, conn, ~STALLED);
 
 	if (conn->seq_ack_to_tap != conn->seq_from_tap)

tcp_conn.h

@@ -85,10 +85,9 @@ struct tcp_tap_conn {
 	uint8_t		flags;
 #define STALLED			BIT(0)
 #define LOCAL			BIT(1)
-#define WND_CLAMPED		BIT(2)
+#define ACTIVE_CLOSE		BIT(2)
-#define ACTIVE_CLOSE		BIT(3)
+#define ACK_TO_TAP_DUE		BIT(3)
-#define ACK_TO_TAP_DUE		BIT(4)
+#define ACK_FROM_TAP_DUE	BIT(4)
-#define ACK_FROM_TAP_DUE	BIT(5)
 
 
 #define TCP_MSS_BITS			14