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: d3192f67c492 ("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>

1 files changed, 3 insertions, 4 deletions

diff --git a/tcp_conn.h b/tcp_conn.h
index 01d31d4..0c6a35b 100644
--- a/tcp_conn.h
+++ b/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(3)
-#define ACK_TO_TAP_DUE		BIT(4)
-#define ACK_FROM_TAP_DUE	BIT(5)
+#define ACTIVE_CLOSE		BIT(2)
+#define ACK_TO_TAP_DUE		BIT(3)
+#define ACK_FROM_TAP_DUE	BIT(4)
 
 
 #define TCP_MSS_BITS			14