tcp: Force TCP_WINDOW_CLAMP before resetting STALLED flag

It looks like we need it as workaround for this situation, readily
reproducible at least with a 6.5 Linux kernel, with default rmem_max
and wmem_max values:

- an iperf3 client on the host sends about 160 KiB, typically
  segmented into five frames by passt. We read this data using
  MSG_PEEK

- the iperf3 server on the guest starts receiving

- meanwhile, the host kernel advertised a zero-sized window to the
  sender, as expected

- eventually, the guest acknowledges all the data sent so far, and
  we drop it from the buffer, courtesy of tcp_sock_consume(), using
  recv() with MSG_TRUNC

- the client, however, doesn't get an updated window value, and
  even keepalive packets are answered with zero-window segments,
  until the connection is closed

It looks like dropping data from a socket using MSG_TRUNC doesn't
cause a recalculation of the window, which would be expected as a
result of any receiving operation that invalidates data on a buffer
(that is, not with MSG_PEEK).

Strangely enough, setting TCP_WINDOW_CLAMP via setsockopt(), even to
the previous value we clamped to, forces a recalculation of the
window which is advertised to the sender.

I couldn't quite confirm this issue by following all the possible
code paths in the kernel, yet. If confirmed, this should be fixed in
the kernel, but meanwhile this workaround looks robust to me (and it
will be needed for backward compatibility anyway).

Reported-by: Matej Hrica <mhrica@redhat.com>
Link: https://bugs.passt.top/show_bug.cgi?id=74
Analysed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

0 files changed, 0 insertions, 0 deletions