[PATCH v2 0/6] tcp: Fixes for issues uncovered by tests with 6.17-rc1 kernels

[Stefano Brivio <sbrivio@redhat.com>]

Starting from Linux kernel commit 1d2fbaad7cd8 ("tcp: stronger
sk_rcvbuf checks"), window limits are enforced more aggressively with
a bigger amount of zero-window updates compared to what happened with
e2142825c120 ("net: tcp: send zero-window ACK when no memory") alone,
and occasional duplicate ACKs can now be seen also for local transfers
with default (208 KiB) socket buffer sizes.

Paul reports that, with 6.17-rc1-ish kernels, Podman tests for the
pasta integration occasionally fail on the "TCP/IPv4 large transfer,
tap" case.

While playing with a reproducer that seems to be matching those
failures:

  while true; do ./pasta --trace -l /tmp/pasta.log -p /tmp/pasta.pcap --config-net -t 5555 -- socat TCP-LISTEN:5555 OPEN:/tmp/large.rcv,trunc & (sleep 0.3; socat -T2 OPEN:large.bin TCP:88.198.0.164:5555; ); wait; diff large.bin /tmp/large.rcv || break; done

and a kernel including that commit, I hit a few different failures,
that should be fixed by this series.

Paul tested v1 of this series and found an additional failure
(transfer timeout), which I could reproduce with a slightly different
command:

  while true; do ./pasta --trace -l /tmp/pasta.log -p /tmp/pasta.pcap --config-net -t 5555 -- socat TCP-LISTEN:5555 EXEC:./write.sh & (sleep 0.3; socat -T2 OPEN:large.bin TCP:88.198.0.164:5555; ); wait; diff large.bin /tmp/large.rcv || break; done

where write.sh is simply:

  #!/bin/sh
  
  cat > /tmp/large.rcv

so that the connection is not half-closed starting from the beginning,
because socat can't make assumptions about the unidirectional nature
of the traffic. This should now be fixed as well by the new version of
patch 3/6.


v2: in 3/6, rewind sequence also if the zero-window update comes in
    the middle of a batch with non-zero window updates


Stefano Brivio (6):
  tcp: FIN flags have to be retransmitted as well
  tcp: Factor sequence rewind for retransmissions into a new function
  tcp: Rewind sequence when guest shrinks window to zero
  tcp: Fix closing logic for half-closed connections
  tcp: Don't try to transmit right after the peer shrank the window to
    zero
  tcp: Fast re-transmit if half-closed, make TAP_FIN_RCVD path
    consistent

 tcp.c | 179 +++++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 127 insertions(+), 52 deletions(-)

-- 
2.43.0