[CRIU] [PATCH] net: Do not toggle TCP_REPAIR while restoring TCP send queues

[Amey Deshpande]

For an established TCP connection, the send queue is restored in two
steps: in step (1), we retransmit the data that was sent before but not
yet acknowledged, and in step (2), we transmit the data that was never
sent outside before.  The TCP_REPAIR option is disabled before step (2)
and re-enabled after step (2) (without this patch).

If the amount of data to be sent in step (2) is large, the TCP_REPAIR
flag on the socket can remain off for some time (O(milliseconds)).  If a
listen() is called on another socket bound to the same port during this
time window, it fails. This is because -- turning TCP_REPAIR off clears
the SO_REUSEADDR flag on the socket.

There are several possible ways to prevent this problem from happening:
- The simplest option is to *not* toggle TCP_REPAIR option while
  restoring the TCP queues.
- Another way would be to explicitly enable SO_REUSEADDR on the
  socket after turning TCP_REPAIR off.  This still leaves a small time
  window, and such race could still occur.
- A more involved solution would use a mutex per port number, so
  that a listen() on a port number does not happen while SO_REUSEADDR for
  another socket on the same port is off.

This patch removes the toggling of TCP_REPAIR option during restoring
TCP send queues.

Signed-off-by: Amey Deshpande <ameyd at google.com>
---
 sk-tcp.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/sk-tcp.c b/sk-tcp.c
index 3f1556d..02ed1f9 100644
--- a/sk-tcp.c
+++ b/sk-tcp.c
@@ -534,11 +534,8 @@ static int restore_tcp_queues(int sk, TcpStreamEntry *tse, struct cr_img *img)
 	 * they can be restored without any tricks.
 	 */
 	len = tse->unsq_len;
-	tcp_repair_off(sk);
 	if (len && __send_tcp_queue(sk, TCP_SEND_QUEUE, len, img))
 		return -1;
-	if (tcp_repair_on(sk))
-		return -1;
 
 	return 0;
 }
-- 
2.2.0.rc0.207.ga3a616c