[TCP]: Fix shrinking windows with window scaling

[backported from 2.6 commit 607bfbf2d55dd1cfe5368b41c2a81a8c9ccf4723]

When selecting a new window, tcp_select_window() tries not to shrink
the offered window by using the maximum of the remaining offered window
size and the newly calculated window size. The newly calculated window
size is always a multiple of the window scaling factor, the remaining
window size however might not be since it depends on rcv_wup/rcv_nxt.
This means we're effectively shrinking the window when scaling it down.

The dump below shows the problem (scaling factor 2^7):

- Window size of 557 (71296) is advertised, up to 3111907257:

IP 172.2.2.3.33000 > 172.2.2.2.33000: . ack 3111835961 win 557 <...>

- New window size of 514 (65792) is advertised, up to 3111907217, 40 bytes
  below the last end:

IP 172.2.2.3.33000 > 172.2.2.2.33000: . 3113575668:3113577116(1448) ack 3111841425 win 514 <...>

The number 40 results from downscaling the remaining window:

3111907257 - 3111841425 = 65832
65832 / 2^7 = 514
65832 % 2^7 = 40

If the sender uses up the entire window before it is shrunk, this can have
chaotic effects on the connection. When sending ACKs, tcp_acceptable_seq()
will notice that the window has been shrunk since tcp_wnd_end() is before
tp->snd_nxt, which makes it choose tcp_wnd_end() as sequence number.
This will fail the receivers checks in tcp_sequence() however since it
is before it's tp->rcv_wup, making it respond with a dupack.

If both sides are in this condition, this leads to a constant flood of
ACKs until the connection times out.

Make sure the window is never shrunk by aligning the remaining window to
the window scaling factor.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 1 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 142eacceaeee2f..daa40d51b4b467 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -142,6 +142,10 @@ static __inline__ void tcp_event_ack_sent(struct sock *sk)
 	tcp_clear_xmit_timer(sk, TCP_TIME_DACK);
 }
 
+/* from 2.6's ALIGN, used in tcp_select_window() */
+#define ALIGN_WIN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
+#define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))
+
 /* Chose a new window to advertise, update state in tcp_opt for the
  * socket, and return result with RFC1323 scaling applied.  The return
  * value can be stuffed directly into th->window for an outgoing
@@ -162,7 +166,7 @@ static __inline__ u16 tcp_select_window(struct sock *sk)
 		 *
 		 * Relax Will Robinson.
 		 */
-		new_win = cur_win;
+		new_win = ALIGN_WIN(cur_win, 1 << tp->rcv_wscale);
 	}
 	tp->rcv_wnd = new_win;
 	tp->rcv_wup = tp->rcv_nxt;