netdev - incorrect tcp options with window scale

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Date:	Fri, 22 Aug 2008 19:15:20 -0700
From:	Philip Love <love_phil@....com>
To:	netdev@...r.kernel.org
Subject: incorrect tcp options with window scale

Attached is a patch against the latest 2.6.27-rc4 kernel.

When window scaling is enabled, but the tcp buffer sizes are small,  
then the tcp SYN packet created will have an incorrect tcp header  
length. The tcp length is calculated assuming that the window scale  
option will be inserted into the tcp header, but since the buffer  
sizes are too small the window scale is not used. Therefore the last 4  
bytes of the options in the tcp header are left uninitialized.

This can be reproduced by first setting the following variables.

echo 8192 > /proc/sys/net/core/rmem_max
echo "4096 8192 8192" > /proc/sys/net/ipv4/tcp_rmem
echo 1 > /proc/sys/net/ipv4/tcp_window_scaling

Then use wireshark to examine the tcp options in the SYN packet of a  
newly created connection.

*** a/net/ipv4/tcp_output.c	2008-08-20 19:35:56.000000000 -0700
--- b/net/ipv4/tcp_output.c	2008-08-22 12:04:00.000000000 -0700
***************
*** 468,474 ****
  	}
  	if (likely(sysctl_tcp_window_scaling)) {
  		opts->ws = tp->rx_opt.rcv_wscale;
! 		size += TCPOLEN_WSCALE_ALIGNED;
  	}
  	if (likely(sysctl_tcp_sack)) {
  		opts->options |= OPTION_SACK_ADVERTISE;
--- 468,475 ----
  	}
  	if (likely(sysctl_tcp_window_scaling)) {
  		opts->ws = tp->rx_opt.rcv_wscale;
! 		if(opts->ws)
! 			size += TCPOLEN_WSCALE_ALIGNED;
  	}
  	if (likely(sysctl_tcp_sack)) {
  		opts->options |= OPTION_SACK_ADVERTISE;
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