Re: [Bugme-new] [Bug 10767] New: Seg Fault Instead of Swapping

[David Miller <davem@davemloft.net>]

From: "Ilpo_Järvinen" <ilpo.jarvinen@helsinki.fi>
Date: Mon, 26 May 2008 14:24:33 +0300 (EEST)

Ilpo, do you want me to apply this to net-2.6?

> [PATCH] [TCP]: Fix inconsistency source (CA_Open only when !tcp_left_out(tp))
> 
> It is possible that this skip path causes TCP to end up into an
> invalid state where ca_state was left to CA_Open while some
> segments already came into sacked_out. If next valid ACK doesn't
> contain new SACK information TCP fails to enter into
> tcp_fastretrans_alert(). Thus at least high_seq is set
> incorrectly to a too high seqno because some new data segments
> could be sent in between (and also, limited transmit is not
> being correctly invoked there). Reordering in both directions
> can easily cause this situation to occur.
> 
> I guess we would want to use tcp_moderate_cwnd(tp) there as well
> as it may be possible to use this to trigger oversized burst to
> network by sending an old ACK with huge amount of SACK info, but
> I'm a bit unsure about its effects (mainly to FlightSize), so to
> be on the safe side I just currently fixed it minimally to keep
> TCP's state consistent (obviously, such nasty ACKs have been
> possible this far). Though it seems that FlightSize is already
> underestimated by some amount, so probably on the long term we
> might want to trigger recovery there too, if appropriate, to make
> FlightSize calculation to resemble reality at the time when the
> losses where discovered (but such change scares me too much now
> and requires some more thinking anyway how to do that as it
> likely involves some code shuffling).
> 
> This bug was found by Brian Vowell while running my TCP debug
> patch to find cause of another TCP issue (fackets_out
> miscount).
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>