netdev - Re: [Bugme-new] [Bug 8405] New: pppd does stops compresion with "Lost compression sync"

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Date:	Thu, 3 May 2007 19:18:53 +0200
From:	Stefan Wenk <stefan.wenk@....at>
To:	Richard Purdie <rpurdie@...ys.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, netdev@...r.kernel.org,
	"bugme-daemon@...nel-bugs.osdl.org" 
	<bugme-daemon@...zilla.kernel.org>
Subject: Re: [Bugme-new] [Bug 8405] New: pppd does stops compresion with "Lost compression sync"

On Thursday 03 May 2007 02:23, Richard Purdie wrote:
> On Wed, 2007-05-02 at 18:59 +0200, Stefan Wenk wrote:
> > The situation now is similar as without any modifications. Instead of -5
> > (Z_BUF_ERROR)
> > we get back -3 (Z_DATA_ERROR) from zlib_inflate. Here is the kernel log
> >
> > kernel: PPP generic driver version 2.4.2
> > pppd[6101]: pppd 2.4.4 started by root, uid 0
> > pppd[6101]: Using interface ppp0
> > pppd[6101]: Connect: ppp0 <--> /dev/pts/8
> > pppd[6101]: kernel does not support PPP filtering
> > kernel: PPP BSD Compression module registered
> > kernel: PPP Deflate Compression module registered
> > pppd[6101]: Deflate (15) compression enabled
> > pppd[6101]: Cannot determine ethernet address for proxy ARP
> > pppd[6101]: local  IP address 192.168.3.2
> > pppd[6101]: remote IP address 192.168.3.1
> > kernel: z_decompress0: inflate returned -3 (),av in 0, av out 1, t in
> > 1520, t out 3070, dp 0, of 1, is 644, os 1504
> > kernel: z_decompress0: inflate returned -3 ()
> > pppd[6101]: Lost compression sync: disabling compression
> > pppd[6101]: Terminating on signal 2
> > pppd[6101]: Child process pppd (charshunt) (pid 6131) terminated with
> > signal 2 pppd[6101]: Modem hangup
> > pppd[6101]: Connect time 0.2 minutes.
> > pppd[6101]: Sent 2969 bytes, received 2774 bytes.
> > pppd[6101]: Connection terminated.
> > pppd[6101]: Exit.
> >
> > Attached you find the corresponding pppdump file.
>
> Thanks, the interesting bit is that overflow=1. The Z_DATA_ERROR is
> probably coming from zlib_inflateSyncPacket() suggesting a logic error.
>
> I managed to find a null modem cable, create this setup and occasionally
> reproduce the problem. I think the change I suggested was half way there
> but there is another problem. Could you try this patch and see if it
> fixes your errors?
>
> I haven't seen the problem occur here on my box with the patch below
> applied. Having said that, I was testing against a box running 2.6.11.2
> and I did see similar looking compression errors appearing on that just
> before I was about to send this after stressing the link for an hour.
> This could mean there is some more subtle problem (not related to the
> inflate patch). It is sending Reset requests rather than Term Requests
> though so perhaps its some other problem. Lets see what you find with
> the patch below...
>
> Cheers,
>
> Richard
>
> Index: linux-2.6.20-rc4/lib/zlib_inflate/inflate.c
> ===================================================================
> --- linux-2.6.20-rc4.orig/lib/zlib_inflate/inflate.c
> +++ linux-2.6.20-rc4/lib/zlib_inflate/inflate.c
> @@ -743,12 +745,14 @@ int zlib_inflate(z_streamp strm, int flu
>
>      strm->data_type = state->bits + (state->last ? 64 : 0) +
>                        (state->mode == TYPE ? 128 : 0);
> -    if (((in == 0 && out == 0) || flush == Z_FINISH) && ret == Z_OK)
> -        ret = Z_BUF_ERROR;
>
>      if (flush == Z_PACKET_FLUSH && ret == Z_OK &&
> -            (strm->avail_out != 0 || strm->avail_in == 0))
> +            strm->avail_out != 0 && strm->avail_in == 0)
>    return zlib_inflateSyncPacket(strm);
> +
> +    if (((in == 0 && out == 0) || flush == Z_FINISH) && ret == Z_OK)
> +        ret = Z_BUF_ERROR;
> +
>      return ret;
>  }
With this modification the reported problem with the lost compression sync is 
fixed indeed. Thank you! I have tried to stress the nullmodem link and on 
application level everthing looks fine now. But I do also sometimes see CCP 
Reset Requests which are sent by the box with the 2.6.21 kernel.  This 
problem seems to be fixed on the TCP layer, so the applications do not 
recognise. I can't tell is this the ordinary behaviour as I have never looked 
at the PPP level in such detail before. I do agree with you that if this is a 
problem it does not look to be related with the issue which you fixed now.

Ciao Stefan
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