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Re: [lwip-users] Resource starvation

From: Yoav Nissim
Subject: Re: [lwip-users] Resource starvation
Date: Tue, 15 Feb 2011 12:31:08 +0200
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv: Gecko/20100915 Lightning/1.0b2 Thunderbird/3.1.4

Hi Simon.

We've changed our code to call pppSigHup and pppClose using a

This caused a crash in a specific scenario when our serial port is

I believe this is a result of a bug in PPP code:

pppLinkTerminated is always called after lcp phase is set to PHASE_DEAD.
In it, the pppInput thread is "released" by calling pppRecvWakeup, the
application callback is called with PPPERR_PROTOCOL, and openFlag is set
to 0.

Since PPP does not manage the sio_fd (it simply receives an open handle
and does not close it), there must be a point in which PPP lets me know
I can have my sio_fd back. This point AFAICT is when my link_status_cb
is called with PPPERR_PROTOCOL.

The problem (the crash actually) occurs because both pppClose and pppHup
call pppRecvWakeup AFTER calling pppLinkTerminated. Since I release my
resources on pppLinkTerminated, the call to pppRecvWakeup references a
nonexistent sio_fd object.


1. pppHup does not need to call pppRecvWakeup at all.

2. pppRecvWakeup (in the flow of ppClose()) should check openFlag before
using the sio_fd. [It may not be needed at all, but I really don't know
enough about ppp close flow to tell]

Could you please confirm or correct these?



On 13/2/2011 23:28, Simon Goldschmidt wrote:

> Yoav Nissim <address@hidden> wrote:
>> The code in netif_set_ipaddr() does not handle bound PCBs and time_wait
>> PCBs. The listening PCBs in set_ipaddr() simply have their address
>> changed, but in our case we need to clean them up.
> Oh, right.
>> The active PCBs are taken care of by tcp_abort(). This leaves:
>> 1. time_wait PCBs which I believe can simply be removed.
>> 2. listening PCBs which cannot be processed by tcp_abort() - how do I
>> clean them up?
> I think they do have an err callback, but I'll have to check the sources...
>> 3. bound PCBs - I am not sure if I can call tcp_abort() on these, or
>> whether I would manually call TCP_EVENT_ERR(errf, errf_arg, ERR_ABRT),
>> and then have them removed. An application may be taken by surprise here...
> Calling tcp_abort is correct. In fact, netif_set_ipaddr already does this.
>> Not a problem. I just need to be sure I understand - this means that all
>> the functions in ppp.h under the "PUBLIC FUNCTIONS" comment [namely
>> pppOverSerialOpen, pppClose and pppSigHup] must be called using
>> tcpip_callback()?
> Again, I have to check that in the sources but I guess the answer is yes, 
> since every net if driver in lwIP has to do it that way, and PPP is 
> essentially a netif driver to lwIP.
> Simon
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> lwip-users mailing list
> address@hidden
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Yoav Nissim
Software Engineer, Software Tools Division
Jungo Software Technologies
Email: address@hidden
Web: http://www.jungo.com
Phone: +972-74-7212138
Fax: +972-74-7212122
Mobile: +972-54-2271315

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