Re: Virtual device and ARP table

[Eric Dumazet <eric.dumazet@gmail.com>]

Le lundi 07 juin 2010 à 12:21 +0200, Christophe Jelger a écrit :
> Hello,
> 
> I am currently "resurrecting" a Linux module (called LUNAR) which I 
> co-developed in 2007 and I'm having a weird kernel crash. This code 
> basically used to work fine up to 2.6.18 which was the latest version 
> before we stopped our development. I quickly ported it to 2.6.{31,32}: 
> it compiles fine and loads fine, but it crashes/hangs the kernel when 
> it's really being used.
> 
> The module is a virtual device used for MANET routing: with the current 
> version, it basically "captures" DNS requests sent to the virtual 
> interface --> this triggers the sending of a fake DNS reply (see below) 
> and the creation of an ARP table entry for the destination (the MANET 
> route is built at the same time). Packets can then be sent to the 
> destination.
> 
> The problem I'm having is that the kernel quickly hangs after I create a 
> new ARP entry (actually only if it's being used). If the entry I create 
> is set to NUD_PERMANENT, then everything works fine! I use 
> __neigh_lookup_errno to lookup/create the entry and neigh_lookup to 
> set/update the MAC address. Note that the ARP entry is created without 
> problem, but typically even just doing a userspace "arp -a" command can 
> crash the kernel (it also hangs the userspace command!). Doing "arp -na" 
> usually does NOT crash the kernel!
> 
> I guess the problem comes from a combination of ARP + DNS 
> lookups/replies. Note that my kernel module has its own internal fake 
> DNS server which captures lookups and sends replies directly back to the 
> stack. What is amazing: if the ARP entry I create is set to 
> NUD_PERMANENT, then I don't get any crash (however I cannot develop my 
> module with permanent ARP entries).
> 
> I'm wondering if there were any major changes to the neighbor and arp 
> code (between 2.6.18 and 2.6.31) that are somehow causing this problem ?...
> 
> Any hint is very welcome.
> 
> thanks in advance,
> Christophe
> 
> PS: I can easily reproduce the problem, and was trying to debug with 
> qemu and gdb server but so fra no success to clearly identify the 
> problem. Last point: it seems the kernel does not really "crash" but 
> rather ends up in some unstable state and maybe in a loop.
> --

Hi Christophe

You should ask these kind of questions on netdev instead of lkml.

And of course, post your patch, or send us a crystal ball ;)

Yes, many things changed between 2.6.18 and 2.6.34