Panic in ipfw

[Robert Watson]

On Tue, 3 Jul 2007, Ian FREISLICH wrote:

> "Andrey V. Elsukov" wrote:
>>
>>> I got this panic yesterday on a fairly busy firewall.  I have some private 
>>> patches to ip_fw2.c and to the em driver (see the earlier "em0 hijacking 
>>> traffic to port 623" thread).  I don't think this panic is a result of 
>>> those changes.
>>
>>> It occurred round about the time an address was added to an interface.
>>
>> I have a patch that can help you (i guess..). Can you test this patch?
>>
>> http://butcher.heavennet.ru/patches/kernel/inaddr_locking/
>
> Thanks.  Wow, that looks like it touches a lot more than just ipfw. It took 
> about 1.5 years of production at 2.3 billion backets a day to trigger this 
> condition twice.  It's going to be difficult to tell if this patch fixes the 
> problem.

This, FYI, is actually the reason why the locking isn't there now -- when 
prioritizing things to make MPSAFE, performance work, etc, address/ifnet 
lists, with the exception of multicast address lists, it was clear that they 
were basically static data structures.  Andrey's patch addresses problems I've 
wanted to work on for several years, and I'm very pleased he's working on it. 
It's only part of the solution to the long-term problem there -- we have a 
number of other synchronization issues for data structures that are nearly 
almost always static, and some life cycle issues with ifnet registration.  To 
date, I think this is really the first bug report I've seen that I could 
authoritatively point the finger at missing synchronization at the 
ifnet/ifaddr layer as the source.

My hope is that these will be addressed in FreeBSD 8.x, especially with the 
upcoming read-mostly locks, which will have almost zero cost to acquire for 
read protection, exactly what we'd like to see for these code paths.  Some of 
these changes may be mergeable to the 7.x branch, but will need lots of time 
to "burn in", as the risks of such changes are non-trivial.  I did some 
initial work to properly lock down the ifaddr address lists for netinet, and 
found they required moderate rearrangement of the address management ioctl 
paths, which are pretty complex without being rerranged :-).

Robert N M Watson
Computer Laboratory
University of Cambridge