m_pkthdr.rcvif dangling pointer problem

[John-Mark Gurney]

Gleb Smirnoff wrote this message on Thu, Jul 14, 2011 at 19:44 +0400:
>   2) kib@ suggested to allocate ifnets from a UMA_ZONE_NOFREE zone.
> I've made a compilable & working patch:
> 
> http://people.freebsd.org/~glebius/patches/ifnet.no_free
> 
> But on second though I find this a bad idea, this is just fooling
> of INVARIANTS. Yes, we avoid thrashing of freed memory and rewriting
> it by some other kernel allocation. But still out pointer point to
> invalid ifnet. Even, if we make a check for IFF_DYING flag, we still
> can not guarantee that an interface had been re-allocated for a new
> instance. This would be not a panic condition, but subtle bugs in
> firewalls.
> 
>   3) As we now have a straight if_index table that can grow, what about
> storing the if_index in the m_pkthdr? Lookup of interface by index
> is fast enough if done lockless. Doing it lockless isn't perfect, but
> better than current pointer dereferncing. Optionally it could be
> done with locking and with putting a reference. To avoid situation
> with with getting to a re-allocated interface with the same index,
> we can use a unique cookie, that is incremented in if_alloc().

How is this any different than #2?  I assume that if_index's are reused
causing the same issues w/ the firewall that #2 has.

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."