m_pkthdr.rcvif dangling pointer problem

[Julian Elischer]

On 7/14/11 8:44 AM, Gleb Smirnoff wrote:
>    Hi!
>
>    This problem is definitely known and is as old as network stack
> is parallel. Those, who know the problem may skip to next paragraph.
> Short description is the following: every mbuf that is allocated in
> an interface receive procedure has a field where a pointer to the
> corresponding struct ifnet is stored. Everything is okay, until you
> are running a box, where configuration of interfaces is changing
> rapidly, for example a PPP access concentrator. Utilizing any facility
> that can delay packet processing for example netgraph(4) or dummynet(4)
> would make probability of crash much bigger. Running INVARIANTS kernel
> would crash a box with dummynet and disappearing interfaces in a few
> minutes.
>
>    I see three approaches to this problem:
>
>    1) Every m_pkthdr.rcvif should if_ref() the interface. Releasing
> references can be done in the mbuf allocator: mb_dtor_mbuf(),
> mb_dtor_pack(). I'm afraid this approach would degrate performance,
> since adding at least a couple of atomic ops on every mbuf for its
> lifetime. That is +2 atomic ops per packet.

I did that in the 3.x timeframe in a branch.. it's probably in the cvs 
tree still but was expensive.

better idea would be to use ifnet number and a generation number...


>    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().
> Smth like:
>
> struct ifnet *
> mbuf_rcvif(struct mbuf *m)
> {
>          struct ifnet *ifp;
>
>          M_ASSERTPKTHDR(m);
>
>          if (m->m_pkthdr.rcvif_idx == 0)
>                  return (NULL);
>
>          ifp = ifnet_byindex_locked(m->m_pkthdr.rcvif_idx);
>          if (ifp == NULL)
>                  return (NULL);
>          if (ifp->if_unique != m->m_pkthdr.rcvif_unique)
>                  return (NULL);
>
>          return (ifp);
> }
>
> struct ifnet *
> mbuf_rcvif_ref(struct mbuf *m)
> {
>          struct ifnet *ifp;
>
>          M_ASSERTPKTHDR(m);
>
>          if (m->m_pkthdr.rcvif_idx == 0)
>                  return (NULL);
>
>          ifp = ifnet_byindex_ref(m->m_pkthdr.rcvif_idx);
>          if (ifp == NULL)
>                  return (NULL);
>          if (ifp->if_unique != m->m_pkthdr.rcvif_unique) {
>                  if_rele(ifp);
>                  return (NULL);
>          }
>
>          return (ifp);
> }
>
> The size of struct mbuf isn't going to change on amd64:
>
> @@ -111,7 +111,8 @@
>    * Record/packet header in first mbuf of chain; valid only if M_PKTHDR is set.
>    */
>   struct pkthdr {
> -       struct ifnet    *rcvif;         /* rcv interface */
> +       uint32_t         rcvif_idx;     /* rcv interface index */
> +       uint32_t         rcvif_unique;  /* rcv interface unique id */
>
> A proof-of-concept patch is available here:
>
> http://people.freebsd.org/~glebius/patches/pkthdr.rcvif_idx
>
> It doesn't cover entire kernel, LINT won't compile. It covers kern, netinet,
> netinet6, several interface drivers and netgraph.
>
> One of the problems is ongoing namespace pollution: not all code that include
> mbuf.h includes if_var.h and vice versa. I've cut ifnet from m_devget(), but
> my new function do declare it in parameter list :(. To deal with that I had
> to declare functions in mbuf.h but implement them in if.c.
>
> Other problem is net80211 code that abuses the rcvif pointer in mbuf packet
> header for its own purposes casting it to private type. This can be fixed
> utilizing mbuf_tags(9), I think. I haven't made this yet, that's why LINT
> doesn't compile.
>
>    Comments are requested!
>
>    Any alternative ideas on solving the problem are welcome!
>