ip_queue, is this safe:

[Peteris Krumins <peteris@lf.lv>]

Hello,

  I have written an application which uses ip_queue mechanism to quickly
  push (of course only some) packets from kernel space to user space.
  (The packets are queued in PREROUTING, mangle)
  The application is quite big and i bet i have made several mistakes,
  leaving unpleasent buffer overflows and something more.

  what i am interested in is if this is safe to write:

---
  [...]
  struct iphdr *ip;
  ip = (struct iphdr *)packet->payload;

  if (ip->protocol == IPPROTO_TCP) {
      struct tcphdr *tcp;

      tcp = (struct tcphdr *)(packet->payload + ip->ihl*4);
      [...]
  }
  [...]
---

  i am doubting:
  tcp = (struct tcphdr *)(packet->payload + ip->ihl*4);

  because ip->ihl could be faked, couldnt it? The packet would get queued
  and if ip->ihl is something faked, using the tcp pointer i'd get a
  segfault, wouldn't i?


P.Krumins