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Canary and Guard Pages in Ethereal

In order to help detect memory over- and under-flows in ([http://wiki.ethereal.com/EMEMification se_ and ep_]) allocated memory, Ethereal now places canary values (e.g., a gap or no-mans-land) after each allocation. When the memory is later freed, the canary values are checked to make sure that they haven't been changed. If they have, then someone has written past the memory that they allocated. See also [http://gcc.fyxm.net/summit/2003/Stackguard.pdf stackguard].

In addition, guard pages are placed before and after each block of memory. These are entire pages of memory that are mprotect()'d against writes so that if the page is written to (because someone went way beyond the end of their allocated memory), Ethereal will get a memory access violation (and hopefully dump core for post-mortem analysis).

With the guard pages in place, memory allocations now look like:

  +----+--------+---+--------+---+--------+---+-------+----+
  | gp | alloc1 | c | alloc2 | c | alloc3 | c |  ...  | gp |
  +----+--------+---+--------+---+--------+---+-------+----+

("gp" = guard page, "allocn" = allocated memory, "c" = canary).

What should I do if Ethereal detects such a canary violation?

If Ethereal exits stating "Per-packet memory corrupted." and exits (possibly leaving a core file (on *NIX, anyway) that points to epan/emem.c around line 600), then Ethereal detected a canary violation. If you're not a developer, you're not compiling Ethereal yourself, or you don't have the time to debug the issue yourself, please [http://bugs.ethereal.com open a bug] and attach a (preferrably small) capture file that exhibits the problem.

If you can debug the issue yourself, the best thing to do (again, on Linux, anyway) is to: