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From: Aaron W. LaFramboise (aaronrabiddog51_at_[hidden])
Date: 2004-07-31 15:49:40

Tony Juricic wrote:
> Since we are sort of messing with PEF and considering a separate DLL for
> catching thread exit notifications (it won't work with threads forcefully
> terminated with TerminateThread Win32 API) perhaps hooking Win32 API calls
> ExitThread and TerminateThread would be one solution?

I've given this some amount of thought, but to be honest, I don't see
any way this could be done reliably and without making some serious

-I think it might be possible for threads to die some other way. I
don't think anything makes any guarantee that ExitThread and
TerminateThread are the only ways a thread may exit. For example, a
thread may be caused to exit by some lower-level API (perhaps a direct
access to NTDLL).

-Hooking itself is problematic. I don't think that modifying kernel32's
in-memory EAT is going to catch all possible ways ExitThread might be
called. An alternate proposed strategy is to directly alter the first
few bytes of the function the EAT refers to, but this is also not
generally correct (a jmp is typically five bytes, but there might be
some jmp into the middle of that instr), and also is a system global
change on Win9x systems. Besides these problems, modifying in-memory
DLL images in any manner is undesirable, as it will trigger COW
semantics and cause possibly significant increases in memory usage.

-Hooking may interfere with proper operation of debuggers.

Hooking is great strategy for a debugger, or for programs designed to
act as debuggers, but I think it is far to risky a technique to rely on
for normal application programs.

The advantage you mention (besides possibly having a valid PTD at
dtor-time) is that we can execute dtors for threads killed by
TerminateThread. I don't know if this is a good idea. In the general
case, there is no particular guarantee about what context the thread
will be in when you call TerminateThread. In particular, any and all
thread-local data structures may be in an inconsistant state. Executing
dtors in this situation would be wrong, and would lead to undesirable
and unpredictable actions.

Aaron W. LaFramboise

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