a new(?) bug in Olly [776]

I’ve found a bug in Olly leading to crash SEH/VEH-based programs during tracing. (an example-pack could be found at: http://nezumi.org.ru/olly-bug-776.zip, it includes two SEH/VEH programs and requests XP or latter to run VEH, while SEH works everywhere).

Load an example into Olly and start tracing [F7] /* we can skip the first call by pressing [F8] (Step Over), since it has nothing interesting for us */

xor eax,eax/mov eax,[eax] generates an exceptions, catching by OS kernel. the kernel saves registry context (including TF-bit, set by Olly to “1”) and passes control to NTDLL.DLL!KiUserExceptionDispatcher function – the first user-land function executes after an exception. it calls SEH/VEH handler (if there is at lest one).

if the handler returns control to the code, where the exception was raised, registry context (including TF-bit) is restored by OS kernel. everything is fine, yes? hell, no!!!

Olly catches NTDLL.DLL!KiUserExceptionDispatcher and offers us to press Shift?F7/F8/F9. Olly allows to trace KiUserExceptionDispatcher, but totally forgets to clean TF-bit. as result – when control is passed the to original code (to the command follows mov eax,[eax] in our case, since handler adds 2 bytes /* size of mov eax,[eax] */ to EIP), TF-bit is set!!! so, a new exception occurs and SEH/VEH handler is called again and over again, coz, Olly passes this exception to the program, coz it forgets who “owns” this exception!!!

since, SEH/VEH handler doesn’t expect to handle trace exaction, we have an undefined behavior (a crash).


it’s definitely a bug!!! and how we can to fix it?!
well, it’s very simple. look at the prototype of KiUserExceptionDispatcher(EXCPT_REC *pExcptRec, CONTEXT *pContext). everything we need – is just to take pContext and clear TF-bit manually. pContext is the next DWORD on the stack and EFlags has C0h offset, while TF-bit is the first bit of the next word (1 << 8). just clean it and enjoy! press [F9] to Run the program or do whatever you want!

of course, we can create plug-in, doing it automatically, or set-up a conditional breakpoint on KiUserExceptionDispatcher.

I’ve checked the latest 2.00e version [April 19, 2008] and… the bug is still there, damn it!

I reported about this bug to the Olly creator and he admitted it, writing me back:

# Dear Kris,
# thank you for your bug report. I will try to remove it from v2.0 ASAP.
# Sincerely, Olly


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