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Help, my INT 3 is not working

 
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igoticecream
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PostPosted: Mon Sep 20, 2010 9:22 pm    Post subject: Help, my INT 3 is not working Reply with quote

Does anybody knows the reason why, after INT 3 instruction is executed, my VEH does not handle it?

It can handle exceptions like divide by zero, but not breakpoints exceptions. Here's the code:

Code:

#define BUFFER_SIZE 512
#define BREAKPOINT _asm int 3

VOID DbgPrint(LPCTSTR Format, ...)
{
    va_list Arguments;
   va_start(Arguments, Format);
    TCHAR Buffer[BUFFER_SIZE];
   _vstprintf_s(Buffer, BUFFER_SIZE, Format, Arguments);
   va_end(Arguments);
 
    OutputDebugString(Buffer);
   OutputDebugString(TEXT("\r\n"));
}

LONG WINAPI VectoredExceptionHandler(PEXCEPTION_POINTERS pExceptionInfo)
{
   if (pExceptionInfo->ExceptionRecord->ExceptionCode == EXCEPTION_BREAKPOINT)
   {
      DbgPrint(_T("-BREAKPOINT-"));
      DbgPrint(_T("EAX: 0x%08X"),pExceptionInfo->ContextRecord->Eax);
      DbgPrint(_T("EBX: 0x%08X"),pExceptionInfo->ContextRecord->Ebx);
      DbgPrint(_T("ECX: 0x%08X"),pExceptionInfo->ContextRecord->Ecx);
      DbgPrint(_T("EDX: 0x%08X"),pExceptionInfo->ContextRecord->Edx);
      DbgPrint(_T("EDI: 0x%08X"),pExceptionInfo->ContextRecord->Edi);
      DbgPrint(_T("ESI: 0x%08X"),pExceptionInfo->ContextRecord->Esi);
      DbgPrint(_T("EBP: 0x%08X"),pExceptionInfo->ContextRecord->Ebp);
      DbgPrint(_T("ESP: 0x%08X"),pExceptionInfo->ContextRecord->Esp);
      DbgPrint(_T("EIP: 0x%08X"),pExceptionInfo->ContextRecord->Eip);
      return EXCEPTION_CONTINUE_EXECUTION;
   }
   return EXCEPTION_CONTINUE_SEARCH;
}

int _tmain(int argc, _TCHAR* argv[])
{
   PVOID pExceptionHandler = AddVectoredExceptionHandler(1,(PVECTORED_EXCEPTION_HANDLER)VectoredExceptionHandler);
   __asm
   {
      xor eax, eax
      BREAKPOINT
      inc eax
      BREAKPOINT
      xor eax, eax
      BREAKPOINT
   }
   _getch();
   return 0;
}


Thanks guys.

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Dark Byte
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PostPosted: Mon Sep 20, 2010 9:33 pm    Post subject: Reply with quote

Not sure, exactly what is happening or not happening?

also, make sure you do NOT have a debugger attached. If a debugger is attached the breakpoint will not get handled by the exception handler (debug comes before exception handler)

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igoticecream
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PostPosted: Mon Sep 20, 2010 9:44 pm    Post subject: Reply with quote

I ran it without any debugger attached to it, and what I expect the program do:

Execute: "xor eax, eax"
Execute: "int 3"
Output: "EAX: 0x00000000" (and the others registers, but im focus on this one)
Execute: "inc eax"
Execute: "int 3"
Output: "EAX: 0x00000001"
Execute: "xor eax, eax"
Execute: "int 3"
Output: "EAX: 0x00000000"

But my VEH does not handle the breakpoint exception (that should be generated by int 3, right?)

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Dark Byte
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PostPosted: Mon Sep 20, 2010 9:51 pm    Post subject: Reply with quote

try changing
Code:

if (pExceptionInfo->ExceptionRecord->ExceptionCode == EXCEPTION_BREAKPOINT)


with:
Code:

if (pExceptionInfo->ExceptionRecord->ExceptionCode != DBG_PRINTEXCEPTION_C)

That way the OutputDebugString shouldn't cause an infinite loop.

Perhaps int3 doesn't return EXCEPTION_BREAKPOINT but an intermediate kind of breakpoint (e.g : STATUS_WX86_BREAKPOINT )

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igoticecream
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PostPosted: Mon Sep 20, 2010 10:07 pm    Post subject: Reply with quote

You are right Dark Byte, OutputDebugString is causing infinite loop and my VEH gets spammed with 0x40010006 exception code (DBG_PRINTEXCEPTION_C). Can you explain this event? I'm so lost why OutputDebugString cause this.

EDIT: If i remove every call to OutputDebugString my code runs fine.

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PostPosted: Mon Sep 20, 2010 10:10 pm    Post subject: This post has 1 review(s) Reply with quote

That's just how it works. OutputDebugString raises a debug event which is either handled by the exception handler, or handled by the debugger

So just filter out those exceptions(You can just continue when you get them, just don't call DbgPrint while there)

also, if you're interested:
ExceptionInformation[0] contains stringlength
ExceptionInformation[1] contains the pointer to the string

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igoticecream
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PostPosted: Mon Sep 20, 2010 10:16 pm    Post subject: Reply with quote

Ok now is running just fine, thanks for the info dark byte... i didn't know how OutputDebugString worked and yea, that's explain the infinite loop... REP+

Also, i forgot a line of code

"pExceptionInfo->ContextRecord->Eip++;"

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