Cheat Engine

iPromise · Posted: Sat May 28, 2011 11:57 am Post subject: Setting a breakpoint

Hey guys,

How would I set a int3 breakpoint on an address? Any tutorials, i've done lots of research and I havn't found a good topic.

Thanks!

sponge · Posted: Sat May 28, 2011 12:08 pm Post subject:

You place an int3 at that address... then handle the resulting exception when executed.

iPromise · Posted: Sat May 28, 2011 4:09 pm Post subject:

You place the int3 like this?

memcpy ( (void*) dwAddress, (void*) '\x03', 1 );

sponge · Posted: Sat May 28, 2011 4:18 pm Post subject:

int3...

iPromise · Posted: Sat May 28, 2011 5:42 pm Post subject:

I see.

Dark Byte · Posted: Sat May 28, 2011 6:02 pm Post subject:

Just a suggestion:
use

iPromise · Posted: Sat May 28, 2011 6:34 pm Post subject:

After I set the int3 breakpoint, the debugger isn't called? I try to access the address by writing to it and it still won't invoke the debugger. Am I doing something wrong?

Dark Byte · Posted: Sat May 28, 2011 6:41 pm Post subject:

an int3 breakpoint is an EXECUTE breakpoint. You are just overwriting the byte at the given address

If you mean a hardware breakpoint use SetThreadContext and set the address in dr0,1,2 or 3 and set the appropriate bit in DR7

iPromise · Posted: Sat May 28, 2011 8:18 pm Post subject:

Wouldn't a hardware breakpoint stop the whole thread?

See this is what I want to basically do, I want to set a breakpoint on address ex: 00400000 and when the game tries to access that address, it will send a EXCEPTION_BREAKPOINT to my VEH callback, then I can process the pointer from there.

EDIT

I came up with the code needed to set a hardware breakpoint:

Innovation · Posted: Sat May 28, 2011 8:42 pm Post subject:

Debugger flow control: Hardware breakpoints vs software breakpoints

You might want to take into account hardware breakpoints that are already enabled and assert that you are suspending the correct thread (and not the current thread). Also, you mentioned that you wanted it to break on access. 00b is "break on execution."

Dark Byte · Posted: Sat May 28, 2011 8:46 pm Post subject:

That is again an on execute breakpoint

you must set bit 16 to 1 if you want to break on writes to the address (set bit 16 AND 17 to 1 if you want to break on read+write)

and set the CONTEXT_DEBUG_REGISTERS in the ContextFlags of the context structure (when calling GetThreadContext and SetThreadContext)

Also, in your VEH handlr make sure you do not do anything that causes an exception. That includes OutputDebugString

iPromise · Posted: Sat May 28, 2011 9:26 pm Post subject:

Dark Byte · Posted: Sat May 28, 2011 9:33 pm Post subject:

first part: you're putting the 3 at the wrong bit position, write the 3 at startbit 16, (and of course also set bit 0 to 1)

as for the hanging, no idea. Check your veh debugger, or the code after the exception handler.

Innovation · Posted: Sat May 28, 2011 9:37 pm Post subject:

x86 debug register

You need to write to individual bits of the DR7 register.

iPromise · Posted: Sun May 29, 2011 9:52 am Post subject:

Thanks guys I got it working, my searching engine is one step away from being released. I just need to do some testing and make sure everything is stable.

You can lock the topic now.