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CERuleZ
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 Posted: Wed Aug 02, 2017 1:02 pm Post subject: Help my this conditional BP doesn't work |
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I have a call, then the next instruction is the instruction the call jumps to. On this instruction I have a BP. Why doesn't the same conditional BP work on the call instead? I put the conditional BP on the very first instruction the call jumps to, so the BP should also work on the call as well. I'm at a loss here. What am I missing?
| Code: | call edx -> readInteger(ECX+0x00000034)==0x00954150 -> This call goes to the next line. The conditional breakpoint doesn't work here. Why?
sub esp,24 -> readInteger(ECX+0x00000034)==0x00954150 -> works
some instructions
ret 000C |
Thank you.
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Dark Byte
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| Posted: Wed Aug 02, 2017 1:16 pm Post subject: |
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perhaps ECX+34 doesn't point to an address with value 954150 until after the call ? (code breakpoints run before execution)
perhaps a previous (conditional)jump jumps over the call directly to the sub when ECX is valid ?
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CERuleZ
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| Posted: Wed Aug 02, 2017 1:28 pm Post subject: |
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| Dark Byte wrote: | perhaps ECX+34 doesn't point to an address with value 954150 until after the call ? (code breakpoints run before execution)
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on the first instruction call jumps to it already points to the address
| Dark Byte wrote: | | perhaps a previous (conditional)jump jumps over the call directly to the sub when ECX is valid ? |
But then the return instruction would still jump back to the first instruction after the call if it wasn't the call that made the jump?
EDIT
By the way ty for the quick reply.
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Dark Byte
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| Posted: Wed Aug 02, 2017 1:44 pm Post subject: |
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Not sure what you mean with back to the first instruction.
what I meant was:
| Code: |
....
....
cmp ecx,0
jne valid
....
....
call edx //ECX is 0 at this point, so the bp condition is not met (ever)
valid:
//ECX contains a valid value here
sub esp,24
...
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call edx would never be called if ECX is valid
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CERuleZ
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| Posted: Wed Aug 02, 2017 1:57 pm Post subject: |
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| Dark Byte wrote: | Not sure what you mean with back to the first instruction.
what I meant was:
| Code: |
....
....
cmp ecx,0
jne valid
....
....
call edx //ECX is 0 at this point, so the bp condition is not met (ever)
valid:
//ECX contains a valid value here
sub esp,24
...
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call edx would never be called if ECX is valid |
When the return instruction executes, it goes to the line after the call. The return instruction always jumps back to one instruction after the jump, right? What I mean is I have this:
| Code: |
call edx -> goes to "sub esp,24"
mov eax,[ebx]
......
sub esp,24
some instructions
ret 000C -> goes back to "mov eax,[ebx]"
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Then how can the conditional BP work on the call but not on the sub instruction? Unless the return instruction can jump to a instruction that is not the instruction after the call?
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Dark Byte
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| Posted: Wed Aug 02, 2017 2:39 pm Post subject: |
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ah i misread it. i thought you meant call doesn't trigger but after the call it does.
what can cause that is that something inside that call changes the value at ecx+34 (or it changes ecx)
or the call never returns (exception)
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CERuleZ
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| Posted: Wed Aug 02, 2017 2:52 pm Post subject: |
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| Dark Byte wrote: |
what can cause that is that something inside that call changes the value at ecx+34 (or it changes ecx) |
So you're saying there is come code that executes after the call but before the sub?
Like: | Code: |
call edx -> goes to "mystery"
mov eax, [ebx]
...
mysterycode
sub esp,24
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Wait. I mean yes, the ecx is changed somewhere but it's after the sub instruction, so it shouldn't matter. I'm confused
| Quote: |
or the call never returns (exception) |
What do you mean? Then what about the "ret 000C"? 
EDIT
Is it possible that some other thread is changing the value of ECX or what ECX points at between the call and the first instruction the call executes?
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