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Index > Windows > callback function resides on dll gives error access denied

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sleepsleep



Joined: 05 Oct 2006
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sleepsleep
hi there Cool
the idea is like this, i was trying to make a enumwindowsproc in a dll, so that i could resuse it next time.

but the problem is, even a simple enumwindowsproc with code
mov eax,TRUE
ret
would give the below error.
Code:
The instruction at "0x004030bb" referenced memory at "0x0051f238". The memory could not be "written".
    

but if i put the exact enumwindowsproc on the .exe itself, it produces no error... i wonder why?

i attached with the dll and the sample test.exe for testing.


Description: 4 files here,
2 asm source, dll & exe
2 assembled files, dll & exe

Download
Filename: test.zip
Filesize: 1.77 KB
Downloaded: 61 Time(s)

Post 21 Mar 2007, 03:43
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
Maybe "pop [dllH]" is the actual faulty instruction. Why? Because you forgot to add the fixups to the DLL. Add "section '.reloc' fixups data discardable" to the source.

Cheers

[edit] Well you must still fix what I've told you but looking at the executable that use the DLL I found that you call "invoke EnumWindows,enumTopWindowProc,0" but it must be "invoke EnumWindows,[enumTopWindowProc],0"
Post 21 Mar 2007, 04:31
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sleepsleep



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sleepsleep
thanks LocoDelAssembly.
the [enumTopWindowProc] seems to solve the error.

but i don't understand why we need to use [enumTopWindowProc] ?
so, if we write mov eax,enumTopWindowProc
what address actually we get in eax? the pointer of enumTopWindowProc?
the pointer of pointer to enumTopWindowProc? the pointer of pointer of pointer to enumTopWindowProc? Confused
Post 21 Mar 2007, 08:59
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
You get the pointer of pointer to DLL function. Remember that EXE files are not relocated and uses an import table which consists of pointers to functions that Windows must fill on image load. Note that the same happens when calling, you need to use "call dword [function]" instead of "call function" and the reason again is because there is no relocs (and the last one is uncompilable).

DLL also uses import table but are relocatable, and for that you have to add the fixups because if the base address of DLL is occupied already by something else then your DLL will fail.
Post 21 Mar 2007, 13:55
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sleepsleep



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sleepsleep
sorry for my stupid question, i think i get it now Smile
thanks LocoDelAssembly
Post 21 Mar 2007, 16:46
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