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Jul 18th, 2009
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function hook problem

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I am playing with function hooking so I wrote simple program that calls
DisplayMessage Function which prints out Hello..
C++ Syntax (Toggle Plain Text)
  1. void DisplayMessage()
  2. {
  3. 	cout << "Hello";
  4. }

After this one i found the DisplayMessage offset its (0x00131000)
now my target is write a dll to change DisplayMessage function to say bye instead of hello....

and here comes the problem...
DLL code
dll.h'
C++ Syntax (Toggle Plain Text)
  1. #ifndef _DLL_H_
  2. #define _DLL_H_
  3.  
  4. #if BUILDING_DLL
  5. # define DLLIMPORT __declspec (dllexport)
  6. #else /* Not BUILDING_DLL */
  7. # define DLLIMPORT __declspec (dllimport)
  8. #endif /* Not BUILDING_DLL */
  9.  
  10. DLLIMPORT void Hook(void);
  11.  
  12. #endif /* _DLL_H_ *
dll.cpp
C++ Syntax (Toggle Plain Text)
  1.  
  2. void (*PHook)(void);
  3. PHook = (void*)(0x00131000);
  4. PHook(void)
  5. {
  6. 	cout << "Bye";
  7. }
  8.  
  9. void Hook()
  10. { 
  11. 	PHook();	       
  12.  
  13. }
  14.  
  15.  
  16. BOOL APIENTRY DllMain (HINSTANCE hInst     /* Library instance handle. */ ,
  17.                        DWORD reason        /* Reason this function is being called. */ ,
  18.                        LPVOID reserved     /* Not used. */ )
  19. {
  20.     switch (reason)
  21.     {
  22.       case DLL_PROCESS_ATTACH:          
  23.            Hook();
  24.         break;
  25.  
  26.       case DLL_PROCESS_DETACH:
  27.  
  28.         break;
  29.  
  30.       case DLL_THREAD_ATTACH:
  31.  
  32.         break;
  33.  
  34.       case DLL_THREAD_DETACH:
  35.  
  36.         break;
  37.  
  38.     }
  39.     /* Returns TRUE on success, FALSE on failure */
  40.     return TRUE;
  41. }

its wont compile no idea why
heres the errors
C++ Syntax (Toggle Plain Text)
  1. mpiling...
  2. 1>dllmain.cpp
  3. 1>.\dllmain.cpp(11) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
  4. 1>.\dllmain.cpp(11) : error C2373: 'PHook' : redefinition; different type modifiers
  5. 1>        .\dllmain.cpp(10) : see declaration of 'PHook'
  6. 1>.\dllmain.cpp(11) : error C2440: 'initializing' : cannot convert from 'void *' to 'int'
  7. 1>        There is no context in which this conversion is possible
  8. 1>.\dllmain.cpp(13) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
  9. 1>.\dllmain.cpp(13) : error C2365: 'PHook' : redefinition; previous definition was 'data variable'
  10. 1>        .\dllmain.cpp(10) : see declaration of 'PHook'
  11. 1>.\dllmain.cpp(15) : warning C4508: 'PHook' : function should return a value; 'void' return type assumed
  12. 1>.\dllmain.cpp(18) : warning C4273: 'Hook' : inconsistent dll linkage
  13. 1>        c:\users\zippo\documents\visual studio 2008\projects\dynamic link libary\dynamic link libary\dll.h(10) : see previous definition of 'Hook'
  14. 1>.\dllmain.cpp(19) : error C3861: 'PHook': identifier not found


it give compile errors
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laconstantine is offline Offline
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Jul 18th, 2009
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Re: function hook problem

C++ Syntax (Toggle Plain Text)
  1. void (*PHook)(void); // 1
  2. PHook = (void*)(0x00131000); // 2
  3. PHook(void) // 3
  4. {
  5. 	cout << "Bye";
  6. }

The first error is on line //3, you forget the return type of the PHook function.

But all-together what you are doing isn't going to work out, if you're looking at some easy way to hook have a look at Microsoft's Detours library, it provides an easy way for you to hook your functions.
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thelamb is offline Offline
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Jul 18th, 2009
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Re: function hook problem

can you explain why it wont work?because i already hooked regular variables this way
im totaly new to it..
and yes i heard about detours libary but is there anyway to do it without it? because i really want to understand how it works
Last edited by laconstantine; Jul 18th, 2009 at 12:40 pm.
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Re: function hook problem

>>can you explain why it wont work?

It won't work because the addresses will change every time the program is run. Windows hook functions work because you pass it a pointer to the function, not the actually address value.

This should work
C++ Syntax (Toggle Plain Text)
  1. void PHook(void)
  2. {
  3. 	cout << "Bye";
  4. }
  5. void (*fn)(void) = PHook;
  6.  
  7. void Hook()
  8. { 
  9. 	fn();	       
  10.  
  11. }

or this
C++ Syntax (Toggle Plain Text)
  1. void PHook(void)
  2. {
  3. 	cout << "Bye";
  4. }
  5.  
  6. void Hook(void (*fn)() )
  7. { 
  8. 	fn();	       
  9.  
  10. }
  11.  
  12. int main()
  13. {
  14.     Hook( PHook );
  15. }
Last edited by Ancient Dragon; Jul 18th, 2009 at 12:59 pm.
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Jul 18th, 2009
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Re: function hook problem

You didnt get my point right I wana change function of a program by injecting a dll into it.

for example we have 2 progs.
1 is .exe which has function DisplayMessage and main funcs call it..
2.a dynamic link libary

now i want that my dll will have a function pointer which points to .exe function DisplayMessage and then change it.

and the final step inject the dll into .exe

is it possible? without detours libary.
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Re: function hook problem

you might search these links
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