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codecave - game crashes

 
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661089799107
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Joined: 25 Jan 2009
Posts: 186
PostPosted: Sun Jan 23, 2011 2:41 am    Post subject: codecave - game crashes Reply with quote
The game is crashing immediately after changing the opcodes to a jump.

Here are the opcodes that I am trying to change:



After the change:



And the function it jumps to:



Code:

void jump() {
   __asm {
      mov [edi], ecx
      and ecx, 0xF8
   }
}


For some reason the opcodes are different, as you can see above. (83 E1 F8 and 81 E1 F800000)

And here is the function that I'm using to change to a jump: (Both functions 'change' & 'jump' are in a dll, which is injected into the game)

Code:

// change((BYTE*) 0x1038B626, (BYTE*) jump);

void change(BYTE *src, const BYTE *dst) {
   BYTE* jmp = (BYTE*) malloc(5);
   
   DWORD previous;

   VirtualProtect(src, 5, PAGE_READWRITE, &previous);
   
   memcpy(jmp, src, 5);
   jmp += 5;
 
   jmp[0] = 0xE9; // E9 = jump
   *(DWORD*)(jmp+1) = (DWORD)(src+5 - jmp) - 5;
   
   src[0] = 0xE9; // E9 = jump
   *(DWORD*)(src+1) = (DWORD)(dst - src) - 5;
   
   VirtualProtect(src, 5, previous, &previous);
}
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Dark Byte
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PostPosted: Sun Jan 23, 2011 7:14 am    Post subject: Reply with quote
the jump back is generated properly ? And you're not on a system that supports the no execute protection ?


anyhow, try mov ecx,0xfffffff8 instead of f8
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661089799107
Expert Cheater
Reputation: 3

Joined: 25 Jan 2009
Posts: 186
PostPosted: Sun Jan 23, 2011 2:41 pm    Post subject: Reply with quote
Dark Byte wrote:
the jump back is generated properly ? And you're not on a system that supports the no execute protection ?


anyhow, try mov ecx,0xfffffff8 instead of f8


Thanks Smile. The problem was I forgot to add a jump back..

Also changing 0xf8 to 0xfffffff8 fixed that problem as well.

Code:

void jump() {
   static DWORD returnAddress = 0x1038B62B;

   __asm {
      mov [edi], ecx
      and ecx, 0xfffffff8
      
      pop ebp
      jmp returnAddress
   }
}
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