[Source]Hooking via forced exception

Super bored so I decided to do this, now I'm bored again. >_>

The example below shows how forcing an access violation and setting up an exception handler to catch the exception could be used to hook a function. Yep...

Remember, I'm always looking for criticism to improve my code. |:

Well, enjoy?

Code:

#include <windows.h>

/*
xor eax,eax
mov eax,[eax]
nop
*/
BYTE bytes[] = { 0x33,0xC0,0x8B,0x00,0x90 }; //force access violation    

BYTE origBytes[5];
BYTE *addy;

int __stdcall Func(HWND hwnd,LPCTSTR text,LPCTSTR caption,UINT code)
{
    text = "Hooked";
    caption = "Hooked";

    return MessageBox(hwnd,text,caption,code);
}

LONG CALLBACK ExceptionHandler(PEXCEPTION_POINTERS pException)
{
    pException->ContextRecord->Eip = (DWORD)Func;
    for(int i=0;i<sizeof(bytes)+1;i++)
    {
        *(addy+i) = origBytes[i];
    }

    return EXCEPTION_CONTINUE_EXECUTION;
}

void Main()
{
    while(!GetModuleHandle("user32.dll"))
    {
        Sleep(10);
    }

    AddVectoredExceptionHandler(1,&ExceptionHandler);

    addy = (BYTE*)GetProcAddress( LoadLibrary("user32.dll"),"MessageBoxA" );

    DWORD old;
    VirtualProtect(addy,4096,PAGE_EXECUTE_READWRITE,&old);
    for(int i=0;i<sizeof(bytes)+1;i++)
    {
        origBytes[i] = *(addy+i);
        *(addy+i) = bytes[i];
    }
}

bool __stdcall DllMain(HINSTANCE hInst,DWORD dwReason,void* useless)
{
    if(dwReason == DLL_PROCESS_ATTACH)
    {
        CreateThread(0,0,(LPTHREAD_START_ROUTINE)Main,0,0,0);
    }
    return true;
}

This thread was inspired by me.

Uhg, I should have added the exception handler before writing the bytes to cause the exception. |:

kdone.

Heh, neato. Might try this out some time soon =o

Originally Posted by Void

Super bored so I decided to do this, now I'm bored again. >_>

The example below shows how forcing an access violation and setting up an exception handler to catch the exception could be used to hook a function. Yep...

Remember, I'm always looking for criticism to improve my code. |:

Well, enjoy?

[php]#include <windows.h>

/*
xor eax,eax
mov eax,[eax]
*/
BYTE bytes[] = { 0x33,0xC0,0x8B,0x00,0x90 }; //force access violation

BYTE origBytes[5];
BYTE *addy;

int __stdcall Func(HWND hwnd,LPCTSTR text,LPCTSTR caption,UINT code)
{
text = "Hooked";
caption = "Hooked";

return MessageBox(hwnd,text,caption,code);
}

LONG CALLBACK ExceptionHandler(PEXCEPTION_POINTERS pException)
{
pException->ContextRecord->Eip = (DWORD)Func;
for(int i=0;i<sizeof(bytes)+1;i++)
{
*(addy+i) = origBytes[i];
}

return EXCEPTION_CONTINUE_EXECUTION;
}

void Main()
{
while(!GetModuleHandle("user32.dll"))
{
Sleep(10);
}

AddVectoredExceptionHandler(1,&ExceptionHandler);

addy = (BYTE*)GetProcAddress( LoadLibrary("user32.dll"),"MessageBoxA" );

DWORD old;
VirtualProtect(addy,4096,PAGE_EXECUTE_READWRITE,&o ld);
for(int i=0;i<sizeof(bytes)+1;i++)
{
origBytes[i] = *(addy+i);
*(addy+i) = bytes[i];
}
}

bool __stdcall DllMain(HINSTANCE hInst,DWORD dwReason,void* useless)
{
if(dwReason == DLL_PROCESS_ATTACH)
{
CreateThread(0,0,(LPTHREAD_START_ROUTINE)Main,0,0, 0);
}
return true;
}[/php]

What is a BYTE?

Originally Posted by Gab

What is a BYTE?

unsigned char.

Apparently it's also something you do to the dust when you reach 2,000 posts..

If the game has its own exception handler (For a crash log, telling the player something bad happened, etc), would this method not work?
I've not poked exception handlers enough. Is it limited to where the exception happens or something?

Originally Posted by -Raz0r-

Apparently it's also something you do to the dust when you reach 2,000 posts..

If the game has its own exception handler (For a crash log, telling the player something bad happened, etc), would this method not work?
I've not poked exception handlers enough. Is it limited to where the exception happens or something?

Well, the first parameter of AddVectoredExceptionHandler is to tell the exception handler whether its the first handler or the last handler to be called. So assuming this is the last handler created, it should be called first. But Idk too much about it. |:

Edit: Mhm, MSDN: If the FirstHandler parameter is nonzero, the handler is the first handler to be called until a subsequent call to AddVectoredExceptionHandler is used to specify a different handler as the first handler.

I was right.

Still modifying bytes, thus you might as well use a jmp hook ;P
Debug Registers

Originally Posted by Hell_Demon

Still modifying bytes, thus you might as well use a jmp hook ;P
Debug Registers

Yeah, I should have caused an exception in another way, shoulda' just used VirtualProtect and set the rights to page_guard or something..

only problem is that a page spans across quite an address range, so it would trigger for other functions as well(and you can't return to the same function since it'd break again)

[Source]Hooking via forced exception

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