Being a big fan of ADetours, created by Azorbix, I've ported it to ring0 a while ago & now i've enhanced it a bit to make a release.

What it does now:

ADetourKernelFunc:
- auto-detect opcode size
- lock pages in memory before accessing them (needed for detours over pageable memory like win32k.sys)

ARetourKernelFunc:
- you can now decide wheter to free or redirect the trampoline.

Usage:
at the top of your .cpp/.c file:
PHP Code:
#include "adetours.h"

// define Example1 functions and init the Org_Example1 address to 0x101010
// > 1st arg: use page locking before hooking (boolean) TRUE/FALSE
// >             set to TRUE if hooking pageable memory
// > 2nd arg: function name
// > 3rd arg: return type
// > 4th arg: arguments with "( )"
// > 5th arg: original function address
// > 6th arg: calling convention (this is optional) ADETOUR_DEFINE_AND_INIT(TRUE,Example1, BOOLEAN, (IN LONG arg1), 0x101010, NTAPI);
// define Example2 functions and don't init Org_Example2
// > we could also add a ", NTAPI" here,
// > but as it is optional, we don't do it this time,
// > attention: callconvention will default to your compilers default setting doing so ADETOUR_DEFINE(FALSE,Example2, VOID, (IN LONG arg1) ); 

in your .cpp/.c file:
PHP Code:
void AddDetours() {
   // set detour & auto-detect opcode size
   ADETOUR_HOOK(Example1);
   // init Org_Example2
   Org_Example2 = (Example2_Func)0x202020;
   // set detour & use 8 as opcode length (minimal value is 5)
   ADETOUR_HOOK_LEN(Example2, 8);
} void RemoveDetours() {
    // remove detour & don't free trampoline and redirect it to Org_Example1
     ADETOUR_UNHOOK(Example1, false);
     // remove detour with given opcode length & free trampoline
     ADETOUR_UNHOOK_LEN(Example2, 8, true);
}
BOOLEAN NTAPI My_Example1(IN LONG arg1)
{
  return Trmp_Example1(arg1);
}
VOID NTAPI My_Example2(IN LONG arg1)
{
  return Trmp_Example2(arg1);
} 
About Hooking
- don't enable any sti/cli/cr0 asm tricks before hooking/unhooking, adetours will do this for you


Hooking Win32k.sys
- make sure to use page locking
- before hooking/unhooking make sure you are in a GUI-process context,
either attach to one ( check Sheppard's Ring 0 Hack for Diablo 2 ), or better send an IOCTL to your driver and do it from there , cause attaching sometimes causes problems during hooking



Credits
Matthew L (Azorbix)
Dom1n1k
LanceVorgin
P47R!CK
rain
Ms-Rem

Changelog
2.1
- converted to C
- better names for the function arguments

2.0
- can now choose to use page locking or not
- added cr0 / cli / sti tricks
- fixed callconvention macros
- general improvements

1.1
- rewrote page locking functions

1.0
- first release