title: How to create a trampoline function for hook
body: <p>I'm interested in hooking and I decided to see if I could hook some functions. I wasn't interested in using a library like detours because I want to have the experience of doing it on my own. With some sources I found on the internet, I was able to create the code below. It's basic, but it works alright. However when hooking functions that are called by multiple threads it proves to be extremely unstable. If two calls are made at nearly the same time, it'll crash. After some research I think I need to create a trampoline function. After looking for hours all I was not able to find anything other that a general description on what a trampoline was. I could not find anything specifically about writing a trampoline function, or how they really worked. If any one could help me write one, post some sources, or at least point me in the right direction by recommending some articles, sites, books, etc. I would greatly appreciate it.</p>
  
  <p>Below is the code I've written. It's really basic but I hope others might learn from it.</p>
  
  <p>test.cpp</p>
  
  <pre><code>#include "stdafx.h"
  
  Hook hook;
  
  typedef int (WINAPI *tMessageBox)(HWND hWnd, LPCTSTR lpText, LPCTSTR lpCaption, UINT uType);
  
  DWORD hMessageBox(HWND hWnd, LPCTSTR lpText, LPCTSTR lpCaption, UINT uType)
  {
      hook.removeHook();
      tMessageBox oMessageBox = (tMessageBox)hook.funcPtr; 
      int ret =oMessageBox(hWnd, lpText, "Hooked!", uType);
      hook.applyHook(&amp;hMessageBox);
  
      return ret;
  }
  
  void hookMessageBox()
  {
      printf("Hooking MessageBox...\n");
      if(hook.findFunc("User32.dll", "MessageBoxA")) 
      {
          if(hook.applyHook(&amp;hMessageBox))
          {
              printf("hook applied! \n\n");
          } else printf("hook could not be applied\n");
      }   
  }
  </code></pre>
  
  <p>hook.cpp</p>
  
  <pre><code>#include "stdafx.h"
  
  bool Hook::findFunc(char* libName, char* funcName) 
  {
      Hook::funcPtr = (void*)GetProcAddress(GetModuleHandleA(libName), funcName); 
      return (Hook::funcPtr != NULL);
  }
  
  bool Hook::removeHook() 
  {
      DWORD dwProtect;
      if(VirtualProtect(Hook::funcPtr, 6, PAGE_EXECUTE_READWRITE, &amp;dwProtect))
          {
          WriteProcessMemory(GetCurrentProcess(), (LPVOID)Hook::funcPtr, Hook::origData, 6, 0);
          VirtualProtect(Hook::funcPtr, 6, dwProtect, NULL);
          return true;
      } else return false;
  }
  
  bool Hook::reapplyHook() 
  {
      DWORD dwProtect;
      if(VirtualProtect(funcPtr, 6, PAGE_EXECUTE_READWRITE, &amp;dwProtect)) 
          {
          WriteProcessMemory(GetCurrentProcess(), (LPVOID)funcPtr, Hook::hookData, 6, 0);
          VirtualProtect(funcPtr, 6, dwProtect, NULL);
          return true;
      } else return false;
  }
  
  bool Hook::applyHook(void* hook) 
  { 
      return setHookAtAddress(Hook::funcPtr, hook);
  }
  
  bool Hook::setHookAtAddress(void* funcPtr, void* hook) 
  {
      Hook::funcPtr = funcPtr;
      BYTE jmp[6] = { 0xE9, //jmp
                     0x00, 0x00, 0x00, 0x00,  //address
                     0xC3 //retn 
                   };
  
      DWORD dwProtect;
  
      if(VirtualProtect(funcPtr, 6, PAGE_EXECUTE_READWRITE, &amp;dwProtect)) // make memory writable
      {
  
          ReadProcessMemory(GetCurrentProcess(), (LPVOID)funcPtr, Hook::origData, 6, 0); // save old data
          DWORD offset = ((DWORD)hook - (DWORD)funcPtr - 5);  //((to)-(from)-5)
          memcpy(&amp;jmp[1], &amp;offset, 4); // write address into jmp
          memcpy(Hook::hookData, jmp, 6); // save hook data
          WriteProcessMemory(GetCurrentProcess(), (LPVOID)funcPtr, jmp, 6, 0); // write jmp
          VirtualProtect(funcPtr, 6, dwProtect, NULL); // reprotect
  
          return true;
      } else return false;
  }
  </code></pre>