浅谈API HOOK技术（二）

       在这里我将要实现转跳。有人说修改内存内容要进入Ring 0 才可以。可是Windows本身提供了一个写内存的指令WriteProcessMemory。有了这把利器，我们几乎无所不能。如游戏的修改等在这里我们只谈APIHOOK。
function RepointFunction(OldFunc, NewFunc: Pointer): Integer;
var
   IsDone: TList;
   function RepointAddrInModule(hModule: THandle; OldFunc, NewFunc: Pointer): Integer;
   var
      Dos: PImageDosHeader;
      NT: PImageNTHeaders;
      ImportDesc: PImage_Import_Entry;
      RVA: DWORD;
      Func: ^Pointer;
      DLL: string;
      f: Pointer;
      written: DWORD;
   begin
      Result := 0;
      Dos := Pointer(hModule);
      if IsDone.IndexOf(Dos) >= 0 then exit;
      IsDone.Add(Dos);

      OldFunc := LocateFunctionAddress(OldFunc);

      if IsBadReadPtr(Dos, SizeOf(TImageDosHeader)) then exit;
      if Dos.e_magic <> IMAGE_DOS_SIGNATURE then exit;
      NT := Pointer(Integer(Dos) + dos._lfanew);

      RVA := NT^.OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_IMPORT]
         .VirtualAddress;

      if RVA = 0 then exit;
      ImportDesc := pointer(integer(Dos) + RVA);
      while (ImportDesc^.Name <> 0) do
      begin
         DLL := PChar(Integer(Dos) + ImportDesc^.Name);
         RepointAddrInModule(GetModuleHandle(PChar(DLL)), OldFunc, NewFunc);
         Func := Pointer(Integer(DOS) + ImportDesc.LookupTable);
         while Func^ <> nil do
         begin
            f := LocateFunctionAddress(Func^);
            if f = OldFunc then
            begin
               WriteProcessMemory(GetCurrentProcess, Func, @NewFunc, 4, written);
               if Written > 0 then Inc(Result);
            end;
            Inc(Func);
         end;
         Inc(ImportDesc);
      end;
   end;

begin
   IsDone := TList.Create;
   try
      Result := RepointAddrInModule(GetModuleHandle(nil), OldFunc, NewFunc);
   finally
      IsDone.Free;
   end;
end;
有了这两个函数我们几乎可以更改任何API函数。
我们可以先写一个DLL文件。我这里以修改Text相关函数为例：
先定义几个函数：
type
   TTextOutA = function(DC: HDC; X, Y: Integer; Str: PAnsiChar; Count: Integer): BOOL; stdcall;
   TTextOutW = function(DC: HDC; X, Y: Integer; Str: PWideChar; Count: Integer): BOOL; stdcall;
   TTextOut = function(DC: HDC; X, Y: Integer; Str: PChar; Count: Integer): BOOL; stdcall;
   TDrawTextA = function(hDC: HDC; lpString: PAnsiChar; nCount: Integer; var lpRect: TRect; uFormat: UINT): Integer; stdcall;
   TDrawTextW = function(hDC: HDC; lpString: PWideChar; nCount: Integer; var lpRect: TRect; uFormat: UINT): Integer; stdcall;
   TDrawText = function(hDC: HDC; lpString: PChar; nCount: Integer; var lpRect: TRect; uFormat: UINT): Integer; stdcall;
var
   OldTextOutA: TTextOutA;
   OldTextOutW: TTextOutW;
   OldTextOut: TTextOut;
   OldDrawTextA: TDrawTextA;
   OldDrawTextW: TDrawTextW;
   OldDrawText: TDrawText;
......
function MyTextOutA(DC: HDC; X, Y: Integer; Str: PAnsiChar; Count: Integer): BOOL; stdcall;
begin
   OldTextOutA(DC, X, Y, 'ABC', length('ABC'));
end;

function MyTextOutW(DC: HDC; X, Y: Integer; Str: PWideChar; Count: Integer): BOOL; stdcall;
begin
   OldTextOutW(DC, X, Y, 'ABC', length('ABC'));
end;

function MyTextOut(DC: HDC; X, Y: Integer; Str: PChar; Count: Integer): BOOL; stdcall;
begin
   OldTextOut(DC, X, Y, 'ABC', length('ABC'));
end;

function MyDrawTextA(hDC: HDC; lpString: PAnsiChar; nCount: Integer; var lpRect: TRect; uFormat: UINT): Integer; stdcall;
begin
   OldDrawTextA(hDC, 'ABC', length('ABC'), lpRect, uFormat);
end;

function MyDrawTextW(hDC: HDC; lpString: PWideChar; nCount: Integer; var lpRect: TRect; uFormat: UINT): Integer; stdcall;
begin
   OldDrawTextW(hDC, 'ABC', length('ABC'), lpRect, uFormat);
end;

function MyDrawText(hDC: HDC; lpString: PChar; nCount: Integer; var lpRect: TRect; uFormat: UINT): Integer; stdcall;
begin
   OldDrawText(hDC, 'ABC', length('ABC'), lpRect, uFormat);
end;

调用时我们要把原来的函数地址保存下来：
   if @OldTextOutA = nil then
      @OldTextOutA := LocateFunctionAddress(@TextOutA);
   if @OldTextOutW = nil then
      @OldTextOutW := LocateFunctionAddress(@TextOutW);
   if @OldTextOut = nil then
      @OldTextOut := LocateFunctionAddress(@TextOut);
   if @OldDrawTextA = nil then
      @OldDrawTextA := LocateFunctionAddress(@DrawTextA);
   if @OldDrawTextW = nil then
      @OldDrawTextW := LocateFunctionAddress(@DrawTextW);
   if @OldDrawText = nil then
      @OldDrawText := LocateFunctionAddress(@DrawText);
然后很顺其自然的用自己的函数替换掉原来的函数
   RepointFunction(@OldTextOutA, @MyTextOutA);
   RepointFunction(@OldTextOutW, @MyTextOutW);
   RepointFunction(@OldTextOut, @MyTextOut);
   RepointFunction(@OldDrawTextA, @MyDrawTextA);
   RepointFunction(@OldDrawTextW, @MyDrawTextW);
   RepointFunction(@OldDrawText, @MyDrawText);
        在结束时不要忘记恢复原来函数的入口，要不然你会死得很难看哟！好了我们在写一个Demo程序。你会说怎么文字没有变成ABC呀？是呀，你要刷新一下才行。最小化然后在最大化。看看变了没有。  
        要不然你就写代码刷新一下好了。至于去拦截其他进程的API那就用SetWindowsHookEx写一个其他的钩子将DLL映射进去就行了，我就不再浪费口水了。
掌握了该方法你几乎无所不能。你可以修改其它程序。你可以拦截Createwindow等窗口函数改变其他程序的窗口形状、你还可以入侵其它的程序，你还可以......嘿嘿。干了坏事别招出我来就行了。
我还写了个例子，请在CSDN上下载。