Hooking a DLL function in Delphi

Question

I have a coclass which checks the registry to determine if an application is installed, but it does this poorly and doesn't find newer versions of the application. In cases where a competing application has been installed, it will try to open that one. If the competing application has been uninstalled, the program will crash. This coclass is defined in a DLL file I do not have the source code for the library, so I can't just change that. I have been looking into using hooking to replace the function with one that works, but it seems complicated when I look at the MSDN documentation on using SetWindowsHookEx. Can someone please provide an example of how to use SetWindowsHookEx or another method of hooking into Windows?

Thank You

EDIT: I would like to note that I accepted the answer I did because it worked for me. I could not use the other answer at the time the question was asked, but it looks like its just as good.

Answer 1

Here is a short example from my own codebase which shows the most basic hooking technique:

unit MethodHooker;

interface

implementation

uses
  SysUtils, Windows, Classes;

procedure Patch(Address: Pointer; const NewCode; Size: Integer);
var
  NumberOfBytes: DWORD;
begin
  WriteProcessMemory(GetCurrentProcess, Address, @NewCode, Size, NumberOfBytes);
end;

type
  PInstruction = ^TInstruction;
  TInstruction = packed record
    Opcode: Byte;
    Offset: Integer;
  end;

procedure Redirect(OldAddress, NewAddress: Pointer);
var
  NewCode: TInstruction;
begin
  NewCode.Opcode := $E9;//jump relative
  NewCode.Offset := Integer(NewAddress)-Integer(OldAddress)-SizeOf(NewCode);
  Patch(OldAddress, NewCode, SizeOf(NewCode));
end;

function GetCursorPos(var lpPoint: TPoint): BOOL; stdcall;
(* The GetCursorPos API in user32 fails if it is passed a memory address >2GB which
   breaks LARGEADDRESSAWARE apps.  We counter this by calling GetCursorInfo instead
   which does not suffer from the same problem. *)
var
  CursorInfo: TCursorInfo;
begin
  CursorInfo.cbSize := SizeOf(CursorInfo);
  Result := GetCursorInfo(CursorInfo);
  if Result then begin
    lpPoint := CursorInfo.ptScreenPos;
  end else begin
    lpPoint := Point(0, 0);
  end;
end;

initialization
  if not ModuleIsPackage then begin
    if not CheckWin32Version(6, 1) then begin
      //this bug was fixed in Windows 7
      Redirect(@Windows.GetCursorPos, @MethodHooker.GetCursorPos);
    end;

end.

Answer 2

For a very good detouring/hooking unit (can check for jumps and apply new offset!) I would recommend KOLdetours.pas

I use this in many projects, for example my AsmProfiler.

Btw: with detouring you get a "trampoline" so you can call the original function too!

Answer 3

I am the auhtor of koldetours. The license is a free for all: i.e. use as you like EVEN IN COMMERCIAL PROGRAMS. Basically it is real open source, not crippled by any licensing. Just as the code from which it is derived. It clearly states so in the header.