Easyhook 32 bit application

Question

I'm using EasyHook to intercept registry calls. In more detail, I use RegQueryValue to intercept the call that reads a key from registry and change its value with something else. The relevant code looks like:

[UnmanagedFunctionPointer(CallingConvention.StdCall, CharSet = CharSet.Unicode, SetLastError = true)]
delegate int DRegQueryValueExW(
    IntPtr hKey,
    string lpValueName,
    int lpReserved,
    ref Microsoft.Win32.RegistryValueKind lpType,
    StringBuilder lpData,
    ref int lpcbData);

[DllImport("Advapi32.dll", CharSet = CharSet.Unicode, SetLastError = true, CallingConvention = CallingConvention.StdCall)]
static extern int RegQueryValueExW(
    IntPtr hKey,
    string lpValueName,
    int lpReserved,
    ref Microsoft.Win32.RegistryValueKind lpType,
    StringBuilder lpData,
    ref int lpcbData);

int RegQueryValueExW_Hooked(
    IntPtr hKey,
    string lpValueName,
    int lpReserved,
    ref Microsoft.Win32.RegistryValueKind lpType,
    StringBuilder lpData,
    ref int lpcbData)
{
    // todo: change value of lpData and return 0
    return RegQueryValueExW(hKey, lpValueName, lpReserved, ref lpType, lpData, ref lpcbData);                  
}

If I build everything with target x64, this all executes without a problem.

However, if I build it with target x32, it crashes in RegQueryValueExW_Hooked with the error:

Unhandled Exception: System.Runtime.InteropServices.SEHException: External component has thrown an exception. at DummyDCA.Program.Main(String[] args) Unhandled Exception: System.ArgumentOutOfRangeException: Capacity exceeds maximum capacity. Parameter name: capacity at AG.RU.Valuation.Controller.AFMToolbox.Inject.Main.RegQueryValueExW(IntPtrhKey, String lpValueName, Int32 lpReserved, RegistryValueKind& lpType, StringBuilder lpData, Int32& lpcbData) at AG.RU.Valuation.Controller.AFMToolbox.Inject.Main.RegQueryValueExW_Hooked(IntPtr hKey, String lpValueName, Int32 lpReserved, RegistryValueKind& lpType, StringBuilder lpData, Int32& lpcbData)

The problem seems to be lpData of type StringBuilder (some kind of overflow, the StringBuilder is not large enough or something). If I replace StringBuilder with IntPtr, it doesn't crash; but then I have a pointer and not a StringBuilder, so I'm not sure how I can replace the value of lpData.

Does anyone have an idea why this is, and how it should be done?

Thanks!

Answer 1

It seemed that instead of StringBuilder, I had to use IntPtr; and the implementation of RegQueryValueExW_Hooked also was a bit special.

The actual solution is described by Luaan in this thread: Changing the string to which an IntPtr is pointing