Determine path to registry key from HKEY handle in C++

Question

Given a handle to a Windows Registry Key, such as the ones that are set by ::RegOpenKeyEx(), is it possible to determine the full path to that key?

I realize that in a simple application all you have to do is look up 5 or 10 lines and read... but in a complex app like the one I'm debugging, the key I'm interested in can be opened from a series of calls.

Answer 1

Use LoadLibrary and NtQueryKey exported function as in the following code snippet.

#include <windows.h>
#include <string>

typedef LONG NTSTATUS;

#ifndef STATUS_SUCCESS
#define STATUS_SUCCESS ((NTSTATUS)0x00000000L)
#endif

#ifndef STATUS_BUFFER_TOO_SMALL
#define STATUS_BUFFER_TOO_SMALL ((NTSTATUS)0xC0000023L)
#endif

std::wstring GetKeyPathFromKKEY(HKEY key)
{
    std::wstring keyPath;
    if (key != NULL)
    {
        HMODULE dll = LoadLibrary(L"ntdll.dll");
        if (dll != NULL) {
            typedef DWORD (__stdcall *NtQueryKeyType)(
                HANDLE  KeyHandle,
                int KeyInformationClass,
                PVOID  KeyInformation,
                ULONG  Length,
                PULONG  ResultLength);

            NtQueryKeyType func = reinterpret_cast<NtQueryKeyType>(::GetProcAddress(dll, "NtQueryKey"));

            if (func != NULL) {
                DWORD size = 0;
                DWORD result = 0;
                result = func(key, 3, 0, 0, &size);
                if (result == STATUS_BUFFER_TOO_SMALL)
                {
                    size = size + 2;
                    wchar_t* buffer = new (std::nothrow) wchar_t[size/sizeof(wchar_t)]; // size is in bytes
                    if (buffer != NULL)
                    {
                        result = func(key, 3, buffer, size, &size);
                        if (result == STATUS_SUCCESS)
                        {
                            buffer[size / sizeof(wchar_t)] = L'';
                            keyPath = std::wstring(buffer + 2);
                        }

                        delete[] buffer;
                    }
                }
            }

            FreeLibrary(dll);
        }
    }
    return keyPath;
}

int _tmain(int argc, _TCHAR* argv[])
{
    HKEY key = NULL;
    LONG ret = ERROR_SUCCESS;

    ret = RegOpenKey(HKEY_LOCAL_MACHINE, L"SOFTWARE\Microsoft", &key);
    if (ret == ERROR_SUCCESS)
    {
        wprintf_s(L"Key path for %p is '%s'.", key, GetKeyPathFromKKEY(key).c_str());    
        RegCloseKey(key);
    }

    return 0;
}

This will print the key path on the console:

Key path for 00000FDC is 'REGISTRYMACHINESOFTWAREMicrosoft'.