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'\0';
                            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 '\REGISTRY\MACHINE\SOFTWARE\Microsoft'.

Answer 2

Nominally no because it's just a handle and there is no API that I know of to let you do this in the normal Windows API's.

HOWEVER the Native API has lots of functions some of which can give you handles open for given files and the like so there maybe something similar for the Registry. That and RegMon by SysInternals may do something like this but you'll have to Google I'm afraid :/

Answer 3

You can use RegSaveKey and write it to a file, then look at the file.

Alternatively you can keep a global map of HKEYs to LPCWSTRs and add entries when you open them and do lookups whenever.

You may also be able to do something with the !reg command in WinDBG / NTSD, but you can't just give it the HKEY. You'll have to do some other trickery to get the info you want out of it.

Answer 4

For ntsd/windbg:

!handle yourhandle 4

Answer 5

I was excited to find this article and its well liked solution. Until I found that my system's NTDLL.DLL did not have NtQueryKeyType.

After some hunting around, I ran across ZwQueryKey in the DDK forums.

It is in C#, but here is the solution that works for me:

enum KEY_INFORMATION_CLASS
{
    KeyBasicInformation,            // A KEY_BASIC_INFORMATION structure is supplied.
    KeyNodeInformation,             // A KEY_NODE_INFORMATION structure is supplied.
    KeyFullInformation,             // A KEY_FULL_INFORMATION structure is supplied.
    KeyNameInformation,             // A KEY_NAME_INFORMATION structure is supplied.
    KeyCachedInformation,           // A KEY_CACHED_INFORMATION structure is supplied.
    KeyFlagsInformation,            // Reserved for system use.
    KeyVirtualizationInformation,   // A KEY_VIRTUALIZATION_INFORMATION structure is supplied.
    KeyHandleTagsInformation,       // Reserved for system use.
    MaxKeyInfoClass                 // The maximum value in this enumeration type.
}
[StructLayout(LayoutKind.Sequential)]
public struct KEY_NAME_INFORMATION
{
    public UInt32 NameLength;     // The size, in bytes, of the key name string in the Name array.
    public char[] Name;           // An array of wide characters that contains the name of the key.
                                  // This character string is not null-terminated.
                                  // Only the first element in this array is included in the
                                  //    KEY_NAME_INFORMATION structure definition.
                                  //    The storage for the remaining elements in the array immediately
                                  //    follows this element.
}

[DllImport("ntdll.dll", SetLastError = true, CharSet = CharSet.Unicode)]
private static extern int ZwQueryKey(IntPtr hKey, KEY_INFORMATION_CLASS KeyInformationClass, IntPtr lpKeyInformation, int Length, out int ResultLength);

public static String GetHKeyName(IntPtr hKey)
{
    String result = String.Empty;
    IntPtr pKNI = IntPtr.Zero;

    int needed = 0;
    int status = ZwQueryKey(hKey, KEY_INFORMATION_CLASS.KeyNameInformation, IntPtr.Zero, 0, out needed);
    if ((UInt32)status == 0xC0000023)   // STATUS_BUFFER_TOO_SMALL
    {
        pKNI = Marshal.AllocHGlobal(sizeof(UInt32) + needed + 4 /*paranoia*/);
        status = ZwQueryKey(hKey, KEY_INFORMATION_CLASS.KeyNameInformation, pKNI, needed, out needed);
        if (status == 0)    // STATUS_SUCCESS
        {
            char[] bytes = new char[2 + needed + 2];
            Marshal.Copy(pKNI, bytes, 0, needed);
            // startIndex == 2  skips the NameLength field of the structure (2 chars == 4 bytes)
            // needed/2         reduces value from bytes to chars
            //  needed/2 - 2    reduces length to not include the NameLength
            result = new String(bytes, 2, (needed/2)-2);
        }
    }
    Marshal.FreeHGlobal(pKNI);
    return result;
}

I've only ever tried it while running as Administrator, which may be required.

The result is a bit oddly formatted: \REGISTRY\MACHINE\SOFTWARE\company\product for example, instead of HKEY_LOCAL_MACHINE\SOFTWARE\company\product.