WinAPI: OpenProcess() returns error 5 with SeDebugPrivilege enabled for host process

I've got a routine where I process-walk to obtain the HANDLE of each process as I 'walk' down the list (which works fine), but my issue lies when I do:

HANDLE h = OpenProcess(PROCESS_ALL_ACCESS, FALSE, pe32.th32ProcessID) where PROCESS_ALL_ACCESS is the access token, handle inheritance is set to FALSE, and pe32 is a PROCESSENTRY32

GetLastError() returns error code 5, and all the handles that are made are addresses which do not correspond to any appropriate process in Spy++32/64 (I've tried building the application under both platform targets, but as you'd expect, the result is the same).

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The code for setting SeDebugPrivilege for the host process which I'm using is:

BOOL EnableDebugPrivilege(BOOL bEnable)
{
    HANDLE hToken = nullptr;
    LUID luid;

    if (!OpenProcessToken(GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES, &hToken)) return FALSE;
    if (!LookupPrivilegeValue(NULL, SE_DEBUG_NAME, &luid)) return FALSE;

    TOKEN_PRIVILEGES tokenPriv;
    tokenPriv.PrivilegeCount = 1;
    tokenPriv.Privileges[0].Luid = luid;
    tokenPriv.Privileges[0].Attributes = bEnable ? SE_PRIVILEGE_ENABLED : 0;

    if (!AdjustTokenPrivileges(hToken, FALSE, &tokenPriv, sizeof(TOKEN_PRIVILEGES), NULL, NULL)) return FALSE;

    return TRUE;
}

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Some questions that would be helpful to you:

1. I'm running Windows 7 x64 Professional.
2. Yes, devenv.exe is started with "Run as Administrator" privileges, which means that the debugger and the application itself are started under the same affinity.
3. I have tried toggling UAC or running the application with UAC off altogether. Still error code 5.
4. I just attempted doing it with PROCESS_QUERY_LIMITED_INFORMATION and I receive error code 6, or ERROR_INVALID_HANDLE. Also attempted with PROCESS_QUERY_INFORMATION | PROCESS_VM_READ, result is error 5 again.
5. SeDebugPrivilege is enabled, verified with SysInternals' Process Explorer. Additionally, all processes that spawn from devenv/whatever the debugger is called inherit SeDebugPrivilege so...this is weird.

Thank you all very much for your time, I'm reaching wits end with this issue :S

Are you sure you are not passing 0 as a process ID value? The system idle process with ID 0 is included in the snapshot under the name [System Process], but you can't open a handle for it as the documentation for OpenProcess specifically says it'll fail. Well it says a bit more:

If the specified process is the System Process (0x00000000), the function fails and the last error code is ERROR_INVALID_PARAMETER. If the specified process is the Idle process or one of the CSRSS processes, this function fails and the last error code is ERROR_ACCESS_DENIED because their access restrictions prevent user-level code from opening them.

Well, it's not completely true as I was able to open handle to CSRSS (of course, it doesn't actually have the requested rights). But it may fail for some protected processes (audiodg), so you shouldn't not do this. Instead, check the name of the process if it's the one you want.