QEMU Cannot Open Tap Device in Windows 10 Host

[Faisal Al-Humaimidi]

Hello QEMU devs,

I am trying to bridge my QEMU VM to a local adapter in my Windows 10 host, but QEMU complains that the interface name provided by ifname option to the -netdev argument cannot be opened. I have cross-compiled QEMU from source to debug this behavior, for whether or not the interface name can be detected, and apparently QEMU can find the adapter's name, but not the TAP device file. To further explain my point, here's a code snippet from net/tap-win32.c:595 from the QEMU 4.2.0 source code (latest release version as of this writing), specifically tap_win32_open function, and I will highlight where it was failing in this function (look for // THIS IS WHERE IT WILL FAIL. comments):

1. Testing with a correct existing network interface:

static int tap_win32_open(tap_win32_overlapped_t **phandle,
                          const char *preferred_name)
{
    ...

    rc = get_device_guid(device_guid, sizeof(device_guid), name_buffer, sizeof(name_buffer));
    if (rc)
        return -1;

    snprintf (device_path, sizeof(device_path), "%s%s%s",
              USERMODEDEVICEDIR,
              device_guid,
              TAPSUFFIX);

    handle = CreateFile (
        device_path,
        GENERIC_READ | GENERIC_WRITE,
        0,
        0,
        OPEN_EXISTING,
        FILE_ATTRIBUTE_SYSTEM | FILE_FLAG_OVERLAPPED,
        0 );

    if (handle == INVALID_HANDLE_VALUE) {
        return -1; // THIS IS WHERE IT WILL FAIL.
    }

    ...

2. Testing with an incorrect (non-existent) network interface:

static int tap_win32_open(tap_win32_overlapped_t **phandle,
                          const char *preferred_name)
{
    ...

    rc = get_device_guid(device_guid, sizeof(device_guid), name_buffer, sizeof(name_buffer));
    if (rc)
        return -1; // THIS IS WHERE IT WILL FAIL.

    snprintf (device_path, sizeof(device_path), "%s%s%s",
              USERMODEDEVICEDIR,
              device_guid,
              TAPSUFFIX);

    handle = CreateFile (
        device_path,
        GENERIC_READ | GENERIC_WRITE,
        0,
        0,
        OPEN_EXISTING,
        FILE_ATTRIBUTE_SYSTEM | FILE_FLAG_OVERLAPPED,
        0 );

    if (handle == INVALID_HANDLE_VALUE) {
        return -1;
    }

    ...

QEMU uses the prefix USERMODEDEVICEDIR, which is \\.\Global\ and a suffix .tap to the device's GUID to create the device's path in Windows. For example, the network adapter I am dealing with results in the following device path: \\.\Global\{990DA322-3986-4854-AE93-1D6FB0BFA137}.tap. Any idea why CreateFile always results in INVALID_HANDLE_VALUE on the device path? By the way, GetLastError() returns 2, which from docs.microsoft.com means the following:

...
ERROR_FILE_NOT_FOUND

2 (0x2)

The system cannot find the file specified.
...

Regards.