What is the format of reparseData for junctions?

[hach-que]

I want to expose a directory entry in my virtual filesystem as a junction, with the value of the junction being something like \Device\HarddiskVolume3\UES\_emmvbnin7d1nw-000. This matches the junction value you can specify when using the Win32 DeviceIoControl / FSCTL_SET_REPARSE_POINT.

I've tried passing both the Unicode bytes back as the reparseData, as well as recreating the REPARSE_DATA_BUFFER structure and passing that back, but neither seem to work. For example, this code does not produce reparse data that the operating system recognizes:

using (var memory = new MemoryStream())
{
    var target = junctions[fileName];
    var targetByteLength = (ushort)(target.Length * 2);
    memory.Write(BitConverter.GetBytes((uint)2684354563U));
    memory.Write(BitConverter.GetBytes((ushort)(targetByteLength + 12)));
    memory.Write(BitConverter.GetBytes((ushort)0));
    memory.Write(BitConverter.GetBytes((ushort)0));
    memory.Write(BitConverter.GetBytes((ushort)targetByteLength));
    memory.Write(BitConverter.GetBytes((ushort)(targetByteLength + 2)));
    memory.Write(BitConverter.GetBytes((ushort)0));
    memory.Write(Encoding.Unicode.GetBytes(target));
    memory.Write(new byte[] { 0, 0 });
    reparseData = memory.ToArray();
    return STATUS_SUCCESS;
}

For reference, REPARSE_DATA_BUFFER is defined as (though unused in the example above, the layout should match in memory):

[StructLayout(LayoutKind.Sequential)]
private unsafe struct REPARSE_DATA_BUFFER
{
    public uint ReparseTag;
    public ushort ReparseDataLength;
    public ushort Reserved;
    public ushort SubstituteNameOffset;
    public ushort SubstituteNameLength;
    public ushort PrintNameOffset;
    public ushort PrintNameLength;
    public fixed char PathBuffer[8192];
}

The value 2684354563U is IO_REPARSE_TAG_MOUNT_POINT.

What data should I be putting into reparseData to expose a junction like this via WinFsp?

[hach-que]

It turns out the resulting data was missing a few null bytes at the end. The following code works to return junctions:

public override int GetReparsePointByName(string fileName, bool isDirectory, ref byte[] reparseData)
{
    var junctions = _additionalJunctions;
    if (junctions.ContainsKey(fileName))
    {
        var target = junctions[fileName];
        var targetByteLength = (ushort)(target.Length * 2);

        using (var memory = new MemoryStream())
        {
            memory.Write(BitConverter.GetBytes((uint)2684354563U));
            memory.Write(BitConverter.GetBytes((ushort)(targetByteLength + 12)));
            memory.Write(BitConverter.GetBytes((ushort)0));
            memory.Write(BitConverter.GetBytes((ushort)0));
            memory.Write(BitConverter.GetBytes((ushort)targetByteLength));
            memory.Write(BitConverter.GetBytes((ushort)(targetByteLength + 2)));
            memory.Write(BitConverter.GetBytes((ushort)0));
            memory.Write(Encoding.Unicode.GetBytes(target));
            memory.Write(new byte[] { 0, 0 });
            memory.Write(new byte[] { 0, 0 });
            reparseData = memory.ToArray();
        }

        return STATUS_SUCCESS;
    }
    else
    {
        return STATUS_IO_DEVICE_ERROR;
    }
}

[billziss-gh]

Hello @hach-que, glad to see that you have this resolved.