Event log error with iVSSWriterCallBack

Sep 7, 2011 at 12:36 PM
Edited Sep 7, 2011 at 12:37 PM

I am getting errors in the application log when using AlphaVSS

The error is "Volume Shadow Copy Service error: Unexpected error querying for the IVssWriterCallback interface. hr = 0x80070005, Access is denied."  

What's odd is that it doesn't seem to cause any issue with the shadow copy being created.

It must be one of the following statements that generates the error

Dim oVSSImpl As Alphaleonis.Win32.Vss.IVssImplementation = Alphaleonis.Win32.Vss.VssUtils.LoadImplementation()

oVSS = oVSSImpl.CreateVssBackupComponents       


oVSS.SetBackupState(True, True, Alphaleonis.Win32.Vss.VssBackupType.Full, False)
        Using async As Alphaleonis.Win32.Vss.IVssAsync = oVSS.GatherWriterMetadata()


End Using

Sep 7, 2011 at 5:57 PM

On what operating system are you getting this? (Including any service pack(s) and x32/x64)? And is running the program snippet above enough to get the error in the event log?

Regards, Peter.

Sep 7, 2011 at 8:04 PM

I have now narrowed it down to the GatherWriterMetaData statement.  I'm running it on Windows Server 2008 R2 SP1 (x64).

This article may be relevant, I have not been able to test it yet though



Sep 7, 2011 at 8:18 PM
Edited Sep 7, 2011 at 8:19 PM

Ahhh... This sounds like something I encountered when working on the sample application for AlphaVSS (yeah, I know, it's slow progress indeed). Anyway, I discovered that I actually didn't get information from all writers when running "Gather writer metadata" when I compared to what VShadow.exe (the sample in the Windows SDK) produced.  And it turned out to be a problem with the COM security stuff (which I don't know much about to be honest).

Now this was a while ago, so I don't remember everything I read about it off the top of my head. All I remember is that calling CoInitializeSecurity from a .NET application is not entierly straightforward or perhaps even possible. Just do a google for it. But my sample application starts with the following commented line:

// WARNING: This call will fail if debugging this application using the Visual Studio Hosting process. It may 
//          also fail under other conditions. It is used to allow communication with all writers. If missing 
//          for example the System Writer will not show up in the IVssBackupComponents.WriterMetadata collection.
//          Posts on the internet seems to suggest that the only way to reliably set this is to use a custom CLR host, which 
//          is out of the scope for this sample.


Perhaps this is a pointer towards the solution also to your problem?

Let me know of your progress.

Regards, Peter.

Sep 8, 2011 at 8:12 AM
Edited Sep 8, 2011 at 8:15 AM

OK, looks like I've fixed it.  At least, I'm not getting the error in the Event Viewer any more

I created a new class called NativeMethods with the code below. 


Imports System.Runtime.InteropServices
Public Class NativeMethods
    ''' <summary>
    ''' Define RPC_C_AUTHN_LEVEL_ constants
    ''' </summary>
    Public Enum RpcAuthnLevel
        [Default] = 0
    End Enum

    ''' <summary>
    ''' Define RPC_C_IMP_LEVEL_ constants
    ''' </summary>
    Public Enum RpcImpLevel
        [Default] = 0
    End Enum

    ''' <summary>
    ''' Define EOAC_ constants
    ''' </summary>
    Public Enum EoAuthnCap
        None = &H0
        MutualAuth = &H1
        StaticCloaking = &H20
        DynamicCloaking = &H40
        AnyAuthority = &H80
        MakeFullSIC = &H100
        [Default] = &H800
        SecureRefs = &H2
        AccessControl = &H4
        AppID = &H8
        Dynamic = &H10
        RequireFullSIC = &H200
        AutoImpersonate = &H400
        NoCustomMarshal = &H2000
        DisableAAA = &H1000
    End Enum
    <DllImport("Ole32.dll", ExactSpelling:=True, EntryPoint:="CoInitializeSecurity", CallingConvention:=CallingConvention.StdCall, SetLastError:=False, PreserveSig:=False)> _
	Private Shared Sub CoInitializeSecurity(ByVal pVoid As IntPtr, ByVal cAuthSvc As Integer, ByVal asAuthSvc As IntPtr, _
						ByVal pReserved1 As IntPtr, ByVal dwAuthnLevel As UInteger, ByVal dwImpLevel As UInteger, _
						ByVal pAuthList As IntPtr, ByVal dwCapabilities As UInteger, ByVal pReserved3 As IntPtr)
	End Sub

	Public Sub New()
		CoInitializeSecurity(IntPtr.Zero, -1, IntPtr.Zero, IntPtr.Zero, CUInt(RpcAuthnLevel.PktPrivacy), CUInt(RpcImpLevel.Impersonate), _
				IntPtr.Zero, CUInt(EoAuthnCap.DynamicCloaking), IntPtr.Zero)
	End Sub

End Class


 Then, the very first statement of my application is :-

Dim NM as New NativeMethods

Hope this helps other people!