WMI Provider EvtWmiInstanceSetInstance not called if class has string members

NTDEV
8

Here’s the .mof

#PRAGMA AUTORECOVER

[WMI,
 Dynamic,
 Provider("WmiProv"),
 Locale("MS\\0x409"),
 Description("My WMI Class with confidential info removed") : amended,
 guid("82DE1B9F-C0EC-4BA4-B81E-D6B9C9EDD2D3")]
class My_WMIClass 
{
    [key, read]
    string InstanceName;

    [read]
    boolean Active;

    [WmiDataId(1),
     read,
     write,
     Description("R/W integer values")]
    uint32 NormalMember;

    [WmiDataId(2),
     read,
     Description("Description"),
     MaxLen(511)]
    string Description;
};

Here’s the header:

// WMI method
#define My_WMIClass _GUID)\
    { 0x82de1b9f,0xc0ec,0x4ba4, { 0xb8,0x1e,0xd6,0xb9,0xc9,0xed,0xd2,0xd3 } }

#if ! (defined(MIDL_PASS))
DEFINE_GUID(My_WMIClass _GUID), \
   0x82de1b9f, 0xc0ec, 0x4ba4, 0xb8, 0x1e, 0xd6, 0xb9, 0xc9, 0xed, 0xd2, 0xd3);
#endif

typedef struct _My_WMIClass
{
   // Normal integer member
   ULONG NormalMember;
#define My_WMIClass_NormalMember_SIZE sizeof(ULONG)
#define My_WMIClass_NormalMember_ID 1

   // Description
   WCHAR Description[511 + 1];
#define My_WMIClass_Description_ID 2
}My_WMIClass, *PMy_WMIClass;

#define My_WMIClass_SIZE (FIELD_OFFSET(My_WMIClass, NormalMember) + sizeof(WCHAR)*512)
WDF_DECLARE_CONTEXT_TYPE_WITH_NAME(My_WMIClass, MyWmiGetData);

Here’s the registration:

NTSTATUS register_wmi_provider(
   _In_ WDFDEVICE device
)
{
   NTSTATUS status;
   WDF_OBJECT_ATTRIBUTES woa;
   WDF_WMI_PROVIDER_CONFIG providerConfig;
   WDF_WMI_INSTANCE_CONFIG instanceConfig;
   DECLARE_CONST_UNICODE_STRING(mofResourceName, MOFRESOURCENAME);

   PAGED_CODE();
   
   //
   // Register the MOF resource names of any customized WMI data providers
   // that are not defined in wmicore.mof.
   //
   status = WdfDeviceAssignMofResourceName(device, &mofResourceName);
   if (!NT_SUCCESS(status)) {
      TraceEvents(TRACE_LEVEL_ERROR, TRACE_WMI,
         "WdfDeviceAssignMofResourceName failed 0x%x", status);
      return status;
   }

   WDF_WMI_PROVIDER_CONFIG_INIT(&providerConfig, &My_WMIClass_GUID);
   providerConfig.MinInstanceBufferSize = My_WMIClass_SIZE;

   WDF_WMI_INSTANCE_CONFIG_INIT_PROVIDER_CONFIG(&instanceConfig, &providerConfig);
   instanceConfig.Register = TRUE;
   instanceConfig.EvtWmiInstanceQueryInstance = wmi_instance_query;
   instanceConfig.EvtWmiInstanceSetInstance = wmi_instance_set;
   instanceConfig.EvtWmiInstanceSetItem = wmi_item_set;

   WDF_OBJECT_ATTRIBUTES_INIT_CONTEXT_TYPE(&woa, My_WMIClass );

   //
   // No need to get the newly created handle because we just reference data
   // from our device extension directly.
   //
   status = WdfWmiInstanceCreate(device,
      &instanceConfig,
      &woa,
      WDF_NO_HANDLE);
   if (!NT_SUCCESS(status)) {
      TraceEvents(TRACE_LEVEL_ERROR, TRACE_WMI,
         "WdfWmiInstanceCreate failed 0x%x", status);
      return status;
   }

   return status;
}

If I remove the Description property, the following PowerShell works:

$x = get-ciminstance MY_WMIClass -Namespace "root/wmi"

$x.NormalMember = 42
Set-CimInstance -CimInstance $x -PassThru

As stated previously, as soon as I add the Description property any attempt to update the instance fails. I have tried changing providerConfig.MinInstanceBufferSize and that doesn’t change the observed behavior.

I still haven’t figured out how to use PowerShell such that it calls my EvtWmiInstanceSetItem callback.