Good evening, I trying to get the Volume GUID from the volume enumerated, at 1st I define the volumeContext structure and use it,
my understanding is, 1st call FltGetVolumeGuidName() to get the BufferSizeNeeded() and then use this value to allocate memory pool and then call again FltGetVolumeGuidName() to fetch Guid for each of the Volume present in the system,
I have observed kernel crash while using (volumeContext->GUIDinfo.Buffer = NULL; )
Kindly suggest, if my approach is correct.

FLT_PREOP_CALLBACK_STATUS PreFileOperationCallback(__inout PFLT_CALLBACK_DATA Data,
    __in PCFLT_RELATED_OBJECTS FltObjects, __deref_out_opt PVOID* CompletionContext)
{
    DbgPrint("\n-------Inside ---PreFileOperationCallback--------");
    NTSTATUS                            status, ntStatus;
    PFLT_VOLUME                         pVolumeList[MAX_VOLUME_CHARS];
    ULONG                               VolumeListSize = MAX_VOLUME_CHARS;
    ULONG                               index = 0;
    PULONG                              NumberVolumesReturned = NULL;
    WCHAR                               wszNameBuffer[MAX_VOLUME_LENGTH] = { 0 };
    FILTER_VOLUME_INFORMATION_CLASS     FilterVolumeStandardInformation;
    FILTER_VOLUME_STANDARD_INFORMATION  FltVolumeStdInfo;
    PULONG                              BufferSizeNeeded;
    SIZE_T                              bytesRequired;
    ULONG                               MEMTAG_VOL_GUID = 0;
    PVOLUME_CONTEXT                     volumeContext = NULL;
    UNICODE_STRING                      volumeGuidName, VolumeNameString;
    PVOID                               VolumeNameBuffer = NULL;
    UNICODE_STRING                      volumebuf[MAX_VOLUME_LENGTH];

    DbgPrint("\n--------Before FltEnumerateVolumes--------");
    ntStatus = FltEnumerateVolumes(fileHandler, pVolumeList, VolumeListSize, &NumberVolumesReturned);   //Get volume information
    if (!NT_SUCCESS(ntStatus)) {
        return ntStatus;
    }
    DbgPrint("\n-----After  FltEnumerateVolumes---------");
    DbgPrint("\nNumberVolumesReturned-%d",NumberVolumesReturned);
    DbgPrint("\n------------------------------------------");
    for (index = 1; index <= NumberVolumesReturned; index++)
    {
        if (index == NumberVolumesReturned) {
            goto Dereference;  //jump the forLoop
        }
        volumeContext->GUIDinfo.Buffer = NULL;                               //kernel crash here  <<<<======
        volumeContext->GUIDinfo.Length = 0;
        volumeContext->GUIDinfo.MaximumLength = 0;
        //fetching correct size
        (void) FltGetVolumeGuidName(pVolumeList[index], &volumeContext->GUIDinfo, &BufferSizeNeeded);
        //Allocating space
        if (NULL == volumeContext->GUIDinfo.Buffer) {
            status = STATUS_INSUFFICIENT_RESOURCES;
            DbgPrint("\n STATUS_INSUFFICIENT_RESOURCES");
            break;
        }
        //Memory allocation 
        volumeContext->GUIDinfo.Buffer = (PWCHAR)ExAllocatePoolWithTag(PagedPool, BufferSizeNeeded, MEMTAG_VOL_GUID);
        volumeContext->GUIDinfo.Length = 0;
        ASSERT(BufferSizeNeeded <= UNICODE_STRING_MAX_BYTES);
        volumeContext->GUIDinfo.MaximumLength = (ULONG)BufferSizeNeeded;

        ntStatus = FltGetVolumeGuidName(pVolumeList[index], &volumeContext->GUIDinfo, &BufferSizeNeeded);
        if (ntStatus == STATUS_BUFFER_TOO_SMALL) {
            DbgPrint("\n STATUS_BUFFER_TOO_SMALL");
            break;
        }else if(ntStatus == STATUS_INSUFFICIENT_RESOURCES) {

            DbgPrint("\n STATUS_INSUFFICIENT_RESOURCES");
            break;
        }
        else if (ntStatus == STATUS_INVALID_DEVICE_REQUEST) {
            DbgPrint("\n STATUS_INVALID_DEVICE_REQUEST");

        }else if (ntStatus == STATUS_FLT_VOLUME_NOT_FOUND) {

            DbgPrint("\n STATUS_FLT_VOLUME_NOT_FOUND");
            break; //break from forLoop
        }
        continue; 
        // The format is \??\Volume{GUID}
        for (index = 0; (L'{' != volumeContext->GUIDinfo.Buffer[index] && index < (volumeContext->GUIDinfo.Length / sizeof(WCHAR))); index++)
            /* nothing */;
        volumeGuidName.Buffer = &volumeContext->GUIDinfo.Buffer[index];
        volumeGuidName.Length = (USHORT)DbgPrint("\nVolumeGuidName Length - %ld",volumeGuidName.Length); 

        DbgPrint("\n-------- GUID Info ---------");
        DbgPrint("\nVolumeGuidName  - %wZ", &volumeContext->GUIDinfo);
        DbgPrint("\nVolumeGuidName Name - %ld",volumeGuidName.Buffer);
        DbgPrint("\nVolumeGuidName Length - %ld",volumeGuidName.Length);
        DbgPrint("\nVolumeGuidName Buffer - { %s }",(PWCHAR)volumeGuidName.Buffer);
        DbgPrint("\n----------------------------");
        status = RtlGUIDFromString(&volumeGuidName, &volumeContext->GUIDinfo);
        if (!NT_SUCCESS(status)) {
            break;
        } 
    }//End of forLoop
Dereference:
    for (index = 0; index < NumberVolumesReturned; index++)
    {
        FltObjectDereference(pVolumeList[index]);
    }
    //return pInstance;  
    return FLT_PREOP_SUCCESS_NO_CALLBACK;
}