Adding upstream version 7.0.14-dfsg.upstream/7.0.14-dfsg

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 3179 insertions, 0 deletions

diff --git a/src/VBox/Storage/VHD.cpp b/src/VBox/Storage/VHD.cpp
new file mode 100644
index 00000000..ad14a147
--- /dev/null
+++ b/src/VBox/Storage/VHD.cpp
@@ -0,0 +1,3179 @@
+/* $Id: VHD.cpp $ */
+/** @file
+ * VHD Disk image, Core Code.
+ */
+
+/*
+ * Copyright (C) 2006-2023 Oracle and/or its affiliates.
+ *
+ * This file is part of VirtualBox base platform packages, as
+ * available from https://www.virtualbox.org.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, in version 3 of the
+ * License.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <https://www.gnu.org/licenses>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+
+/*********************************************************************************************************************************
+*   Header Files                                                                                                                 *
+*********************************************************************************************************************************/
+#define LOG_GROUP LOG_GROUP_VD_VHD
+#include <VBox/vd-plugin.h>
+#include <VBox/err.h>
+
+#include <VBox/log.h>
+#include <VBox/version.h>
+#include <iprt/asm.h>
+#include <iprt/assert.h>
+#include <iprt/mem.h>
+#include <iprt/uuid.h>
+#include <iprt/path.h>
+#include <iprt/string.h>
+#include <iprt/utf16.h>
+
+#include "VDBackends.h"
+
+#define VHD_RELATIVE_MAX_PATH 512
+#define VHD_ABSOLUTE_MAX_PATH 512
+
+#define VHD_SECTOR_SIZE 512
+#define VHD_BLOCK_SIZE  (2 * _1M)
+
+/** The maximum VHD size is 2TB due to the 32bit sector numbers in the BAT.
+ * Note that this is the maximum file size including all footers and headers
+ * and not the maximum virtual disk size presented to the guest.
+ */
+#define VHD_MAX_SIZE    (2 * _1T)
+/** Maximum number of 512 byte sectors for a VHD image. */
+#define VHD_MAX_SECTORS (VHD_MAX_SIZE / VHD_SECTOR_SIZE)
+
+/* This is common to all VHD disk types and is located at the end of the image */
+#pragma pack(1)
+typedef struct VHDFooter
+{
+    char     Cookie[8];
+    uint32_t Features;
+    uint32_t Version;
+    uint64_t DataOffset;
+    uint32_t Timestamp;
+    uint8_t  CreatorApp[4];
+    uint32_t CreatorVer;
+    uint32_t CreatorOS;
+    uint64_t OrigSize;
+    uint64_t CurSize;
+    uint16_t DiskGeometryCylinder;
+    uint8_t  DiskGeometryHeads;
+    uint8_t  DiskGeometrySectors;
+    uint32_t DiskType;
+    uint32_t Checksum;
+    char     UniqueID[16];
+    uint8_t  SavedState;
+    uint8_t  Reserved[427];
+} VHDFooter;
+#pragma pack()
+
+/* this really is spelled with only one n */
+#define VHD_FOOTER_COOKIE "conectix"
+#define VHD_FOOTER_COOKIE_SIZE 8
+
+#define VHD_FOOTER_FEATURES_NOT_ENABLED   0
+#define VHD_FOOTER_FEATURES_TEMPORARY     1
+#define VHD_FOOTER_FEATURES_RESERVED      2
+
+#define VHD_FOOTER_FILE_FORMAT_VERSION    0x00010000
+#define VHD_FOOTER_DATA_OFFSET_FIXED      UINT64_C(0xffffffffffffffff)
+#define VHD_FOOTER_DISK_TYPE_FIXED        2
+#define VHD_FOOTER_DISK_TYPE_DYNAMIC      3
+#define VHD_FOOTER_DISK_TYPE_DIFFERENCING 4
+
+#define VHD_MAX_LOCATOR_ENTRIES           8
+#define VHD_PLATFORM_CODE_NONE            0
+#define VHD_PLATFORM_CODE_WI2R            0x57693272
+#define VHD_PLATFORM_CODE_WI2K            0x5769326B
+#define VHD_PLATFORM_CODE_W2RU            0x57327275
+#define VHD_PLATFORM_CODE_W2KU            0x57326B75
+#define VHD_PLATFORM_CODE_MAC             0x4D163220
+#define VHD_PLATFORM_CODE_MACX            0x4D163258
+
+/* Header for expanding disk images. */
+#pragma pack(1)
+typedef struct VHDParentLocatorEntry
+{
+    uint32_t u32Code;
+    uint32_t u32DataSpace;
+    uint32_t u32DataLength;
+    uint32_t u32Reserved;
+    uint64_t u64DataOffset;
+} VHDPLE, *PVHDPLE;
+
+typedef struct VHDDynamicDiskHeader
+{
+    char     Cookie[8];
+    uint64_t DataOffset;
+    uint64_t TableOffset;
+    uint32_t HeaderVersion;
+    uint32_t MaxTableEntries;
+    uint32_t BlockSize;
+    uint32_t Checksum;
+    uint8_t  ParentUuid[16];
+    uint32_t ParentTimestamp;
+    uint32_t Reserved0;
+    uint16_t ParentUnicodeName[256];
+    VHDPLE   ParentLocatorEntry[VHD_MAX_LOCATOR_ENTRIES];
+    uint8_t  Reserved1[256];
+} VHDDynamicDiskHeader;
+#pragma pack()
+
+#define VHD_DYNAMIC_DISK_HEADER_COOKIE "cxsparse"
+#define VHD_DYNAMIC_DISK_HEADER_COOKIE_SIZE 8
+#define VHD_DYNAMIC_DISK_HEADER_VERSION 0x00010000
+
+/**
+ * Complete VHD image data structure.
+ */
+typedef struct VHDIMAGE
+{
+    /** Image file name. */
+    const char       *pszFilename;
+    /** Opaque storage handle. */
+    PVDIOSTORAGE      pStorage;
+
+    /** Pointer to the per-disk VD interface list. */
+    PVDINTERFACE      pVDIfsDisk;
+    /** Pointer to the per-image VD interface list. */
+    PVDINTERFACE      pVDIfsImage;
+    /** Error interface. */
+    PVDINTERFACEERROR pIfError;
+    /** I/O interface. */
+    PVDINTERFACEIOINT pIfIo;
+
+    /** Open flags passed by VBoxHDD layer. */
+    unsigned        uOpenFlags;
+    /** Image flags defined during creation or determined during open. */
+    unsigned        uImageFlags;
+    /** Total size of the image. */
+    uint64_t        cbSize;
+
+    /** Physical geometry of this image. */
+    VDGEOMETRY      PCHSGeometry;
+    /** Logical geometry of this image. */
+    VDGEOMETRY      LCHSGeometry;
+
+    /** Image UUID. */
+    RTUUID          ImageUuid;
+    /** Parent image UUID. */
+    RTUUID          ParentUuid;
+
+    /** Parent's time stamp at the time of image creation. */
+    uint32_t        u32ParentTimestamp;
+    /** Relative path to the parent image. */
+    char            *pszParentFilename;
+
+    /** The Block Allocation Table. */
+    uint32_t        *pBlockAllocationTable;
+    /** Number of entries in the table. */
+    uint32_t        cBlockAllocationTableEntries;
+
+    /** Size of one data block. */
+    uint32_t        cbDataBlock;
+    /** Sectors per data block. */
+    uint32_t        cSectorsPerDataBlock;
+    /** Length of the sector bitmap in bytes. */
+    uint32_t        cbDataBlockBitmap;
+    /** A copy of the disk footer. */
+    VHDFooter       vhdFooterCopy;
+    /** Current end offset of the file (without the disk footer). */
+    uint64_t        uCurrentEndOfFile;
+    /** Size of the data block bitmap in sectors. */
+    uint32_t        cDataBlockBitmapSectors;
+    /** Start of the block allocation table. */
+    uint64_t        uBlockAllocationTableOffset;
+    /** Buffer to hold block's bitmap for bit search operations. */
+    uint8_t         *pu8Bitmap;
+    /** Offset to the next data structure (dynamic disk header). */
+    uint64_t        u64DataOffset;
+    /** Flag to force dynamic disk header update. */
+    bool            fDynHdrNeedsUpdate;
+    /** The static region list. */
+    VDREGIONLIST    RegionList;
+} VHDIMAGE, *PVHDIMAGE;
+
+/**
+ * Structure tracking the expansion process of the image
+ * for async access.
+ */
+typedef struct VHDIMAGEEXPAND
+{
+    /** Flag indicating the status of each step. */
+    volatile uint32_t fFlags;
+    /** The index in the  block allocation table which is written. */
+    uint32_t          idxBatAllocated;
+    /** Big endian representation of the block index
+     * which is written in the BAT. */
+    uint32_t          idxBlockBe;
+    /** Old end of the file - used for rollback in case of an error. */
+    uint64_t          cbEofOld;
+    /** Sector bitmap written to the new block - variable in size. */
+    uint8_t           au8Bitmap[1];
+} VHDIMAGEEXPAND, *PVHDIMAGEEXPAND;
+
+/**
+ * Flag defines
+ */
+#define VHDIMAGEEXPAND_STEP_IN_PROGRESS (0x0)
+#define VHDIMAGEEXPAND_STEP_FAILED      (0x2)
+#define VHDIMAGEEXPAND_STEP_SUCCESS     (0x3)
+/** All steps completed successfully. */
+#define VHDIMAGEEXPAND_ALL_SUCCESS      (0xff)
+/** All steps completed (no success indicator) */
+#define VHDIMAGEEXPAND_ALL_COMPLETE     (0xaa)
+
+/** Every status field has 2 bits so we can encode 4 steps in one byte. */
+#define VHDIMAGEEXPAND_STATUS_MASK              0x03
+#define VHDIMAGEEXPAND_BLOCKBITMAP_STATUS_SHIFT 0x00
+#define VHDIMAGEEXPAND_USERBLOCK_STATUS_SHIFT   0x02
+#define VHDIMAGEEXPAND_FOOTER_STATUS_SHIFT      0x04
+#define VHDIMAGEEXPAND_BAT_STATUS_SHIFT         0x06
+
+/**
+ * Helper macros to get and set the status field.
+ */
+#define VHDIMAGEEXPAND_STATUS_GET(fFlags, cShift) \
+    (((fFlags) >> (cShift)) & VHDIMAGEEXPAND_STATUS_MASK)
+#define VHDIMAGEEXPAND_STATUS_SET(fFlags, cShift, uVal) \
+    ASMAtomicOrU32(&(fFlags), ((uVal) & VHDIMAGEEXPAND_STATUS_MASK) << (cShift))
+
+
+/*********************************************************************************************************************************
+*   Static Variables                                                                                                             *
+*********************************************************************************************************************************/
+
+/** NULL-terminated array of supported file extensions. */
+static const VDFILEEXTENSION s_aVhdFileExtensions[] =
+{
+    {"vhd", VDTYPE_HDD},
+    {NULL, VDTYPE_INVALID}
+};
+
+
+/*********************************************************************************************************************************
+*   Internal Functions                                                                                                           *
+*********************************************************************************************************************************/
+
+
+/**
+ * Internal: Compute and update header checksum.
+ */
+static uint32_t vhdChecksum(void *pHeader, uint32_t cbSize)
+{
+    uint32_t u32ChkSum = 0;
+    for (uint32_t i = 0; i < cbSize; i++)
+        u32ChkSum += ((unsigned char *)pHeader)[i];
+    return ~u32ChkSum;
+}
+
+/**
+ * Internal: Convert filename to UTF16 with appropriate endianness.
+ */
+static int vhdFilenameToUtf16(const char *pszFilename, uint16_t *pu16Buf,
+                              uint32_t cbBufSize, uint32_t *pcbActualSize,
+                              bool fBigEndian)
+{
+    int      rc;
+    PRTUTF16 pTmp16 = NULL;
+    size_t   cTmp16Len;
+
+    rc = RTStrToUtf16(pszFilename, &pTmp16);
+    if (RT_SUCCESS(rc))
+    {
+        cTmp16Len = RTUtf16Len(pTmp16);
+        if (cTmp16Len * sizeof(*pTmp16) <= cbBufSize)
+        {
+            if (fBigEndian)
+                for (unsigned i = 0; i < cTmp16Len; i++)
+                    pu16Buf[i] = RT_H2BE_U16(pTmp16[i]);
+            else
+                memcpy(pu16Buf, pTmp16, cTmp16Len * sizeof(*pTmp16));
+            if (pcbActualSize)
+                *pcbActualSize = (uint32_t)(cTmp16Len * sizeof(*pTmp16));
+        }
+        else
+            rc = VERR_FILENAME_TOO_LONG;
+    }
+
+    if (pTmp16)
+        RTUtf16Free(pTmp16);
+    return rc;
+}
+
+/**
+ * Internal: Update one locator entry.
+ */
+static int vhdLocatorUpdate(PVHDIMAGE pImage, PVHDPLE pLocator, const char *pszFilename)
+{
+    int      rc = VINF_SUCCESS;
+    uint32_t cb = 0;
+    uint32_t cbMaxLen = RT_BE2H_U32(pLocator->u32DataSpace);
+    void     *pvBuf = RTMemTmpAllocZ(cbMaxLen);
+    char     *pszTmp;
+
+    if (!pvBuf)
+        return VERR_NO_MEMORY;
+
+    switch (RT_BE2H_U32(pLocator->u32Code))
+    {
+        case VHD_PLATFORM_CODE_WI2R:
+        {
+            if (RTPathStartsWithRoot(pszFilename))
+            {
+                /* Convert to relative path. */
+                char szPath[RTPATH_MAX];
+                rc = RTPathCalcRelative(szPath, sizeof(szPath), pImage->pszFilename, true /*fFromFile*/, pszFilename);
+                if (RT_SUCCESS(rc))
+                {
+                    /* Update plain relative name. */
+                    cb = (uint32_t)strlen(szPath);
+                    if (cb > cbMaxLen)
+                    {
+                        rc = VERR_FILENAME_TOO_LONG;
+                        break;
+                    }
+                    memcpy(pvBuf, szPath, cb);
+                }
+            }
+            else
+            {
+                /* Update plain relative name. */
+                cb = (uint32_t)strlen(pszFilename);
+                if (cb > cbMaxLen)
+                {
+                    rc = VERR_FILENAME_TOO_LONG;
+                    break;
+                }
+                memcpy(pvBuf, pszFilename, cb);
+            }
+            if (RT_SUCCESS(rc))
+                pLocator->u32DataLength = RT_H2BE_U32(cb);
+            break;
+        }
+        case VHD_PLATFORM_CODE_WI2K:
+            /* Update plain absolute name. */
+            rc = RTPathAbs(pszFilename, (char *)pvBuf, cbMaxLen);
+            if (RT_SUCCESS(rc))
+            {
+                cb = (uint32_t)strlen((const char *)pvBuf);
+                pLocator->u32DataLength = RT_H2BE_U32(cb);
+            }
+            break;
+        case VHD_PLATFORM_CODE_W2RU:
+            if (RTPathStartsWithRoot(pszFilename))
+            {
+                /* Convert to relative path. */
+                char szPath[RTPATH_MAX];
+                rc = RTPathCalcRelative(szPath, sizeof(szPath), pImage->pszFilename, true /*fFromFile*/, pszFilename);
+                if (RT_SUCCESS(rc))
+                    rc = vhdFilenameToUtf16(szPath, (uint16_t *)pvBuf, cbMaxLen, &cb, false);
+            }
+            else
+            {
+                /* Update unicode relative name. */
+                rc = vhdFilenameToUtf16(pszFilename, (uint16_t *)pvBuf, cbMaxLen, &cb, false);
+            }
+
+            if (RT_SUCCESS(rc))
+                pLocator->u32DataLength = RT_H2BE_U32(cb);
+            break;
+        case VHD_PLATFORM_CODE_W2KU:
+            /* Update unicode absolute name. */
+            pszTmp = (char*)RTMemTmpAllocZ(cbMaxLen);
+            if (!pszTmp)
+            {
+                rc = VERR_NO_MEMORY;
+                break;
+            }
+            rc = RTPathAbs(pszFilename, pszTmp, cbMaxLen);
+            if (RT_FAILURE(rc))
+            {
+                RTMemTmpFree(pszTmp);
+                break;
+            }
+            rc = vhdFilenameToUtf16(pszTmp, (uint16_t *)pvBuf, cbMaxLen, &cb, false);
+            RTMemTmpFree(pszTmp);
+            if (RT_SUCCESS(rc))
+                pLocator->u32DataLength = RT_H2BE_U32(cb);
+            break;
+        default:
+            rc = VERR_NOT_IMPLEMENTED;
+            break;
+    }
+
+    if (RT_SUCCESS(rc))
+    {
+        Assert(cb > 0);
+        rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage,
+                                    RT_BE2H_U64(pLocator->u64DataOffset),
+                                    pvBuf, cb);
+    }
+
+    if (pvBuf)
+        RTMemTmpFree(pvBuf);
+    return rc;
+}
+
+/**
+ * Internal: Update dynamic disk header from VHDIMAGE.
+ */
+static int vhdDynamicHeaderUpdate(PVHDIMAGE pImage)
+{
+    VHDDynamicDiskHeader ddh;
+    int rc, i;
+
+    if (!pImage)
+        return VERR_VD_NOT_OPENED;
+
+    rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage,
+                               pImage->u64DataOffset, &ddh, sizeof(ddh));
+    if (RT_FAILURE(rc))
+        return rc;
+    if (memcmp(ddh.Cookie, VHD_DYNAMIC_DISK_HEADER_COOKIE, VHD_DYNAMIC_DISK_HEADER_COOKIE_SIZE) != 0)
+        return VERR_VD_VHD_INVALID_HEADER;
+
+    uint32_t u32Checksum = RT_BE2H_U32(ddh.Checksum);
+    ddh.Checksum = 0;
+    if (u32Checksum != vhdChecksum(&ddh, sizeof(ddh)))
+        return VERR_VD_VHD_INVALID_HEADER;
+
+    /* Update parent's timestamp. */
+    ddh.ParentTimestamp = RT_H2BE_U32(pImage->u32ParentTimestamp);
+    /* Update parent's filename. */
+    if (pImage->pszParentFilename)
+    {
+        rc = vhdFilenameToUtf16(RTPathFilename(pImage->pszParentFilename),
+             ddh.ParentUnicodeName, sizeof(ddh.ParentUnicodeName) - 1, NULL, true);
+        if (RT_FAILURE(rc))
+            return rc;
+    }
+
+    /* Update parent's locators. */
+    for (i = 0; i < VHD_MAX_LOCATOR_ENTRIES; i++)
+    {
+        /* Skip empty locators */
+        if (   ddh.ParentLocatorEntry[i].u32Code != RT_H2BE_U32(VHD_PLATFORM_CODE_NONE)
+            && pImage->pszParentFilename)
+        {
+            rc = vhdLocatorUpdate(pImage, &ddh.ParentLocatorEntry[i], pImage->pszParentFilename);
+            if (RT_FAILURE(rc))
+                return rc;
+        }
+    }
+    /* Update parent's UUID */
+    memcpy(ddh.ParentUuid, pImage->ParentUuid.au8, sizeof(ddh.ParentUuid));
+
+    /* Update data offset and number of table entries. */
+    ddh.MaxTableEntries = RT_H2BE_U32(pImage->cBlockAllocationTableEntries);
+
+    ddh.Checksum = 0;
+    ddh.Checksum = RT_H2BE_U32(vhdChecksum(&ddh, sizeof(ddh)));
+    rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage,
+                                pImage->u64DataOffset, &ddh, sizeof(ddh));
+    return rc;
+}
+
+/**
+ * Internal: Update the VHD footer.
+ */
+static int vhdUpdateFooter(PVHDIMAGE pImage)
+{
+    int rc = VINF_SUCCESS;
+
+    /* Update fields which can change. */
+    pImage->vhdFooterCopy.CurSize              = RT_H2BE_U64(pImage->cbSize);
+    pImage->vhdFooterCopy.DiskGeometryCylinder = RT_H2BE_U16(pImage->PCHSGeometry.cCylinders);
+    pImage->vhdFooterCopy.DiskGeometryHeads    = pImage->PCHSGeometry.cHeads;
+    pImage->vhdFooterCopy.DiskGeometrySectors  = pImage->PCHSGeometry.cSectors;
+
+    pImage->vhdFooterCopy.Checksum = 0;
+    pImage->vhdFooterCopy.Checksum = RT_H2BE_U32(vhdChecksum(&pImage->vhdFooterCopy, sizeof(VHDFooter)));
+
+    if (pImage->pBlockAllocationTable)
+        rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage, 0,
+                                    &pImage->vhdFooterCopy, sizeof(VHDFooter));
+
+    if (RT_SUCCESS(rc))
+        rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage,
+                                    pImage->uCurrentEndOfFile, &pImage->vhdFooterCopy,
+                                    sizeof(VHDFooter));
+
+    return rc;
+}
+
+/**
+ * Internal. Flush image data to disk.
+ */
+static int vhdFlushImage(PVHDIMAGE pImage)
+{
+    int rc = VINF_SUCCESS;
+
+    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
+        return VINF_SUCCESS;
+
+    if (pImage->pBlockAllocationTable)
+    {
+        /*
+         * This is an expanding image. Write the BAT and copy of the disk footer.
+         */
+        size_t   cbBlockAllocationTableToWrite = pImage->cBlockAllocationTableEntries * sizeof(uint32_t);
+        uint32_t *pBlockAllocationTableToWrite = (uint32_t *)RTMemAllocZ(cbBlockAllocationTableToWrite);
+
+        if (!pBlockAllocationTableToWrite)
+            return VERR_NO_MEMORY;
+
+        /*
+         * The BAT entries have to be stored in big endian format.
+         */
+        for (unsigned i = 0; i < pImage->cBlockAllocationTableEntries; i++)
+            pBlockAllocationTableToWrite[i] = RT_H2BE_U32(pImage->pBlockAllocationTable[i]);
+
+        /*
+         * Write the block allocation table after the copy of the disk footer and the dynamic disk header.
+         */
+        vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage, pImage->uBlockAllocationTableOffset,
+                               pBlockAllocationTableToWrite, cbBlockAllocationTableToWrite);
+        if (pImage->fDynHdrNeedsUpdate)
+            rc = vhdDynamicHeaderUpdate(pImage);
+        RTMemFree(pBlockAllocationTableToWrite);
+    }
+
+    if (RT_SUCCESS(rc))
+        rc = vhdUpdateFooter(pImage);
+
+    if (RT_SUCCESS(rc))
+        rc = vdIfIoIntFileFlushSync(pImage->pIfIo, pImage->pStorage);
+
+    return rc;
+}
+
+/**
+ * Internal. Free all allocated space for representing an image except pImage,
+ * and optionally delete the image from disk.
+ */
+static int vhdFreeImage(PVHDIMAGE pImage, bool fDelete)
+{
+    int rc = VINF_SUCCESS;
+
+    /* Freeing a never allocated image (e.g. because the open failed) is
+     * not signalled as an error. After all nothing bad happens. */
+    if (pImage)
+    {
+        if (pImage->pStorage)
+        {
+            /* No point updating the file that is deleted anyway. */
+            if (!fDelete)
+                vhdFlushImage(pImage);
+
+            rc = vdIfIoIntFileClose(pImage->pIfIo, pImage->pStorage);
+            pImage->pStorage = NULL;
+        }
+
+        if (pImage->pszParentFilename)
+        {
+            RTStrFree(pImage->pszParentFilename);
+            pImage->pszParentFilename = NULL;
+        }
+        if (pImage->pBlockAllocationTable)
+        {
+            RTMemFree(pImage->pBlockAllocationTable);
+            pImage->pBlockAllocationTable = NULL;
+        }
+        if (pImage->pu8Bitmap)
+        {
+            RTMemFree(pImage->pu8Bitmap);
+            pImage->pu8Bitmap = NULL;
+        }
+
+        if (fDelete && pImage->pszFilename)
+        {
+            int rc2 = vdIfIoIntFileDelete(pImage->pIfIo, pImage->pszFilename);
+            if (RT_SUCCESS(rc))
+                rc = rc2;
+        }
+    }
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/* 946684800 is the number of seconds between 1/1/1970 and 1/1/2000 */
+#define VHD_TO_UNIX_EPOCH_SECONDS UINT64_C(946684800)
+
+static uint32_t vhdRtTime2VhdTime(PCRTTIMESPEC pRtTimestamp)
+{
+    uint64_t u64Seconds = RTTimeSpecGetSeconds(pRtTimestamp);
+    return (uint32_t)(u64Seconds - VHD_TO_UNIX_EPOCH_SECONDS);
+}
+
+static void vhdTime2RtTime(PRTTIMESPEC pRtTimestamp, uint32_t u32VhdTimestamp)
+{
+    RTTimeSpecSetSeconds(pRtTimestamp, VHD_TO_UNIX_EPOCH_SECONDS + u32VhdTimestamp);
+}
+
+/**
+ * Internal: Allocates the block bitmap rounding up to the next 32bit or 64bit boundary.
+ *           Can be freed with RTMemFree. The memory is zeroed.
+ */
+DECLINLINE(uint8_t *)vhdBlockBitmapAllocate(PVHDIMAGE pImage)
+{
+#ifdef RT_ARCH_AMD64
+    return (uint8_t *)RTMemAllocZ(pImage->cbDataBlockBitmap + 8);
+#else
+    return (uint8_t *)RTMemAllocZ(pImage->cbDataBlockBitmap + 4);
+#endif
+}
+
+/**
+ * Internal: called when the async expansion process completed (failure or success).
+ *           Will do the necessary rollback if an error occurred.
+ */
+static int vhdAsyncExpansionComplete(PVHDIMAGE pImage, PVDIOCTX pIoCtx, PVHDIMAGEEXPAND pExpand)
+{
+    int rc = VINF_SUCCESS;
+    uint32_t fFlags = ASMAtomicReadU32(&pExpand->fFlags);
+    bool fIoInProgress = false;
+
+    /* Quick path, check if everything succeeded. */
+    if (fFlags == VHDIMAGEEXPAND_ALL_SUCCESS)
+    {
+        pImage->pBlockAllocationTable[pExpand->idxBatAllocated] = RT_BE2H_U32(pExpand->idxBlockBe);
+        RTMemFree(pExpand);
+    }
+    else
+    {
+        uint32_t uStatus;
+
+        uStatus = VHDIMAGEEXPAND_STATUS_GET(pExpand->fFlags, VHDIMAGEEXPAND_BAT_STATUS_SHIFT);
+        if (   uStatus == VHDIMAGEEXPAND_STEP_FAILED
+            || uStatus == VHDIMAGEEXPAND_STEP_SUCCESS)
+        {
+            /* Undo and restore the old value. */
+            pImage->pBlockAllocationTable[pExpand->idxBatAllocated] = ~0U;
+
+            /* Restore the old value on the disk.
+             * No need for a completion callback because we can't
+             * do anything if this fails. */
+            if (uStatus == VHDIMAGEEXPAND_STEP_SUCCESS)
+            {
+                rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
+                                              pImage->uBlockAllocationTableOffset
+                                            + pExpand->idxBatAllocated * sizeof(uint32_t),
+                                            &pImage->pBlockAllocationTable[pExpand->idxBatAllocated],
+                                            sizeof(uint32_t), pIoCtx, NULL, NULL);
+                fIoInProgress |= rc == VERR_VD_ASYNC_IO_IN_PROGRESS;
+            }
+        }
+
+        /* Restore old size (including the footer because another application might
+         * fill up the free space making it impossible to add the footer)
+         * and add the footer at the right place again. */
+        rc = vdIfIoIntFileSetSize(pImage->pIfIo, pImage->pStorage,
+                                  pExpand->cbEofOld + sizeof(VHDFooter));
+        AssertRC(rc);
+
+        pImage->uCurrentEndOfFile = pExpand->cbEofOld;
+        rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
+                                    pImage->uCurrentEndOfFile,
+                                    &pImage->vhdFooterCopy, sizeof(VHDFooter),
+                                    pIoCtx, NULL, NULL);
+        fIoInProgress |= rc == VERR_VD_ASYNC_IO_IN_PROGRESS;
+    }
+
+    return fIoInProgress ? VERR_VD_ASYNC_IO_IN_PROGRESS : rc;
+}
+
+static int vhdAsyncExpansionStepCompleted(void *pBackendData, PVDIOCTX pIoCtx, void *pvUser, int rcReq, unsigned iStep)
+{
+   PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+   PVHDIMAGEEXPAND pExpand = (PVHDIMAGEEXPAND)pvUser;
+
+   LogFlowFunc(("pBackendData=%#p pIoCtx=%#p pvUser=%#p rcReq=%Rrc iStep=%u\n",
+                pBackendData, pIoCtx, pvUser, rcReq, iStep));
+
+   if (RT_SUCCESS(rcReq))
+       VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, iStep, VHDIMAGEEXPAND_STEP_SUCCESS);
+   else
+       VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, iStep, VHDIMAGEEXPAND_STEP_FAILED);
+
+   if ((pExpand->fFlags & VHDIMAGEEXPAND_ALL_COMPLETE) == VHDIMAGEEXPAND_ALL_COMPLETE)
+       return vhdAsyncExpansionComplete(pImage, pIoCtx, pExpand);
+
+   return VERR_VD_ASYNC_IO_IN_PROGRESS;
+}
+
+static DECLCALLBACK(int) vhdAsyncExpansionDataBlockBitmapComplete(void *pBackendData, PVDIOCTX pIoCtx, void *pvUser, int rcReq)
+{
+    return vhdAsyncExpansionStepCompleted(pBackendData, pIoCtx, pvUser, rcReq, VHDIMAGEEXPAND_BLOCKBITMAP_STATUS_SHIFT);
+}
+
+static DECLCALLBACK(int) vhdAsyncExpansionDataComplete(void *pBackendData, PVDIOCTX pIoCtx, void *pvUser, int rcReq)
+{
+    return vhdAsyncExpansionStepCompleted(pBackendData, pIoCtx, pvUser, rcReq, VHDIMAGEEXPAND_USERBLOCK_STATUS_SHIFT);
+}
+
+static DECLCALLBACK(int) vhdAsyncExpansionBatUpdateComplete(void *pBackendData, PVDIOCTX pIoCtx, void *pvUser, int rcReq)
+{
+    return vhdAsyncExpansionStepCompleted(pBackendData, pIoCtx, pvUser, rcReq, VHDIMAGEEXPAND_BAT_STATUS_SHIFT);
+}
+
+static DECLCALLBACK(int) vhdAsyncExpansionFooterUpdateComplete(void *pBackendData, PVDIOCTX pIoCtx, void *pvUser, int rcReq)
+{
+    return vhdAsyncExpansionStepCompleted(pBackendData, pIoCtx, pvUser, rcReq, VHDIMAGEEXPAND_FOOTER_STATUS_SHIFT);
+}
+
+static int vhdLoadDynamicDisk(PVHDIMAGE pImage, uint64_t uDynamicDiskHeaderOffset)
+{
+    VHDDynamicDiskHeader vhdDynamicDiskHeader;
+    int rc = VINF_SUCCESS;
+    uint32_t *pBlockAllocationTable;
+    uint64_t uBlockAllocationTableOffset;
+    unsigned i = 0;
+
+    Log(("Open a dynamic disk.\n"));
+
+    /*
+     * Read the dynamic disk header.
+     */
+    rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage, uDynamicDiskHeaderOffset,
+                               &vhdDynamicDiskHeader, sizeof(VHDDynamicDiskHeader));
+    if (memcmp(vhdDynamicDiskHeader.Cookie, VHD_DYNAMIC_DISK_HEADER_COOKIE, VHD_DYNAMIC_DISK_HEADER_COOKIE_SIZE))
+        return VERR_INVALID_PARAMETER;
+
+    pImage->cbDataBlock = RT_BE2H_U32(vhdDynamicDiskHeader.BlockSize);
+    LogFlowFunc(("BlockSize=%u\n", pImage->cbDataBlock));
+    pImage->cBlockAllocationTableEntries = RT_BE2H_U32(vhdDynamicDiskHeader.MaxTableEntries);
+    LogFlowFunc(("MaxTableEntries=%lu\n", pImage->cBlockAllocationTableEntries));
+    AssertMsg(!(pImage->cbDataBlock % VHD_SECTOR_SIZE), ("%s: Data block size is not a multiple of %!\n", __FUNCTION__, VHD_SECTOR_SIZE));
+
+    /*
+     * Bail out if the number of BAT entries exceeds the number of sectors for a maximum image.
+     * Lower the number of sectors in the BAT as a few sectors are already occupied by the footers
+     * and headers.
+     */
+    if (pImage->cBlockAllocationTableEntries > (VHD_MAX_SECTORS - 2))
+        return VERR_VD_VHD_INVALID_HEADER;
+
+    pImage->cSectorsPerDataBlock = pImage->cbDataBlock / VHD_SECTOR_SIZE;
+    LogFlowFunc(("SectorsPerDataBlock=%u\n", pImage->cSectorsPerDataBlock));
+
+    /*
+     * Every block starts with a bitmap indicating which sectors are valid and which are not.
+     * We store the size of it to be able to calculate the real offset.
+     */
+    pImage->cbDataBlockBitmap = pImage->cSectorsPerDataBlock / 8;
+    pImage->cDataBlockBitmapSectors = pImage->cbDataBlockBitmap / VHD_SECTOR_SIZE;
+    /* Round up to full sector size */
+    if (pImage->cbDataBlockBitmap % VHD_SECTOR_SIZE > 0)
+        pImage->cDataBlockBitmapSectors++;
+    LogFlowFunc(("cbDataBlockBitmap=%u\n", pImage->cbDataBlockBitmap));
+    LogFlowFunc(("cDataBlockBitmapSectors=%u\n", pImage->cDataBlockBitmapSectors));
+
+    pImage->pu8Bitmap = vhdBlockBitmapAllocate(pImage);
+    if (!pImage->pu8Bitmap)
+        return VERR_NO_MEMORY;
+
+    pBlockAllocationTable = (uint32_t *)RTMemAllocZ(pImage->cBlockAllocationTableEntries * sizeof(uint32_t));
+    if (!pBlockAllocationTable)
+        return VERR_NO_MEMORY;
+
+    /*
+     * Read the table.
+     */
+    uBlockAllocationTableOffset = RT_BE2H_U64(vhdDynamicDiskHeader.TableOffset);
+    LogFlowFunc(("uBlockAllocationTableOffset=%llu\n", uBlockAllocationTableOffset));
+    pImage->uBlockAllocationTableOffset = uBlockAllocationTableOffset;
+    rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage,
+                               uBlockAllocationTableOffset, pBlockAllocationTable,
+                               pImage->cBlockAllocationTableEntries * sizeof(uint32_t));
+    if (RT_FAILURE(rc))
+    {
+        RTMemFree(pBlockAllocationTable);
+        return rc;
+    }
+
+    /*
+     * Because the offset entries inside the allocation table are stored big endian
+     * we need to convert them into host endian.
+     */
+    pImage->pBlockAllocationTable = (uint32_t *)RTMemAllocZ(pImage->cBlockAllocationTableEntries * sizeof(uint32_t));
+    if (!pImage->pBlockAllocationTable)
+    {
+        RTMemFree(pBlockAllocationTable);
+        return VERR_NO_MEMORY;
+    }
+
+    for (i = 0; i < pImage->cBlockAllocationTableEntries; i++)
+        pImage->pBlockAllocationTable[i] = RT_BE2H_U32(pBlockAllocationTable[i]);
+
+    RTMemFree(pBlockAllocationTable);
+
+    if (pImage->uImageFlags & VD_IMAGE_FLAGS_DIFF)
+        memcpy(pImage->ParentUuid.au8, vhdDynamicDiskHeader.ParentUuid, sizeof(pImage->ParentUuid));
+
+    return rc;
+}
+
+static int vhdOpenImage(PVHDIMAGE pImage, unsigned uOpenFlags)
+{
+    uint64_t FileSize;
+    VHDFooter vhdFooter;
+
+    pImage->uOpenFlags = uOpenFlags;
+
+    pImage->pIfError = VDIfErrorGet(pImage->pVDIfsDisk);
+    pImage->pIfIo = VDIfIoIntGet(pImage->pVDIfsImage);
+    AssertPtrReturn(pImage->pIfIo, VERR_INVALID_PARAMETER);
+
+    /*
+     * Open the image.
+     */
+    int rc = vdIfIoIntFileOpen(pImage->pIfIo, pImage->pszFilename,
+                               VDOpenFlagsToFileOpenFlags(uOpenFlags,
+                                                          false /* fCreate */),
+                               &pImage->pStorage);
+    if (RT_FAILURE(rc))
+    {
+        /* Do NOT signal an appropriate error here, as the VD layer has the
+         * choice of retrying the open if it failed. */
+        return rc;
+    }
+
+    rc = vdIfIoIntFileGetSize(pImage->pIfIo, pImage->pStorage, &FileSize);
+    pImage->uCurrentEndOfFile = FileSize - sizeof(VHDFooter);
+
+    rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage, pImage->uCurrentEndOfFile,
+                               &vhdFooter, sizeof(VHDFooter));
+    if (RT_SUCCESS(rc))
+    {
+        if (memcmp(vhdFooter.Cookie, VHD_FOOTER_COOKIE, VHD_FOOTER_COOKIE_SIZE) != 0)
+        {
+            /*
+             * There is also a backup header at the beginning in case the image got corrupted.
+             * Such corrupted images are detected here to let the open handler repair it later.
+             */
+            rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage, 0,
+                                       &vhdFooter, sizeof(VHDFooter));
+            if (RT_SUCCESS(rc))
+            {
+                if (memcmp(vhdFooter.Cookie, VHD_FOOTER_COOKIE, VHD_FOOTER_COOKIE_SIZE) != 0)
+                    rc = VERR_VD_VHD_INVALID_HEADER;
+                else
+                    rc = VERR_VD_IMAGE_CORRUPTED;
+            }
+        }
+    }
+
+    if (RT_FAILURE(rc))
+    {
+        vhdFreeImage(pImage, false);
+        return rc;
+    }
+
+    switch (RT_BE2H_U32(vhdFooter.DiskType))
+    {
+        case VHD_FOOTER_DISK_TYPE_FIXED:
+            pImage->uImageFlags |= VD_IMAGE_FLAGS_FIXED;
+            break;
+        case VHD_FOOTER_DISK_TYPE_DYNAMIC:
+            pImage->uImageFlags &= ~VD_IMAGE_FLAGS_FIXED;
+            break;
+        case VHD_FOOTER_DISK_TYPE_DIFFERENCING:
+            pImage->uImageFlags |= VD_IMAGE_FLAGS_DIFF;
+            pImage->uImageFlags &= ~VD_IMAGE_FLAGS_FIXED;
+            break;
+        default:
+            vhdFreeImage(pImage, false);
+            return VERR_NOT_IMPLEMENTED;
+    }
+
+    pImage->cbSize       = RT_BE2H_U64(vhdFooter.CurSize);
+    pImage->LCHSGeometry.cCylinders   = 0;
+    pImage->LCHSGeometry.cHeads       = 0;
+    pImage->LCHSGeometry.cSectors     = 0;
+    pImage->PCHSGeometry.cCylinders   = RT_BE2H_U16(vhdFooter.DiskGeometryCylinder);
+    pImage->PCHSGeometry.cHeads       = vhdFooter.DiskGeometryHeads;
+    pImage->PCHSGeometry.cSectors     = vhdFooter.DiskGeometrySectors;
+
+    /*
+     * Copy of the disk footer.
+     * If we allocate new blocks in differencing disks on write access
+     * the footer is overwritten. We need to write it at the end of the file.
+     */
+    memcpy(&pImage->vhdFooterCopy, &vhdFooter, sizeof(VHDFooter));
+
+    /*
+     * Is there a better way?
+     */
+    memcpy(&pImage->ImageUuid, &vhdFooter.UniqueID, 16);
+
+    pImage->u64DataOffset = RT_BE2H_U64(vhdFooter.DataOffset);
+    LogFlowFunc(("DataOffset=%llu\n", pImage->u64DataOffset));
+
+    if (!(pImage->uImageFlags & VD_IMAGE_FLAGS_FIXED))
+        rc = vhdLoadDynamicDisk(pImage, pImage->u64DataOffset);
+
+    if (RT_SUCCESS(rc))
+    {
+        PVDREGIONDESC pRegion = &pImage->RegionList.aRegions[0];
+        pImage->RegionList.fFlags   = 0;
+        pImage->RegionList.cRegions = 1;
+
+        pRegion->offRegion            = 0; /* Disk start. */
+        pRegion->cbBlock              = 512;
+        pRegion->enmDataForm          = VDREGIONDATAFORM_RAW;
+        pRegion->enmMetadataForm      = VDREGIONMETADATAFORM_NONE;
+        pRegion->cbData               = 512;
+        pRegion->cbMetadata           = 0;
+        pRegion->cRegionBlocksOrBytes = pImage->cbSize;
+    }
+    else
+        vhdFreeImage(pImage, false);
+    return rc;
+}
+
+/**
+ * Internal: Checks if a sector in the block bitmap is set
+ */
+DECLINLINE(bool) vhdBlockBitmapSectorContainsData(PVHDIMAGE pImage, uint32_t cBlockBitmapEntry)
+{
+    uint32_t iBitmap = (cBlockBitmapEntry / 8); /* Byte in the block bitmap. */
+
+    /*
+     * The index of the bit in the byte of the data block bitmap.
+     * The most significant bit stands for a lower sector number.
+     */
+    uint8_t  iBitInByte = (8-1) - (cBlockBitmapEntry % 8);
+    uint8_t *puBitmap = pImage->pu8Bitmap + iBitmap;
+
+    AssertMsg(puBitmap < (pImage->pu8Bitmap + pImage->cbDataBlockBitmap),
+                ("VHD: Current bitmap position exceeds maximum size of the bitmap\n"));
+
+    return ((*puBitmap) & RT_BIT(iBitInByte)) != 0;
+}
+
+/**
+ * Internal: Sets the given sector in the sector bitmap.
+ */
+DECLINLINE(bool) vhdBlockBitmapSectorSet(PVHDIMAGE pImage, uint8_t *pu8Bitmap, uint32_t cBlockBitmapEntry)
+{
+    RT_NOREF1(pImage);
+    uint32_t iBitmap = (cBlockBitmapEntry / 8); /* Byte in the block bitmap. */
+
+    /*
+     * The index of the bit in the byte of the data block bitmap.
+     * The most significant bit stands for a lower sector number.
+     */
+    uint8_t  iBitInByte = (8-1) - (cBlockBitmapEntry % 8);
+    uint8_t  *puBitmap  = pu8Bitmap + iBitmap;
+
+    AssertMsg(puBitmap < (pu8Bitmap + pImage->cbDataBlockBitmap),
+                ("VHD: Current bitmap position exceeds maximum size of the bitmap\n"));
+
+    bool fClear = ((*puBitmap) & RT_BIT(iBitInByte)) == 0;
+    *puBitmap |= RT_BIT(iBitInByte);
+    return fClear;
+}
+
+/**
+ * Internal: Derive drive geometry from its size.
+ */
+static void vhdSetDiskGeometry(PVHDIMAGE pImage, uint64_t cbSize)
+{
+    uint64_t u64TotalSectors = cbSize / VHD_SECTOR_SIZE;
+    uint32_t u32CylinderTimesHeads, u32Heads, u32SectorsPerTrack;
+
+    if (u64TotalSectors > 65535 * 16 * 255)
+    {
+        /* ATA disks limited to 127 GB. */
+        u64TotalSectors = 65535 * 16 * 255;
+    }
+
+    if (u64TotalSectors >= 65535 * 16 * 63)
+    {
+        u32SectorsPerTrack    = 255;
+        u32Heads              = 16;
+        u32CylinderTimesHeads = u64TotalSectors / u32SectorsPerTrack;
+    }
+    else
+    {
+        u32SectorsPerTrack    = 17;
+        u32CylinderTimesHeads = u64TotalSectors / u32SectorsPerTrack;
+
+        u32Heads = (u32CylinderTimesHeads + 1023) / 1024;
+
+        if (u32Heads < 4)
+        {
+            u32Heads = 4;
+        }
+        if (u32CylinderTimesHeads >= (u32Heads * 1024) || u32Heads > 16)
+        {
+            u32SectorsPerTrack    = 31;
+            u32Heads              = 16;
+            u32CylinderTimesHeads = u64TotalSectors / u32SectorsPerTrack;
+        }
+        if (u32CylinderTimesHeads >= (u32Heads * 1024))
+        {
+            u32SectorsPerTrack    = 63;
+            u32Heads              = 16;
+            u32CylinderTimesHeads = u64TotalSectors / u32SectorsPerTrack;
+        }
+    }
+    pImage->PCHSGeometry.cCylinders = u32CylinderTimesHeads / u32Heads;
+    pImage->PCHSGeometry.cHeads     = u32Heads;
+    pImage->PCHSGeometry.cSectors   = u32SectorsPerTrack;
+    pImage->LCHSGeometry.cCylinders = 0;
+    pImage->LCHSGeometry.cHeads     = 0;
+    pImage->LCHSGeometry.cSectors   = 0;
+}
+
+
+static uint32_t vhdAllocateParentLocators(PVHDIMAGE pImage, VHDDynamicDiskHeader *pDDH, uint64_t u64Offset)
+{
+    RT_NOREF1(pImage);
+    PVHDPLE pLocator = pDDH->ParentLocatorEntry;
+
+    /*
+     * The VHD spec states that the DataSpace field holds the number of sectors
+     * required to store the parent locator path.
+     * As it turned out VPC and Hyper-V store the amount of bytes reserved for the
+     * path and not the number of sectors.
+     */
+
+    /* Unicode absolute Windows path. */
+    pLocator->u32Code = RT_H2BE_U32(VHD_PLATFORM_CODE_W2KU);
+    pLocator->u32DataSpace = RT_H2BE_U32(VHD_ABSOLUTE_MAX_PATH * sizeof(RTUTF16));
+    pLocator->u64DataOffset = RT_H2BE_U64(u64Offset);
+    pLocator++;
+    u64Offset += VHD_ABSOLUTE_MAX_PATH * sizeof(RTUTF16);
+    /* Unicode relative Windows path. */
+    pLocator->u32Code = RT_H2BE_U32(VHD_PLATFORM_CODE_W2RU);
+    pLocator->u32DataSpace = RT_H2BE_U32(VHD_RELATIVE_MAX_PATH * sizeof(RTUTF16));
+    pLocator->u64DataOffset = RT_H2BE_U64(u64Offset);
+    u64Offset += VHD_RELATIVE_MAX_PATH * sizeof(RTUTF16);
+    return u64Offset;
+}
+
+/**
+ * Internal: Additional code for dynamic VHD image creation.
+ */
+static int vhdCreateDynamicImage(PVHDIMAGE pImage, uint64_t cbSize)
+{
+    int rc;
+    VHDDynamicDiskHeader DynamicDiskHeader;
+    uint32_t u32BlockAllocationTableSectors;
+    void    *pvTmp = NULL;
+
+    memset(&DynamicDiskHeader, 0, sizeof(DynamicDiskHeader));
+
+    pImage->u64DataOffset           = sizeof(VHDFooter);
+    pImage->cbDataBlock             = VHD_BLOCK_SIZE; /* 2 MB */
+    pImage->cSectorsPerDataBlock    = pImage->cbDataBlock / VHD_SECTOR_SIZE;
+    pImage->cbDataBlockBitmap       = pImage->cSectorsPerDataBlock / 8;
+    pImage->cDataBlockBitmapSectors = pImage->cbDataBlockBitmap / VHD_SECTOR_SIZE;
+    /* Align to sector boundary */
+    if (pImage->cbDataBlockBitmap % VHD_SECTOR_SIZE > 0)
+        pImage->cDataBlockBitmapSectors++;
+    pImage->pu8Bitmap               = vhdBlockBitmapAllocate(pImage);
+    if (!pImage->pu8Bitmap)
+        return vdIfError(pImage->pIfError, VERR_NO_MEMORY, RT_SRC_POS, N_("VHD: cannot allocate memory for bitmap storage"));
+
+    /* Initialize BAT. */
+    pImage->uBlockAllocationTableOffset = (uint64_t)sizeof(VHDFooter) + sizeof(VHDDynamicDiskHeader);
+    pImage->cBlockAllocationTableEntries = (uint32_t)((cbSize + pImage->cbDataBlock - 1) / pImage->cbDataBlock); /* Align table to the block size. */
+    u32BlockAllocationTableSectors = (pImage->cBlockAllocationTableEntries * sizeof(uint32_t) + VHD_SECTOR_SIZE - 1) / VHD_SECTOR_SIZE;
+    pImage->pBlockAllocationTable = (uint32_t *)RTMemAllocZ(pImage->cBlockAllocationTableEntries * sizeof(uint32_t));
+    if (!pImage->pBlockAllocationTable)
+        return vdIfError(pImage->pIfError, VERR_NO_MEMORY, RT_SRC_POS, N_("VHD: cannot allocate memory for BAT"));
+
+    for (unsigned i = 0; i < pImage->cBlockAllocationTableEntries; i++)
+    {
+        pImage->pBlockAllocationTable[i] = 0xFFFFFFFF; /* It is actually big endian. */
+    }
+
+    /* Round up to the sector size. */
+    if (pImage->uImageFlags & VD_IMAGE_FLAGS_DIFF) /* fix hyper-v unreadable error */
+        pImage->uCurrentEndOfFile = vhdAllocateParentLocators(pImage, &DynamicDiskHeader,
+                                                              pImage->uBlockAllocationTableOffset + u32BlockAllocationTableSectors * VHD_SECTOR_SIZE);
+    else
+        pImage->uCurrentEndOfFile = pImage->uBlockAllocationTableOffset + u32BlockAllocationTableSectors * VHD_SECTOR_SIZE;
+
+    /* Set dynamic image size. */
+    pvTmp = RTMemTmpAllocZ(pImage->uCurrentEndOfFile + sizeof(VHDFooter));
+    if (!pvTmp)
+        return vdIfError(pImage->pIfError, VERR_NO_MEMORY, RT_SRC_POS, N_("VHD: cannot set the file size for '%s'"), pImage->pszFilename);
+
+    rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage, 0, pvTmp,
+                          pImage->uCurrentEndOfFile + sizeof(VHDFooter));
+    if (RT_FAILURE(rc))
+    {
+        RTMemTmpFree(pvTmp);
+        return vdIfError(pImage->pIfError, rc, RT_SRC_POS, N_("VHD: cannot set the file size for '%s'"), pImage->pszFilename);
+    }
+
+    RTMemTmpFree(pvTmp);
+
+    /* Initialize and write the dynamic disk header. */
+    memcpy(DynamicDiskHeader.Cookie, VHD_DYNAMIC_DISK_HEADER_COOKIE, sizeof(DynamicDiskHeader.Cookie));
+    DynamicDiskHeader.DataOffset      = UINT64_C(0xFFFFFFFFFFFFFFFF); /* Initially the disk has no data. */
+    DynamicDiskHeader.TableOffset     = RT_H2BE_U64(pImage->uBlockAllocationTableOffset);
+    DynamicDiskHeader.HeaderVersion   = RT_H2BE_U32(VHD_DYNAMIC_DISK_HEADER_VERSION);
+    DynamicDiskHeader.BlockSize       = RT_H2BE_U32(pImage->cbDataBlock);
+    DynamicDiskHeader.MaxTableEntries = RT_H2BE_U32(pImage->cBlockAllocationTableEntries);
+    /* Compute and update checksum. */
+    DynamicDiskHeader.Checksum = 0;
+    DynamicDiskHeader.Checksum = RT_H2BE_U32(vhdChecksum(&DynamicDiskHeader, sizeof(DynamicDiskHeader)));
+
+    rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage, sizeof(VHDFooter),
+                                &DynamicDiskHeader, sizeof(DynamicDiskHeader));
+    if (RT_FAILURE(rc))
+        return vdIfError(pImage->pIfError, rc, RT_SRC_POS, N_("VHD: cannot write dynamic disk header to image '%s'"), pImage->pszFilename);
+
+    /* Write BAT. */
+    rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage, pImage->uBlockAllocationTableOffset,
+                                pImage->pBlockAllocationTable,
+                                pImage->cBlockAllocationTableEntries * sizeof(uint32_t));
+    if (RT_FAILURE(rc))
+        return vdIfError(pImage->pIfError, rc, RT_SRC_POS, N_("VHD: cannot write BAT to image '%s'"), pImage->pszFilename);
+
+    return rc;
+}
+
+/**
+ * Internal: The actual code for VHD image creation, both fixed and dynamic.
+ */
+static int vhdCreateImage(PVHDIMAGE pImage, uint64_t cbSize,
+                          unsigned uImageFlags, const char *pszComment,
+                          PCVDGEOMETRY pPCHSGeometry,
+                          PCVDGEOMETRY pLCHSGeometry, PCRTUUID pUuid,
+                          unsigned uOpenFlags,
+                          PVDINTERFACEPROGRESS pIfProgress,
+                          unsigned uPercentStart, unsigned uPercentSpan)
+{
+    RT_NOREF3(pszComment, pPCHSGeometry, pLCHSGeometry);
+    VHDFooter Footer;
+    RTTIMESPEC now;
+
+    pImage->uOpenFlags = uOpenFlags;
+    pImage->uImageFlags = uImageFlags;
+    pImage->pIfError = VDIfErrorGet(pImage->pVDIfsDisk);
+
+    int rc = vdIfIoIntFileOpen(pImage->pIfIo, pImage->pszFilename,
+                               VDOpenFlagsToFileOpenFlags(uOpenFlags & ~VD_OPEN_FLAGS_READONLY,
+                                                          true /* fCreate */),
+                               &pImage->pStorage);
+    if (RT_SUCCESS(rc))
+    {
+        pImage->cbSize = cbSize;
+        pImage->ImageUuid = *pUuid;
+        RTUuidClear(&pImage->ParentUuid);
+        vhdSetDiskGeometry(pImage, cbSize);
+
+        /* Initialize the footer. */
+        memset(&Footer, 0, sizeof(Footer));
+        memcpy(Footer.Cookie, VHD_FOOTER_COOKIE, sizeof(Footer.Cookie));
+        Footer.Features = RT_H2BE_U32(0x2);
+        Footer.Version  = RT_H2BE_U32(VHD_FOOTER_FILE_FORMAT_VERSION);
+        Footer.Timestamp = RT_H2BE_U32(vhdRtTime2VhdTime(RTTimeNow(&now)));
+        memcpy(Footer.CreatorApp, "vbox", sizeof(Footer.CreatorApp));
+        Footer.CreatorVer = RT_H2BE_U32(VBOX_VERSION);
+#ifdef RT_OS_DARWIN
+        Footer.CreatorOS  = RT_H2BE_U32(0x4D616320); /* "Mac " */
+#else /* Virtual PC supports only two platforms atm, so everything else will be Wi2k. */
+        Footer.CreatorOS  = RT_H2BE_U32(0x5769326B); /* "Wi2k" */
+#endif
+        Footer.OrigSize   = RT_H2BE_U64(cbSize);
+        Footer.CurSize    = Footer.OrigSize;
+        Footer.DiskGeometryCylinder = RT_H2BE_U16(pImage->PCHSGeometry.cCylinders);
+        Footer.DiskGeometryHeads    = pImage->PCHSGeometry.cHeads;
+        Footer.DiskGeometrySectors  = pImage->PCHSGeometry.cSectors;
+        memcpy(Footer.UniqueID, pImage->ImageUuid.au8, sizeof(Footer.UniqueID));
+        Footer.SavedState = 0;
+
+        if (uImageFlags & VD_IMAGE_FLAGS_FIXED)
+        {
+            Footer.DiskType = RT_H2BE_U32(VHD_FOOTER_DISK_TYPE_FIXED);
+            /*
+             * Initialize fixed image.
+             * "The size of the entire file is the size of the hard disk in
+             * the guest operating system plus the size of the footer."
+             */
+            pImage->u64DataOffset     = VHD_FOOTER_DATA_OFFSET_FIXED;
+            pImage->uCurrentEndOfFile = cbSize;
+            rc = vdIfIoIntFileSetAllocationSize(pImage->pIfIo, pImage->pStorage, pImage->uCurrentEndOfFile + sizeof(VHDFooter),
+                                                0 /* fFlags */, pIfProgress,
+                                                uPercentStart, uPercentSpan);
+            if (RT_FAILURE(rc))
+                rc =  vdIfError(pImage->pIfError, rc, RT_SRC_POS, N_("VHD: cannot set the file size for '%s'"), pImage->pszFilename);
+        }
+        else
+        {
+            /*
+             * Initialize dynamic image.
+             *
+             * The overall structure of dynamic disk is:
+             *
+             * [Copy of hard disk footer (512 bytes)]
+             * [Dynamic disk header (1024 bytes)]
+             * [BAT (Block Allocation Table)]
+             * [Parent Locators]
+             * [Data block 1]
+             * [Data block 2]
+             * ...
+             * [Data block N]
+             * [Hard disk footer (512 bytes)]
+             */
+            Footer.DiskType   = (uImageFlags & VD_IMAGE_FLAGS_DIFF)
+                                  ? RT_H2BE_U32(VHD_FOOTER_DISK_TYPE_DIFFERENCING)
+                                  : RT_H2BE_U32(VHD_FOOTER_DISK_TYPE_DYNAMIC);
+            /* We are half way thorough with creation of image, let the caller know. */
+            vdIfProgress(pIfProgress, (uPercentStart + uPercentSpan) / 2);
+
+            rc = vhdCreateDynamicImage(pImage, cbSize);
+        }
+
+        if (RT_SUCCESS(rc))
+        {
+            /* Compute and update the footer checksum. */
+            Footer.DataOffset = RT_H2BE_U64(pImage->u64DataOffset);
+            Footer.Checksum   = 0;
+            Footer.Checksum   = RT_H2BE_U32(vhdChecksum(&Footer, sizeof(Footer)));
+
+            pImage->vhdFooterCopy = Footer;
+
+            /* Store the footer */
+            rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage, pImage->uCurrentEndOfFile,
+                                        &Footer, sizeof(Footer));
+            if (RT_SUCCESS(rc))
+            {
+                /* Dynamic images contain a copy of the footer at the very beginning of the file. */
+                if (!(uImageFlags & VD_IMAGE_FLAGS_FIXED))
+                {
+                    /* Write the copy of the footer. */
+                    rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage, 0, &Footer, sizeof(Footer));
+                    if (RT_FAILURE(rc))
+                        rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS, N_("VHD: cannot write a copy of footer to image '%s'"), pImage->pszFilename);
+                }
+            }
+            else
+                rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS, N_("VHD: cannot write footer to image '%s'"), pImage->pszFilename);
+        }
+    }
+    else
+        rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS, N_("VHD: cannot create image '%s'"), pImage->pszFilename);
+
+    if (RT_SUCCESS(rc))
+        vdIfProgress(pIfProgress, uPercentStart + uPercentSpan);
+
+    if (RT_SUCCESS(rc))
+    {
+        PVDREGIONDESC pRegion = &pImage->RegionList.aRegions[0];
+        pImage->RegionList.fFlags   = 0;
+        pImage->RegionList.cRegions = 1;
+
+        pRegion->offRegion            = 0; /* Disk start. */
+        pRegion->cbBlock              = 512;
+        pRegion->enmDataForm          = VDREGIONDATAFORM_RAW;
+        pRegion->enmMetadataForm      = VDREGIONMETADATAFORM_NONE;
+        pRegion->cbData               = 512;
+        pRegion->cbMetadata           = 0;
+        pRegion->cRegionBlocksOrBytes = pImage->cbSize;
+    }
+    else
+        vhdFreeImage(pImage, rc != VERR_ALREADY_EXISTS);
+    return rc;
+}
+
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnProbe} */
+static DECLCALLBACK(int) vhdProbe(const char *pszFilename, PVDINTERFACE pVDIfsDisk,
+                                  PVDINTERFACE pVDIfsImage, VDTYPE enmDesiredType, VDTYPE *penmType)
+{
+    RT_NOREF(pVDIfsDisk, enmDesiredType);
+    LogFlowFunc(("pszFilename=\"%s\" pVDIfsDisk=%#p pVDIfsImage=%#p\n", pszFilename, pVDIfsDisk, pVDIfsImage));
+    PVDIOSTORAGE pStorage;
+    PVDINTERFACEIOINT pIfIo = VDIfIoIntGet(pVDIfsImage);
+    AssertPtrReturn(pIfIo, VERR_INVALID_PARAMETER);
+
+    int rc = vdIfIoIntFileOpen(pIfIo, pszFilename,
+                               VDOpenFlagsToFileOpenFlags(VD_OPEN_FLAGS_READONLY,
+                                                          false /* fCreate */),
+                               &pStorage);
+    if (RT_SUCCESS(rc))
+    {
+        uint64_t cbFile;
+
+        rc = vdIfIoIntFileGetSize(pIfIo, pStorage, &cbFile);
+        if (   RT_SUCCESS(rc)
+            && cbFile >= sizeof(VHDFooter))
+        {
+            VHDFooter vhdFooter;
+
+            rc = vdIfIoIntFileReadSync(pIfIo, pStorage, cbFile - sizeof(VHDFooter),
+                                       &vhdFooter, sizeof(VHDFooter));
+            if (RT_SUCCESS(rc))
+            {
+                if (memcmp(vhdFooter.Cookie, VHD_FOOTER_COOKIE, VHD_FOOTER_COOKIE_SIZE) != 0)
+                {
+                    /*
+                     * There is also a backup header at the beginning in case the image got corrupted.
+                     * Such corrupted images are detected here to let the open handler repair it later.
+                     */
+                    rc = vdIfIoIntFileReadSync(pIfIo, pStorage, 0, &vhdFooter, sizeof(VHDFooter));
+                    if (   RT_FAILURE(rc)
+                        || (memcmp(vhdFooter.Cookie, VHD_FOOTER_COOKIE, VHD_FOOTER_COOKIE_SIZE) != 0))
+                           rc = VERR_VD_VHD_INVALID_HEADER;
+                }
+
+                if (RT_SUCCESS(rc))
+                    *penmType = VDTYPE_HDD;
+            }
+            else
+                rc = VERR_VD_VHD_INVALID_HEADER;
+        }
+        else if (RT_SUCCESS(rc))
+            rc = VERR_VD_VHD_INVALID_HEADER;
+
+        vdIfIoIntFileClose(pIfIo, pStorage);
+    }
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnOpen} */
+static DECLCALLBACK(int) vhdOpen(const char *pszFilename, unsigned uOpenFlags,
+                                 PVDINTERFACE pVDIfsDisk, PVDINTERFACE pVDIfsImage,
+                                 VDTYPE enmType, void **ppBackendData)
+{
+    RT_NOREF1(enmType); /**< @todo r=klaus make use of the type info. */
+
+    LogFlowFunc(("pszFilename=\"%s\" uOpenFlags=%#x pVDIfsDisk=%#p pVDIfsImage=%#p enmType=%u ppBackendData=%#p\n",
+                 pszFilename, uOpenFlags, pVDIfsDisk, pVDIfsImage, enmType, ppBackendData));
+    int rc = VINF_SUCCESS;
+
+    /* Check open flags. All valid flags are supported. */
+    AssertReturn(!(uOpenFlags & ~VD_OPEN_FLAGS_MASK), VERR_INVALID_PARAMETER);
+    AssertPtrReturn(pszFilename, VERR_INVALID_POINTER);
+    AssertReturn(*pszFilename != '\0', VERR_INVALID_PARAMETER);
+
+
+    PVHDIMAGE pImage = (PVHDIMAGE)RTMemAllocZ(RT_UOFFSETOF(VHDIMAGE, RegionList.aRegions[1]));
+    if (RT_LIKELY(pImage))
+    {
+        pImage->pszFilename = pszFilename;
+        pImage->pStorage = NULL;
+        pImage->pVDIfsDisk = pVDIfsDisk;
+        pImage->pVDIfsImage = pVDIfsImage;
+
+        rc = vhdOpenImage(pImage, uOpenFlags);
+        if (RT_SUCCESS(rc))
+            *ppBackendData = pImage;
+        else
+            RTMemFree(pImage);
+    }
+    else
+        rc = VERR_NO_MEMORY;
+
+    LogFlowFunc(("returns %Rrc (pBackendData=%#p)\n", rc, *ppBackendData));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnCreate} */
+static DECLCALLBACK(int) vhdCreate(const char *pszFilename, uint64_t cbSize,
+                                   unsigned uImageFlags, const char *pszComment,
+                                   PCVDGEOMETRY pPCHSGeometry, PCVDGEOMETRY pLCHSGeometry,
+                                   PCRTUUID pUuid, unsigned uOpenFlags,
+                                   unsigned uPercentStart, unsigned uPercentSpan,
+                                   PVDINTERFACE pVDIfsDisk, PVDINTERFACE pVDIfsImage,
+                                   PVDINTERFACE pVDIfsOperation, VDTYPE enmType,
+                                   void **ppBackendData)
+{
+    LogFlowFunc(("pszFilename=\"%s\" cbSize=%llu uImageFlags=%#x pszComment=\"%s\" pPCHSGeometry=%#p pLCHSGeometry=%#p Uuid=%RTuuid uOpenFlags=%#x uPercentStart=%u uPercentSpan=%u pVDIfsDisk=%#p pVDIfsImage=%#p pVDIfsOperation=%#p enmType=%u ppBackendData=%#p",
+                 pszFilename, cbSize, uImageFlags, pszComment, pPCHSGeometry, pLCHSGeometry, pUuid, uOpenFlags, uPercentStart, uPercentSpan, pVDIfsDisk, pVDIfsImage, pVDIfsOperation, enmType, ppBackendData));
+    int rc;
+    PVDINTERFACEPROGRESS pIfProgress = VDIfProgressGet(pVDIfsOperation);
+
+    /* Check the VD container type. */
+    if (enmType != VDTYPE_HDD)
+        return VERR_VD_INVALID_TYPE;
+
+    /* Check open flags. All valid flags are supported. */
+    AssertReturn(!(uOpenFlags & ~VD_OPEN_FLAGS_MASK), VERR_INVALID_PARAMETER);
+    AssertPtrReturn(pszFilename, VERR_INVALID_POINTER);
+    AssertReturn(*pszFilename != '\0', VERR_INVALID_PARAMETER);
+    AssertPtrReturn(pPCHSGeometry, VERR_INVALID_POINTER);
+    AssertPtrReturn(pLCHSGeometry, VERR_INVALID_POINTER);
+    /** @todo Check the values of other params */
+
+    PVHDIMAGE pImage = (PVHDIMAGE)RTMemAllocZ(RT_UOFFSETOF(VHDIMAGE, RegionList.aRegions[1]));
+    if (RT_LIKELY(pImage))
+    {
+        pImage->pszFilename = pszFilename;
+        pImage->pStorage = NULL;
+        pImage->pVDIfsDisk = pVDIfsDisk;
+        pImage->pVDIfsImage = pVDIfsImage;
+
+        /* Get I/O interface. */
+        pImage->pIfIo = VDIfIoIntGet(pImage->pVDIfsImage);
+        if (RT_LIKELY(RT_VALID_PTR(pImage->pIfIo)))
+        {
+            rc = vhdCreateImage(pImage, cbSize, uImageFlags, pszComment,
+                                pPCHSGeometry, pLCHSGeometry, pUuid, uOpenFlags,
+                                pIfProgress, uPercentStart, uPercentSpan);
+            if (RT_SUCCESS(rc))
+            {
+                /* So far the image is opened in read/write mode. Make sure the
+                 * image is opened in read-only mode if the caller requested that. */
+                if (uOpenFlags & VD_OPEN_FLAGS_READONLY)
+                {
+                    vhdFreeImage(pImage, false);
+                    rc = vhdOpenImage(pImage, uOpenFlags);
+                }
+
+                if (RT_SUCCESS(rc))
+                    *ppBackendData = pImage;
+            }
+        }
+        else
+            rc = VERR_INVALID_PARAMETER;
+
+        if (RT_FAILURE(rc))
+            RTMemFree(pImage);
+    }
+    else
+        rc = VERR_NO_MEMORY;
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnRename} */
+static DECLCALLBACK(int) vhdRename(void *pBackendData, const char *pszFilename)
+{
+    LogFlowFunc(("pBackendData=%#p pszFilename=%#p\n", pBackendData, pszFilename));
+    int rc = VINF_SUCCESS;
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    /* Check arguments. */
+    AssertReturn((pImage && pszFilename && *pszFilename), VERR_INVALID_PARAMETER);
+
+    /* Close the image. */
+    rc = vhdFreeImage(pImage, false);
+    if (RT_SUCCESS(rc))
+    {
+        /* Rename the file. */
+        rc = vdIfIoIntFileMove(pImage->pIfIo, pImage->pszFilename, pszFilename, 0);
+        if (RT_SUCCESS(rc))
+        {
+            /* Update pImage with the new information. */
+            pImage->pszFilename = pszFilename;
+
+            /* Open the old file with new name. */
+            rc = vhdOpenImage(pImage, pImage->uOpenFlags);
+        }
+        else
+        {
+            /* The move failed, try to reopen the original image. */
+            int rc2 = vhdOpenImage(pImage, pImage->uOpenFlags);
+            if (RT_FAILURE(rc2))
+                rc = rc2;
+        }
+    }
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnClose} */
+static DECLCALLBACK(int) vhdClose(void *pBackendData, bool fDelete)
+{
+    LogFlowFunc(("pBackendData=%#p fDelete=%d\n", pBackendData, fDelete));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    int rc = vhdFreeImage(pImage, fDelete);
+    RTMemFree(pImage);
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnRead} */
+static DECLCALLBACK(int) vhdRead(void *pBackendData, uint64_t uOffset, size_t cbToRead,
+                                 PVDIOCTX pIoCtx, size_t *pcbActuallyRead)
+{
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+    int rc = VINF_SUCCESS;
+
+    LogFlowFunc(("pBackendData=%p uOffset=%#llx pIoCtx=%#p cbToRead=%u pcbActuallyRead=%p\n",
+                 pBackendData, uOffset, pIoCtx, cbToRead, pcbActuallyRead));
+
+    AssertPtr(pImage);
+    Assert(uOffset % 512 == 0);
+    Assert(cbToRead % 512 == 0);
+    AssertPtrReturn(pIoCtx, VERR_INVALID_POINTER);
+    AssertReturn(cbToRead, VERR_INVALID_PARAMETER);
+    AssertReturn(uOffset + cbToRead <= pImage->cbSize, VERR_INVALID_PARAMETER);
+
+    /*
+     * If we have a dynamic disk image, we need to find the data block and sector to read.
+     */
+    if (pImage->pBlockAllocationTable)
+    {
+        /*
+         * Get the data block first.
+         */
+        uint32_t cBlockAllocationTableEntry = (uOffset / VHD_SECTOR_SIZE) / pImage->cSectorsPerDataBlock;
+        uint32_t cBATEntryIndex = (uOffset / VHD_SECTOR_SIZE) % pImage->cSectorsPerDataBlock;
+        uint64_t uVhdOffset;
+
+        LogFlowFunc(("cBlockAllocationTableEntry=%u cBatEntryIndex=%u\n", cBlockAllocationTableEntry, cBATEntryIndex));
+        LogFlowFunc(("BlockAllocationEntry=%u\n", pImage->pBlockAllocationTable[cBlockAllocationTableEntry]));
+
+        /*
+         * Clip read range to remain in this data block.
+         */
+        cbToRead = RT_MIN(cbToRead, (pImage->cbDataBlock - (cBATEntryIndex * VHD_SECTOR_SIZE)));
+
+        /*
+         * If the block is not allocated the content of the entry is ~0
+         */
+        if (pImage->pBlockAllocationTable[cBlockAllocationTableEntry] == ~0U)
+            rc = VERR_VD_BLOCK_FREE;
+        else
+        {
+            uVhdOffset = ((uint64_t)pImage->pBlockAllocationTable[cBlockAllocationTableEntry] + pImage->cDataBlockBitmapSectors + cBATEntryIndex) * VHD_SECTOR_SIZE;
+            LogFlowFunc(("uVhdOffset=%llu cbToRead=%u\n", uVhdOffset, cbToRead));
+
+            /* Read in the block's bitmap. */
+            PVDMETAXFER pMetaXfer;
+            rc = vdIfIoIntFileReadMeta(pImage->pIfIo, pImage->pStorage,
+                                       ((uint64_t)pImage->pBlockAllocationTable[cBlockAllocationTableEntry]) * VHD_SECTOR_SIZE,
+                                       pImage->pu8Bitmap, pImage->cbDataBlockBitmap,
+                                       pIoCtx, &pMetaXfer, NULL, NULL);
+
+            if (RT_SUCCESS(rc))
+            {
+                uint32_t cSectors = 0;
+
+                vdIfIoIntMetaXferRelease(pImage->pIfIo, pMetaXfer);
+                if (vhdBlockBitmapSectorContainsData(pImage, cBATEntryIndex))
+                {
+                    cBATEntryIndex++;
+                    cSectors = 1;
+
+                    /*
+                     * The first sector being read is marked dirty, read as much as we
+                     * can from child. Note that only sectors that are marked dirty
+                     * must be read from child.
+                     */
+                    while (   (cSectors < (cbToRead / VHD_SECTOR_SIZE))
+                           && vhdBlockBitmapSectorContainsData(pImage, cBATEntryIndex))
+                    {
+                        cBATEntryIndex++;
+                        cSectors++;
+                    }
+
+                    cbToRead = cSectors * VHD_SECTOR_SIZE;
+
+                    LogFlowFunc(("uVhdOffset=%llu cbToRead=%u\n", uVhdOffset, cbToRead));
+                    rc = vdIfIoIntFileReadUser(pImage->pIfIo, pImage->pStorage,
+                                               uVhdOffset, pIoCtx, cbToRead);
+                }
+                else
+                {
+                    /*
+                     * The first sector being read is marked clean, so we should read from
+                     * our parent instead, but only as much as there are the following
+                     * clean sectors, because the block may still contain dirty sectors
+                     * further on. We just need to compute the number of clean sectors
+                     * and pass it to our caller along with the notification that they
+                     * should be read from the parent.
+                     */
+                    cBATEntryIndex++;
+                    cSectors = 1;
+
+                    while (   (cSectors < (cbToRead / VHD_SECTOR_SIZE))
+                           && !vhdBlockBitmapSectorContainsData(pImage, cBATEntryIndex))
+                    {
+                        cBATEntryIndex++;
+                        cSectors++;
+                    }
+
+                    cbToRead = cSectors * VHD_SECTOR_SIZE;
+                    LogFunc(("Sectors free: uVhdOffset=%llu cbToRead=%u\n", uVhdOffset, cbToRead));
+                    rc = VERR_VD_BLOCK_FREE;
+                }
+            }
+            else
+                AssertMsg(rc == VERR_VD_NOT_ENOUGH_METADATA, ("Reading block bitmap failed rc=%Rrc\n", rc));
+        }
+    }
+    else
+        rc = vdIfIoIntFileReadUser(pImage->pIfIo, pImage->pStorage, uOffset, pIoCtx, cbToRead);
+
+    if (pcbActuallyRead)
+        *pcbActuallyRead = cbToRead;
+
+    LogFlowFunc(("returns rc=%Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnWrite} */
+static DECLCALLBACK(int) vhdWrite(void *pBackendData, uint64_t uOffset, size_t cbToWrite,
+                         PVDIOCTX pIoCtx, size_t *pcbWriteProcess, size_t *pcbPreRead,
+                         size_t *pcbPostRead, unsigned fWrite)
+{
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+    int rc = VINF_SUCCESS;
+
+    LogFlowFunc(("pBackendData=%p uOffset=%llu pIoCtx=%#p cbToWrite=%u pcbWriteProcess=%p pcbPreRead=%p pcbPostRead=%p fWrite=%u\n",
+             pBackendData, uOffset, pIoCtx, cbToWrite, pcbWriteProcess, pcbPreRead, pcbPostRead, fWrite));
+
+    AssertPtr(pImage);
+    Assert(!(uOffset % VHD_SECTOR_SIZE));
+    Assert(!(cbToWrite % VHD_SECTOR_SIZE));
+    AssertPtrReturn(pIoCtx, VERR_INVALID_POINTER);
+    AssertReturn(cbToWrite, VERR_INVALID_PARAMETER);
+    AssertReturn(uOffset + cbToWrite <= RT_ALIGN_64(pImage->cbSize, pImage->cbDataBlock), VERR_INVALID_PARAMETER); /* The image size might not be on a data block size boundary. */
+
+    if (pImage->pBlockAllocationTable)
+    {
+        /*
+         * Get the data block first.
+         */
+        uint32_t cSector = uOffset / VHD_SECTOR_SIZE;
+        uint32_t cBlockAllocationTableEntry = cSector / pImage->cSectorsPerDataBlock;
+        uint32_t cBATEntryIndex = cSector % pImage->cSectorsPerDataBlock;
+        uint64_t uVhdOffset;
+
+        /*
+         * Clip write range.
+         */
+        cbToWrite = RT_MIN(cbToWrite, (pImage->cbDataBlock - (cBATEntryIndex * VHD_SECTOR_SIZE)));
+
+        /*
+         * If the block is not allocated the content of the entry is ~0
+         * and we need to allocate a new block. Note that while blocks are
+         * allocated with a relatively big granularity, each sector has its
+         * own bitmap entry, indicating whether it has been written or not.
+         * So that means for the purposes of the higher level that the
+         * granularity is invisible. This means there's no need to return
+         * VERR_VD_BLOCK_FREE unless the block hasn't been allocated yet.
+         */
+        if (pImage->pBlockAllocationTable[cBlockAllocationTableEntry] == ~0U)
+        {
+            /* Check if the block allocation should be suppressed. */
+            if (   (fWrite & VD_WRITE_NO_ALLOC)
+                || (cbToWrite != pImage->cbDataBlock))
+            {
+                *pcbPreRead = cBATEntryIndex * VHD_SECTOR_SIZE;
+                *pcbPostRead = pImage->cSectorsPerDataBlock * VHD_SECTOR_SIZE - cbToWrite - *pcbPreRead;
+
+                if (pcbWriteProcess)
+                    *pcbWriteProcess = cbToWrite;
+                return VERR_VD_BLOCK_FREE;
+            }
+
+            PVHDIMAGEEXPAND pExpand;
+            pExpand = (PVHDIMAGEEXPAND)RTMemAllocZ(RT_UOFFSETOF_DYN(VHDIMAGEEXPAND,
+                                                                    au8Bitmap[pImage->cDataBlockBitmapSectors * VHD_SECTOR_SIZE]));
+            bool fIoInProgress = false;
+
+            if (!pExpand)
+                return VERR_NO_MEMORY;
+
+            pExpand->cbEofOld = pImage->uCurrentEndOfFile;
+            pExpand->idxBatAllocated = cBlockAllocationTableEntry;
+            pExpand->idxBlockBe = RT_H2BE_U32(pImage->uCurrentEndOfFile / VHD_SECTOR_SIZE);
+
+            /* Set the bits for all sectors having been written. */
+            for (uint32_t iSector = 0; iSector < (cbToWrite / VHD_SECTOR_SIZE); iSector++)
+            {
+                /* No need to check for a changed value because this is an initial write. */
+                vhdBlockBitmapSectorSet(pImage, pExpand->au8Bitmap, cBATEntryIndex);
+                cBATEntryIndex++;
+            }
+
+            do
+            {
+                /*
+                 * Start with the sector bitmap.
+                 */
+                rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
+                                            pImage->uCurrentEndOfFile,
+                                            pExpand->au8Bitmap,
+                                            pImage->cDataBlockBitmapSectors * VHD_SECTOR_SIZE, pIoCtx,
+                                            vhdAsyncExpansionDataBlockBitmapComplete,
+                                            pExpand);
+                if (RT_SUCCESS(rc))
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_BLOCKBITMAP_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_SUCCESS);
+                else if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
+                    fIoInProgress = true;
+                else
+                {
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_BLOCKBITMAP_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_FAILED);
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_USERBLOCK_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_FAILED);
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_BAT_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_FAILED);
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_FOOTER_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_FAILED);
+                    break;
+                }
+
+
+                /*
+                 * Write the new block at the current end of the file.
+                 */
+                rc = vdIfIoIntFileWriteUser(pImage->pIfIo, pImage->pStorage,
+                                            pImage->uCurrentEndOfFile + (pImage->cDataBlockBitmapSectors + (cSector % pImage->cSectorsPerDataBlock)) * VHD_SECTOR_SIZE,
+                                            pIoCtx, cbToWrite,
+                                            vhdAsyncExpansionDataComplete,
+                                            pExpand);
+                if (RT_SUCCESS(rc))
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_USERBLOCK_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_SUCCESS);
+                else if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
+                    fIoInProgress = true;
+                else
+                {
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_USERBLOCK_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_FAILED);
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_BAT_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_FAILED);
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_FOOTER_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_FAILED);
+                    break;
+                }
+
+                /*
+                 * Write entry in the BAT.
+                 */
+                rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
+                                            pImage->uBlockAllocationTableOffset + cBlockAllocationTableEntry * sizeof(uint32_t),
+                                            &pExpand->idxBlockBe, sizeof(uint32_t), pIoCtx,
+                                            vhdAsyncExpansionBatUpdateComplete,
+                                            pExpand);
+                if (RT_SUCCESS(rc))
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_BAT_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_SUCCESS);
+                else if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
+                    fIoInProgress = true;
+                else
+                {
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_BAT_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_FAILED);
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_FOOTER_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_FAILED);
+                    break;
+                }
+
+                /*
+                 * Set the new end of the file and link the new block into the BAT.
+                 */
+                pImage->uCurrentEndOfFile += pImage->cDataBlockBitmapSectors * VHD_SECTOR_SIZE + pImage->cbDataBlock;
+
+                /* Update the footer. */
+                rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
+                                            pImage->uCurrentEndOfFile,
+                                            &pImage->vhdFooterCopy,
+                                            sizeof(VHDFooter), pIoCtx,
+                                            vhdAsyncExpansionFooterUpdateComplete,
+                                            pExpand);
+                if (RT_SUCCESS(rc))
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_FOOTER_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_SUCCESS);
+                else if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
+                    fIoInProgress = true;
+                else
+                {
+                    VHDIMAGEEXPAND_STATUS_SET(pExpand->fFlags, VHDIMAGEEXPAND_FOOTER_STATUS_SHIFT, VHDIMAGEEXPAND_STEP_FAILED);
+                    break;
+                }
+
+            } while (0);
+
+            if (!fIoInProgress)
+                vhdAsyncExpansionComplete(pImage, pIoCtx, pExpand);
+            else
+                rc = VERR_VD_ASYNC_IO_IN_PROGRESS;
+        }
+        else
+        {
+            /*
+             * Calculate the real offset in the file.
+             */
+            uVhdOffset = ((uint64_t)pImage->pBlockAllocationTable[cBlockAllocationTableEntry] + pImage->cDataBlockBitmapSectors + cBATEntryIndex) * VHD_SECTOR_SIZE;
+
+            /* Read in the block's bitmap. */
+            PVDMETAXFER pMetaXfer;
+            rc = vdIfIoIntFileReadMeta(pImage->pIfIo, pImage->pStorage,
+                                       ((uint64_t)pImage->pBlockAllocationTable[cBlockAllocationTableEntry]) * VHD_SECTOR_SIZE,
+                                       pImage->pu8Bitmap,
+                                       pImage->cbDataBlockBitmap, pIoCtx,
+                                       &pMetaXfer, NULL, NULL);
+            if (RT_SUCCESS(rc))
+            {
+                vdIfIoIntMetaXferRelease(pImage->pIfIo, pMetaXfer);
+
+                /* Write data. */
+                rc = vdIfIoIntFileWriteUser(pImage->pIfIo, pImage->pStorage,
+                                            uVhdOffset, pIoCtx, cbToWrite,
+                                            NULL, NULL);
+                if (RT_SUCCESS(rc) || rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
+                {
+                    bool fChanged = false;
+
+                    /* Set the bits for all sectors having been written. */
+                    for (uint32_t iSector = 0; iSector < (cbToWrite / VHD_SECTOR_SIZE); iSector++)
+                    {
+                        fChanged |= vhdBlockBitmapSectorSet(pImage, pImage->pu8Bitmap, cBATEntryIndex);
+                        cBATEntryIndex++;
+                    }
+
+                    /* Only write the bitmap if it was changed. */
+                    if (fChanged)
+                    {
+                        /*
+                         * Write the bitmap back.
+                         *
+                         * @note We don't have a completion callback here because we
+                         * can't do anything if the write fails for some reason.
+                         * The error will propagated to the device/guest
+                         * by the generic VD layer already and we don't need
+                         * to rollback anything here.
+                         */
+                        rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
+                                                    ((uint64_t)pImage->pBlockAllocationTable[cBlockAllocationTableEntry]) * VHD_SECTOR_SIZE,
+                                                     pImage->pu8Bitmap,
+                                                     pImage->cbDataBlockBitmap,
+                                                     pIoCtx, NULL, NULL);
+                    }
+                }
+            }
+        }
+    }
+    else
+        rc = vdIfIoIntFileWriteUser(pImage->pIfIo, pImage->pStorage,
+                                    uOffset, pIoCtx, cbToWrite, NULL, NULL);
+
+    if (pcbWriteProcess)
+        *pcbWriteProcess = cbToWrite;
+
+    /* Stay on the safe side. Do not run the risk of confusing the higher
+     * level, as that can be pretty lethal to image consistency. */
+    *pcbPreRead = 0;
+    *pcbPostRead = 0;
+
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnFlush} */
+static DECLCALLBACK(int) vhdFlush(void *pBackendData, PVDIOCTX pIoCtx)
+{
+    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    /* No need to write anything here. Data is always updated on a write. */
+    int rc = vdIfIoIntFileFlush(pImage->pIfIo, pImage->pStorage, pIoCtx, NULL, NULL);
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetVersion} */
+static DECLCALLBACK(unsigned) vhdGetVersion(void *pBackendData)
+{
+    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, 0);
+
+    unsigned uVersion = 1; /**< @todo use correct version */
+
+    LogFlowFunc(("returns %u\n", uVersion));
+    return uVersion;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetFileSize} */
+static DECLCALLBACK(uint64_t) vhdGetFileSize(void *pBackendData)
+{
+    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+    uint64_t cb = 0;
+
+    AssertPtrReturn(pImage, 0);
+
+    if (pImage->pStorage)
+        cb = pImage->uCurrentEndOfFile + sizeof(VHDFooter);
+
+    LogFlowFunc(("returns %lld\n", cb));
+    return cb;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetPCHSGeometry} */
+static DECLCALLBACK(int) vhdGetPCHSGeometry(void *pBackendData, PVDGEOMETRY pPCHSGeometry)
+{
+    LogFlowFunc(("pBackendData=%#p pPCHSGeometry=%#p\n", pBackendData, pPCHSGeometry));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+    int rc = VINF_SUCCESS;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    if (pImage->PCHSGeometry.cCylinders)
+        *pPCHSGeometry = pImage->PCHSGeometry;
+    else
+        rc = VERR_VD_GEOMETRY_NOT_SET;
+
+    LogFlowFunc(("returns %Rrc (CHS=%u/%u/%u)\n", rc, pImage->PCHSGeometry.cCylinders,
+                 pImage->PCHSGeometry.cHeads, pImage->PCHSGeometry.cSectors));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnSetPCHSGeometry} */
+static DECLCALLBACK(int) vhdSetPCHSGeometry(void *pBackendData, PCVDGEOMETRY pPCHSGeometry)
+{
+    LogFlowFunc(("pBackendData=%#p pPCHSGeometry=%#p PCHS=%u/%u/%u\n",
+                 pBackendData, pPCHSGeometry, pPCHSGeometry->cCylinders, pPCHSGeometry->cHeads, pPCHSGeometry->cSectors));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+    int rc = VINF_SUCCESS;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
+        rc = VERR_VD_IMAGE_READ_ONLY;
+    else
+        pImage->PCHSGeometry = *pPCHSGeometry;
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetLCHSGeometry} */
+static DECLCALLBACK(int) vhdGetLCHSGeometry(void *pBackendData, PVDGEOMETRY pLCHSGeometry)
+{
+    LogFlowFunc(("pBackendData=%#p pLCHSGeometry=%#p\n", pBackendData, pLCHSGeometry));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+    int rc = VINF_SUCCESS;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    if (pImage->LCHSGeometry.cCylinders)
+        *pLCHSGeometry = pImage->LCHSGeometry;
+    else
+        rc = VERR_VD_GEOMETRY_NOT_SET;
+
+    LogFlowFunc(("returns %Rrc (CHS=%u/%u/%u)\n", rc, pImage->LCHSGeometry.cCylinders,
+                 pImage->LCHSGeometry.cHeads, pImage->LCHSGeometry.cSectors));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnSetLCHSGeometry} */
+static DECLCALLBACK(int) vhdSetLCHSGeometry(void *pBackendData, PCVDGEOMETRY pLCHSGeometry)
+{
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+    int rc = VINF_SUCCESS;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
+        rc = VERR_VD_IMAGE_READ_ONLY;
+    else
+        pImage->LCHSGeometry = *pLCHSGeometry;
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @copydoc VDIMAGEBACKEND::pfnQueryRegions */
+static DECLCALLBACK(int) vhdQueryRegions(void *pBackendData, PCVDREGIONLIST *ppRegionList)
+{
+    LogFlowFunc(("pBackendData=%#p ppRegionList=%#p\n", pBackendData, ppRegionList));
+    PVHDIMAGE pThis = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pThis, VERR_VD_NOT_OPENED);
+
+    *ppRegionList = &pThis->RegionList;
+    LogFlowFunc(("returns %Rrc\n", VINF_SUCCESS));
+    return VINF_SUCCESS;
+}
+
+/** @copydoc VDIMAGEBACKEND::pfnRegionListRelease */
+static DECLCALLBACK(void) vhdRegionListRelease(void *pBackendData, PCVDREGIONLIST pRegionList)
+{
+    RT_NOREF1(pRegionList);
+    LogFlowFunc(("pBackendData=%#p pRegionList=%#p\n", pBackendData, pRegionList));
+    PVHDIMAGE pThis = (PVHDIMAGE)pBackendData;
+    AssertPtr(pThis); RT_NOREF(pThis);
+
+    /* Nothing to do here. */
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetImageFlags} */
+static DECLCALLBACK(unsigned) vhdGetImageFlags(void *pBackendData)
+{
+    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, 0);
+
+    LogFlowFunc(("returns %#x\n", pImage->uImageFlags));
+    return pImage->uImageFlags;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetOpenFlags} */
+static DECLCALLBACK(unsigned) vhdGetOpenFlags(void *pBackendData)
+{
+    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, 0);
+
+    LogFlowFunc(("returns %#x\n", pImage->uOpenFlags));
+    return pImage->uOpenFlags;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnSetOpenFlags} */
+static DECLCALLBACK(int) vhdSetOpenFlags(void *pBackendData, unsigned uOpenFlags)
+{
+    LogFlowFunc(("pBackendData=%#p\n uOpenFlags=%#x", pBackendData, uOpenFlags));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+    int rc = VINF_SUCCESS;
+
+    /* Image must be opened and the new flags must be valid. */
+    if (!pImage || (uOpenFlags & ~(  VD_OPEN_FLAGS_READONLY | VD_OPEN_FLAGS_INFO
+                                   | VD_OPEN_FLAGS_ASYNC_IO | VD_OPEN_FLAGS_SHAREABLE
+                                   | VD_OPEN_FLAGS_SEQUENTIAL | VD_OPEN_FLAGS_SKIP_CONSISTENCY_CHECKS)))
+        rc = VERR_INVALID_PARAMETER;
+    else
+    {
+        /* Implement this operation via reopening the image. */
+        rc = vhdFreeImage(pImage, false);
+        if (RT_SUCCESS(rc))
+            rc = vhdOpenImage(pImage, uOpenFlags);
+    }
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetComment} */
+static DECLCALLBACK(int) vhdGetComment(void *pBackendData, char *pszComment,
+                                       size_t cbComment)
+{
+    RT_NOREF2(pszComment, cbComment);
+    LogFlowFunc(("pBackendData=%#p pszComment=%#p cbComment=%zu\n", pBackendData, pszComment, cbComment));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    LogFlowFunc(("returns %Rrc comment='%s'\n", VERR_NOT_SUPPORTED, pszComment));
+    return VERR_NOT_SUPPORTED;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnSetComment} */
+static DECLCALLBACK(int) vhdSetComment(void *pBackendData, const char *pszComment)
+{
+    RT_NOREF1(pszComment);
+    LogFlowFunc(("pBackendData=%#p pszComment=\"%s\"\n", pBackendData, pszComment));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    int rc;
+    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
+        rc = VERR_VD_IMAGE_READ_ONLY;
+    else
+        rc = VERR_NOT_SUPPORTED;
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetUuid} */
+static DECLCALLBACK(int) vhdGetUuid(void *pBackendData, PRTUUID pUuid)
+{
+    LogFlowFunc(("pBackendData=%#p pUuid=%#p\n", pBackendData, pUuid));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    *pUuid = pImage->ImageUuid;
+
+    LogFlowFunc(("returns %Rrc (%RTuuid)\n", VINF_SUCCESS, pUuid));
+    return VINF_SUCCESS;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnSetUuid} */
+static DECLCALLBACK(int) vhdSetUuid(void *pBackendData, PCRTUUID pUuid)
+{
+    LogFlowFunc(("pBackendData=%#p Uuid=%RTuuid\n", pBackendData, pUuid));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+    int rc = VINF_SUCCESS;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    if (!(pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY))
+    {
+        pImage->ImageUuid = *pUuid;
+        /* Update the footer copy. It will get written to disk when the image is closed. */
+        memcpy(&pImage->vhdFooterCopy.UniqueID, pUuid, 16);
+        /* Update checksum. */
+        pImage->vhdFooterCopy.Checksum = 0;
+        pImage->vhdFooterCopy.Checksum = RT_H2BE_U32(vhdChecksum(&pImage->vhdFooterCopy, sizeof(VHDFooter)));
+
+        /* Need to update the dynamic disk header to update the disk footer copy at the beginning. */
+        if (!(pImage->uImageFlags & VD_IMAGE_FLAGS_FIXED))
+            pImage->fDynHdrNeedsUpdate = true;
+    }
+    else
+        rc = VERR_VD_IMAGE_READ_ONLY;
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetModificationUuid} */
+static DECLCALLBACK(int) vhdGetModificationUuid(void *pBackendData, PRTUUID pUuid)
+{
+    RT_NOREF1(pUuid);
+    LogFlowFunc(("pBackendData=%#p pUuid=%#p\n", pBackendData, pUuid));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    LogFlowFunc(("returns %Rrc (%RTuuid)\n", VERR_NOT_SUPPORTED, pUuid));
+    return VERR_NOT_SUPPORTED;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnSetModificationUuid} */
+static DECLCALLBACK(int) vhdSetModificationUuid(void *pBackendData, PCRTUUID pUuid)
+{
+    RT_NOREF1(pUuid);
+    LogFlowFunc(("pBackendData=%#p Uuid=%RTuuid\n", pBackendData, pUuid));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    int rc;
+    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
+        rc = VERR_VD_IMAGE_READ_ONLY;
+    else
+        rc = VERR_NOT_SUPPORTED;
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetParentUuid} */
+static DECLCALLBACK(int) vhdGetParentUuid(void *pBackendData, PRTUUID pUuid)
+{
+    LogFlowFunc(("pBackendData=%#p pUuid=%#p\n", pBackendData, pUuid));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    *pUuid = pImage->ParentUuid;
+
+    LogFlowFunc(("returns %Rrc (%RTuuid)\n", VINF_SUCCESS, pUuid));
+    return VINF_SUCCESS;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnSetParentUuid} */
+static DECLCALLBACK(int) vhdSetParentUuid(void *pBackendData, PCRTUUID pUuid)
+{
+    LogFlowFunc(("pBackendData=%#p Uuid=%RTuuid\n", pBackendData, pUuid));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+    int rc = VINF_SUCCESS;
+
+    if (pImage && pImage->pStorage)
+    {
+        if (!(pImage->uImageFlags & VD_IMAGE_FLAGS_FIXED))
+        {
+            pImage->ParentUuid = *pUuid;
+            pImage->fDynHdrNeedsUpdate = true;
+        }
+        else
+            rc = VERR_VD_IMAGE_READ_ONLY;
+    }
+    else
+        rc = VERR_VD_NOT_OPENED;
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetParentModificationUuid} */
+static DECLCALLBACK(int) vhdGetParentModificationUuid(void *pBackendData, PRTUUID pUuid)
+{
+    RT_NOREF1(pUuid);
+    LogFlowFunc(("pBackendData=%#p pUuid=%#p\n", pBackendData, pUuid));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    LogFlowFunc(("returns %Rrc (%RTuuid)\n", VERR_NOT_SUPPORTED, pUuid));
+    return VERR_NOT_SUPPORTED;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnSetParentModificationUuid} */
+static DECLCALLBACK(int) vhdSetParentModificationUuid(void *pBackendData, PCRTUUID pUuid)
+{
+    RT_NOREF1(pUuid);
+    LogFlowFunc(("pBackendData=%#p Uuid=%RTuuid\n", pBackendData, pUuid));
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    int rc;
+    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
+        rc = VERR_VD_IMAGE_READ_ONLY;
+    else
+        rc = VERR_NOT_SUPPORTED;
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnDump} */
+static DECLCALLBACK(void) vhdDump(void *pBackendData)
+{
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturnVoid(pImage);
+    vdIfErrorMessage(pImage->pIfError, "Header: Geometry PCHS=%u/%u/%u LCHS=%u/%u/%u cbSector=%u\n",
+                     pImage->PCHSGeometry.cCylinders, pImage->PCHSGeometry.cHeads, pImage->PCHSGeometry.cSectors,
+                     pImage->LCHSGeometry.cCylinders, pImage->LCHSGeometry.cHeads, pImage->LCHSGeometry.cSectors,
+                     VHD_SECTOR_SIZE);
+    vdIfErrorMessage(pImage->pIfError, "Header: uuidCreation={%RTuuid}\n", &pImage->ImageUuid);
+    vdIfErrorMessage(pImage->pIfError, "Header: uuidParent={%RTuuid}\n", &pImage->ParentUuid);
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetTimestamp} */
+static DECLCALLBACK(int) vhdGetTimestamp(void *pBackendData, PRTTIMESPEC pTimestamp)
+{
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    int rc = vdIfIoIntFileGetModificationTime(pImage->pIfIo, pImage->pszFilename, pTimestamp);
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetParentTimestamp} */
+static DECLCALLBACK(int) vhdGetParentTimestamp(void *pBackendData, PRTTIMESPEC pTimestamp)
+{
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    vhdTime2RtTime(pTimestamp, pImage->u32ParentTimestamp);
+    LogFlowFunc(("returns %Rrc\n", VINF_SUCCESS));
+    return VINF_SUCCESS;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnSetParentTimestamp} */
+static DECLCALLBACK(int) vhdSetParentTimestamp(void *pBackendData, PCRTTIMESPEC pTimestamp)
+{
+    int rc = VINF_SUCCESS;
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
+        rc = VERR_VD_IMAGE_READ_ONLY;
+    else
+    {
+        pImage->u32ParentTimestamp = vhdRtTime2VhdTime(pTimestamp);
+        pImage->fDynHdrNeedsUpdate = true;
+    }
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnGetParentFilename} */
+static DECLCALLBACK(int) vhdGetParentFilename(void *pBackendData, char **ppszParentFilename)
+{
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+    *ppszParentFilename = RTStrDup(pImage->pszParentFilename);
+
+    LogFlowFunc(("returns %Rrc\n", VINF_SUCCESS));
+    return VINF_SUCCESS;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnSetParentFilename} */
+static DECLCALLBACK(int) vhdSetParentFilename(void *pBackendData, const char *pszParentFilename)
+{
+    int rc = VINF_SUCCESS;
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+
+    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);
+
+    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
+        rc = VERR_VD_IMAGE_READ_ONLY;
+    else
+    {
+        if (pImage->pszParentFilename)
+            RTStrFree(pImage->pszParentFilename);
+        pImage->pszParentFilename = RTStrDup(pszParentFilename);
+        if (!pImage->pszParentFilename)
+            rc = VERR_NO_MEMORY;
+        else
+            pImage->fDynHdrNeedsUpdate = true;
+    }
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnCompact} */
+static DECLCALLBACK(int) vhdCompact(void *pBackendData, unsigned uPercentStart,
+                                    unsigned uPercentSpan, PVDINTERFACE pVDIfsDisk,
+                                    PVDINTERFACE pVDIfsImage, PVDINTERFACE pVDIfsOperation)
+{
+    RT_NOREF2(pVDIfsDisk, pVDIfsImage);
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+    int rc = VINF_SUCCESS;
+    void *pvBuf = NULL;
+    uint32_t *paBlocks = NULL;
+    PVDINTERFACEPROGRESS pIfProgress = VDIfProgressGet(pVDIfsOperation);
+
+    PFNVDPARENTREAD pfnParentRead = NULL;
+    void *pvParent = NULL;
+    PVDINTERFACEPARENTSTATE pIfParentState = VDIfParentStateGet(pVDIfsOperation);
+    if (pIfParentState)
+    {
+        pfnParentRead = pIfParentState->pfnParentRead;
+        pvParent = pIfParentState->Core.pvUser;
+    }
+
+    do
+    {
+        AssertBreakStmt(pImage, rc = VERR_INVALID_PARAMETER);
+
+        AssertBreakStmt(!(pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY),
+                        rc = VERR_VD_IMAGE_READ_ONLY);
+
+        /* Reject fixed images as they don't have a BAT. */
+        if (pImage->uImageFlags & VD_IMAGE_FLAGS_FIXED)
+        {
+            rc = VERR_NOT_SUPPORTED;
+            break;
+        }
+
+        if (pfnParentRead)
+        {
+            pvParent = RTMemTmpAlloc(pImage->cbDataBlock);
+            AssertBreakStmt(pvParent, rc = VERR_NO_MEMORY);
+        }
+        pvBuf = RTMemTmpAlloc(pImage->cbDataBlock);
+        AssertBreakStmt(pvBuf, rc = VERR_NO_MEMORY);
+
+        unsigned cBlocksAllocated = 0;
+        unsigned cBlocksToMove    = 0;
+        unsigned cBlocks          = pImage->cBlockAllocationTableEntries;
+        uint32_t offBlocksStart   = ~0U; /* Start offset of data blocks in sectors. */
+        uint32_t *paBat           = pImage->pBlockAllocationTable;
+
+        /* Count the number of allocated blocks and find the start offset for the data blocks. */
+        for (unsigned i = 0; i < cBlocks; i++)
+            if (paBat[i] != ~0U)
+            {
+                cBlocksAllocated++;
+                if (paBat[i] < offBlocksStart)
+                    offBlocksStart = paBat[i];
+            }
+
+        if (!cBlocksAllocated)
+        {
+            /* Nothing to do. */
+            rc = VINF_SUCCESS;
+            break;
+        }
+
+        paBlocks = (uint32_t *)RTMemTmpAllocZ(cBlocksAllocated * sizeof(uint32_t));
+        AssertBreakStmt(paBlocks, rc = VERR_NO_MEMORY);
+
+        /* Invalidate the back resolving array. */
+        for (unsigned i = 0; i < cBlocksAllocated; i++)
+            paBlocks[i] = ~0U;
+
+        /* Fill the back resolving table. */
+        for (unsigned i = 0; i < cBlocks; i++)
+            if (paBat[i] != ~0U)
+            {
+                unsigned idxBlock = (paBat[i] - offBlocksStart) / pImage->cSectorsPerDataBlock;
+                if (   idxBlock < cBlocksAllocated
+                    && paBlocks[idxBlock] == ~0U)
+                    paBlocks[idxBlock] = i;
+                else
+                {
+                    /* The image is in an inconsistent state. Don't go further. */
+                    rc = VERR_INVALID_STATE;
+                    break;
+                }
+            }
+
+        if (RT_FAILURE(rc))
+            break;
+
+        /* Find redundant information and update the block pointers
+         * accordingly, creating bubbles. Keep disk up to date, as this
+         * enables cancelling. */
+        for (unsigned i = 0; i < cBlocks; i++)
+        {
+            if (paBat[i] != ~0U)
+            {
+                unsigned idxBlock = (paBat[i] - offBlocksStart) / pImage->cSectorsPerDataBlock;
+
+                /* Block present in image file, read relevant data. */
+                uint64_t u64Offset = ((uint64_t)paBat[i] + pImage->cDataBlockBitmapSectors) * VHD_SECTOR_SIZE;
+                rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage,
+                                           u64Offset, pvBuf, pImage->cbDataBlock);
+                if (RT_FAILURE(rc))
+                    break;
+
+                if (ASMBitFirstSet((volatile void *)pvBuf, (uint32_t)pImage->cbDataBlock * 8) == -1)
+                {
+                    paBat[i] = UINT32_MAX;
+                    paBlocks[idxBlock] = ~0U;
+                    /* Adjust progress info, one block to be relocated. */
+                    cBlocksToMove++;
+                }
+                else if (pfnParentRead)
+                {
+                    rc = pfnParentRead(pvParent, (uint64_t)i * pImage->cbDataBlock, pvParent, pImage->cbDataBlock);
+                    if (RT_FAILURE(rc))
+                        break;
+                    if (!memcmp(pvParent, pvBuf, pImage->cbDataBlock))
+                    {
+                        paBat[i] = ~0U;
+                        paBlocks[idxBlock] = ~0U;
+                        /* Adjust progress info, one block to be relocated. */
+                        cBlocksToMove++;
+                    }
+                }
+            }
+
+            vdIfProgress(pIfProgress, (uint64_t)i * uPercentSpan / (cBlocks + cBlocksToMove) + uPercentStart);
+        }
+
+        if (RT_SUCCESS(rc))
+        {
+            /* Fill bubbles with other data (if available). */
+            unsigned cBlocksMoved = 0;
+            unsigned uBlockUsedPos = cBlocksAllocated;
+            size_t   cbBlock = pImage->cbDataBlock + pImage->cbDataBlockBitmap; /** < Size of whole block containing the bitmap and the user data. */
+
+            /* Allocate data buffer to hold the data block and allocation bitmap in front of the actual data. */
+            RTMemTmpFree(pvBuf);
+            pvBuf = RTMemTmpAllocZ(cbBlock);
+            AssertBreakStmt(pvBuf, rc = VERR_NO_MEMORY);
+
+            for (unsigned i = 0; i < cBlocksAllocated; i++)
+            {
+                unsigned uBlock = paBlocks[i];
+                if (uBlock == ~0U)
+                {
+                    unsigned uBlockData = ~0U;
+                    while (uBlockUsedPos > i && uBlockData == ~0U)
+                    {
+                        uBlockUsedPos--;
+                        uBlockData = paBlocks[uBlockUsedPos];
+                    }
+                    /* Terminate early if there is no block which needs copying. */
+                    if (uBlockUsedPos == i)
+                        break;
+                    uint64_t u64Offset = (uint64_t)uBlockUsedPos * cbBlock
+                                       + (offBlocksStart * VHD_SECTOR_SIZE);
+                    rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage,
+                                               u64Offset, pvBuf, cbBlock);
+                    if (RT_FAILURE(rc))
+                        break;
+
+                    u64Offset = (uint64_t)i * cbBlock
+                                       + (offBlocksStart * VHD_SECTOR_SIZE);
+                    rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage,
+                                                u64Offset, pvBuf, cbBlock);
+                    if (RT_FAILURE(rc))
+                        break;
+
+                    paBat[uBlockData] = i*(pImage->cSectorsPerDataBlock + pImage->cDataBlockBitmapSectors) + offBlocksStart;
+
+                    /* Truncate the file but leave enough room for the footer to avoid
+                     * races if other processes fill the whole harddisk. */
+                    rc = vdIfIoIntFileSetSize(pImage->pIfIo, pImage->pStorage,
+                                              pImage->uCurrentEndOfFile - cbBlock + VHD_SECTOR_SIZE);
+                    if (RT_FAILURE(rc))
+                        break;
+
+                    /* Update pointers and write footer. */
+                    pImage->uCurrentEndOfFile -= cbBlock;
+
+                    /* We're kinda screwed if this failes. */
+                    rc = vhdUpdateFooter(pImage);
+                    if (RT_FAILURE(rc))
+                        break;
+
+                    paBlocks[i] = uBlockData;
+                    paBlocks[uBlockUsedPos] = ~0U;
+                    cBlocksMoved++;
+                }
+
+                rc = vdIfProgress(pIfProgress, (uint64_t)(cBlocks + cBlocksMoved) * uPercentSpan / (cBlocks + cBlocksToMove) + uPercentStart);
+            }
+        }
+
+        /* Write the new BAT in any case. */
+        rc = vhdFlushImage(pImage);
+    } while (0);
+
+    if (paBlocks)
+        RTMemTmpFree(paBlocks);
+    if (pvParent)
+        RTMemTmpFree(pvParent);
+    if (pvBuf)
+        RTMemTmpFree(pvBuf);
+
+    if (RT_SUCCESS(rc))
+        vdIfProgress(pIfProgress, uPercentStart + uPercentSpan);
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnResize} */
+static DECLCALLBACK(int) vhdResize(void *pBackendData, uint64_t cbSize,
+                                   PCVDGEOMETRY pPCHSGeometry, PCVDGEOMETRY pLCHSGeometry,
+                                   unsigned uPercentStart, unsigned uPercentSpan,
+                                   PVDINTERFACE pVDIfsDisk, PVDINTERFACE pVDIfsImage,
+                                   PVDINTERFACE pVDIfsOperation)
+{
+    RT_NOREF5(uPercentSpan, uPercentStart, pVDIfsDisk, pVDIfsImage, pVDIfsOperation);
+    PVHDIMAGE pImage = (PVHDIMAGE)pBackendData;
+    int rc = VINF_SUCCESS;
+
+    /* Making the image smaller is not supported at the moment. */
+    if (cbSize < pImage->cbSize)
+        rc = VERR_VD_SHRINK_NOT_SUPPORTED;
+    else if (pImage->uImageFlags & VD_IMAGE_FLAGS_FIXED)
+        rc = VERR_NOT_SUPPORTED;
+    else if (cbSize > pImage->cbSize)
+    {
+        unsigned cBlocksAllocated = 0;
+        size_t cbBlock = pImage->cbDataBlock + pImage->cbDataBlockBitmap;     /** < Size of a block including the sector bitmap. */
+        uint32_t cBlocksNew = cbSize / pImage->cbDataBlock;                   /** < New number of blocks in the image after the resize */
+        if (cbSize % pImage->cbDataBlock)
+            cBlocksNew++;
+
+        uint32_t cBlocksOld      = pImage->cBlockAllocationTableEntries;      /** < Number of blocks before the resize. */
+        uint64_t cbBlockspaceNew = RT_ALIGN_32(cBlocksNew * sizeof(uint32_t), VHD_SECTOR_SIZE);                         /** < Required space for the block array after the resize. */
+        uint64_t offStartDataNew = RT_ALIGN_32(pImage->uBlockAllocationTableOffset + cbBlockspaceNew, VHD_SECTOR_SIZE); /** < New start offset for block data after the resize */
+        uint64_t offStartDataOld = ~0ULL;
+
+        /* Go through the BAT and find the data start offset. */
+        for (unsigned idxBlock = 0; idxBlock < pImage->cBlockAllocationTableEntries; idxBlock++)
+        {
+            if (pImage->pBlockAllocationTable[idxBlock] != ~0U)
+            {
+                uint64_t offStartBlock = (uint64_t)pImage->pBlockAllocationTable[idxBlock] * VHD_SECTOR_SIZE;
+                if (offStartBlock < offStartDataOld)
+                    offStartDataOld = offStartBlock;
+                cBlocksAllocated++;
+            }
+        }
+
+        if (   offStartDataOld != offStartDataNew
+            && cBlocksAllocated > 0)
+        {
+            /* Calculate how many sectors nee to be relocated. */
+            uint64_t cbOverlapping = offStartDataNew - offStartDataOld;
+            unsigned cBlocksReloc = (unsigned)(cbOverlapping / cbBlock);
+            if (cbOverlapping % cbBlock)
+                cBlocksReloc++;
+
+            cBlocksReloc = RT_MIN(cBlocksReloc, cBlocksAllocated);
+            offStartDataNew = offStartDataOld;
+
+            /* Do the relocation. */
+            LogFlow(("Relocating %u blocks\n", cBlocksReloc));
+
+            /*
+             * Get the blocks we need to relocate first, they are appended to the end
+             * of the image.
+             */
+            void *pvBuf = NULL, *pvZero = NULL;
+            do
+            {
+                /* Allocate data buffer. */
+                pvBuf = RTMemAllocZ(cbBlock);
+                if (!pvBuf)
+                {
+                    rc = VERR_NO_MEMORY;
+                    break;
+                }
+
+                /* Allocate buffer for overwriting with zeroes. */
+                pvZero = RTMemAllocZ(cbBlock);
+                if (!pvZero)
+                {
+                    rc = VERR_NO_MEMORY;
+                    break;
+                }
+
+                for (unsigned i = 0; i < cBlocksReloc; i++)
+                {
+                    uint32_t uBlock = offStartDataNew / VHD_SECTOR_SIZE;
+
+                    /* Search the index in the block table. */
+                    for (unsigned idxBlock = 0; idxBlock < cBlocksOld; idxBlock++)
+                    {
+                        if (pImage->pBlockAllocationTable[idxBlock] == uBlock)
+                        {
+                            /* Read data and append to the end of the image. */
+                            rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage,
+                                                       offStartDataNew, pvBuf, cbBlock);
+                            if (RT_FAILURE(rc))
+                                break;
+
+                            rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage,
+                                                        pImage->uCurrentEndOfFile, pvBuf, cbBlock);
+                            if (RT_FAILURE(rc))
+                                break;
+
+                            /* Zero out the old block area. */
+                            rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage,
+                                                        offStartDataNew, pvZero, cbBlock);
+                            if (RT_FAILURE(rc))
+                                break;
+
+                            /* Update block counter. */
+                            pImage->pBlockAllocationTable[idxBlock] = pImage->uCurrentEndOfFile / VHD_SECTOR_SIZE;
+
+                            pImage->uCurrentEndOfFile += cbBlock;
+
+                            /* Continue with the next block. */
+                            break;
+                        }
+                    }
+
+                    if (RT_FAILURE(rc))
+                        break;
+
+                    offStartDataNew += cbBlock;
+                }
+            } while (0);
+
+            if (pvBuf)
+                RTMemFree(pvBuf);
+            if (pvZero)
+                RTMemFree(pvZero);
+        }
+
+        /*
+         * Relocation done, expand the block array and update the header with
+         * the new data.
+         */
+        if (RT_SUCCESS(rc))
+        {
+            uint32_t *paBlocksNew = (uint32_t *)RTMemRealloc(pImage->pBlockAllocationTable, cBlocksNew * sizeof(uint32_t));
+            if (paBlocksNew)
+            {
+                pImage->pBlockAllocationTable = paBlocksNew;
+
+                /* Mark the new blocks as unallocated. */
+                for (unsigned idxBlock = cBlocksOld; idxBlock < cBlocksNew; idxBlock++)
+                    pImage->pBlockAllocationTable[idxBlock] = ~0U;
+            }
+            else
+                rc = VERR_NO_MEMORY;
+
+            if (RT_SUCCESS(rc))
+            {
+                /* Write the block array before updating the rest. */
+                rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage,
+                                            pImage->uBlockAllocationTableOffset,
+                                            pImage->pBlockAllocationTable,
+                                            cBlocksNew * sizeof(uint32_t));
+            }
+
+            if (RT_SUCCESS(rc))
+            {
+                /* Update size and new block count. */
+                pImage->cBlockAllocationTableEntries = cBlocksNew;
+                pImage->cbSize = cbSize;
+
+                /* Update geometry. */
+                pImage->PCHSGeometry = *pPCHSGeometry;
+                pImage->LCHSGeometry = *pLCHSGeometry;
+            }
+        }
+
+        /* Update header information in base image file. */
+        pImage->fDynHdrNeedsUpdate = true;
+        vhdFlushImage(pImage);
+    }
+    /* Same size doesn't change the image at all. */
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+/** @interface_method_impl{VDIMAGEBACKEND,pfnRepair} */
+static DECLCALLBACK(int) vhdRepair(const char *pszFilename, PVDINTERFACE pVDIfsDisk,
+                                   PVDINTERFACE pVDIfsImage, uint32_t fFlags)
+{
+    LogFlowFunc(("pszFilename=\"%s\" pVDIfsDisk=%#p pVDIfsImage=%#p\n", pszFilename, pVDIfsDisk, pVDIfsImage));
+    int rc;
+    PVDINTERFACEERROR pIfError;
+    PVDINTERFACEIOINT pIfIo;
+    PVDIOSTORAGE pStorage = NULL;
+    uint64_t cbFile;
+    VHDFooter vhdFooter;
+    VHDDynamicDiskHeader dynamicDiskHeader;
+    uint32_t *paBat = NULL;
+    uint32_t *pu32BlockBitmap = NULL;
+
+    pIfIo = VDIfIoIntGet(pVDIfsImage);
+    AssertPtrReturn(pIfIo, VERR_INVALID_PARAMETER);
+
+    pIfError = VDIfErrorGet(pVDIfsDisk);
+
+    do
+    {
+        uint64_t offDynamicDiskHeader = 0;
+        uint64_t offBat = 0;
+        uint64_t offFooter = 0;
+        uint32_t cBatEntries = 0;
+        bool fDynamic = false;
+        bool fRepairFooter = false;
+        bool fRepairBat = false;
+        bool fRepairDynHeader = false;
+
+        rc = vdIfIoIntFileOpen(pIfIo, pszFilename,
+                               VDOpenFlagsToFileOpenFlags(  fFlags & VD_REPAIR_DRY_RUN
+                                                          ? VD_OPEN_FLAGS_READONLY
+                                                          : 0,
+                                                          false /* fCreate */),
+                               &pStorage);
+        if (RT_FAILURE(rc))
+        {
+            rc = vdIfError(pIfError, rc, RT_SRC_POS, "Failed to open image \"%s\"", pszFilename);
+            break;
+        }
+
+        rc = vdIfIoIntFileGetSize(pIfIo, pStorage, &cbFile);
+        if (RT_FAILURE(rc))
+        {
+            rc = vdIfError(pIfError, rc, RT_SRC_POS, "Failed to query image size");
+            break;
+        }
+
+        if (cbFile < sizeof(VHDFooter))
+        {
+            rc = vdIfError(pIfError, VERR_VD_INVALID_SIZE, RT_SRC_POS,
+                           "Image must be at least %u bytes (got %llu)",
+                           sizeof(VHDFooter), cbFile);
+            break;
+        }
+
+        rc = vdIfIoIntFileReadSync(pIfIo, pStorage, cbFile - sizeof(VHDFooter),
+                                   &vhdFooter, sizeof(VHDFooter));
+        if (RT_FAILURE(rc))
+        {
+            rc = vdIfError(pIfError, rc, RT_SRC_POS, "Failed to read footer of image");
+            break;
+        }
+
+        if (memcmp(vhdFooter.Cookie, VHD_FOOTER_COOKIE, VHD_FOOTER_COOKIE_SIZE) != 0)
+        {
+            /* Dynamic images have a backup at the beginning of the image. */
+            rc = vdIfIoIntFileReadSync(pIfIo, pStorage, 0,
+                                       &vhdFooter, sizeof(VHDFooter));
+            if (RT_FAILURE(rc))
+            {
+                rc = vdIfError(pIfError, rc, RT_SRC_POS, "Failed to read header of image");
+                break;
+            }
+
+            /*
+             * Check for the header, if this fails the image is either completely corrupted
+             * and impossible to repair or in another format.
+             */
+            if (memcmp(vhdFooter.Cookie, VHD_FOOTER_COOKIE, VHD_FOOTER_COOKIE_SIZE) != 0)
+            {
+                rc = vdIfError(pIfError, VERR_VD_IMAGE_REPAIR_IMPOSSIBLE, RT_SRC_POS,
+                               "No valid VHD structures found");
+                break;
+            }
+            else
+                vdIfErrorMessage(pIfError, "Missing footer structure, using backup\n");
+
+            /* Remember to fix the footer structure. */
+            fRepairFooter = true;
+        }
+
+        offFooter = cbFile - sizeof(VHDFooter);
+
+        /* Verify that checksums match. */
+        uint32_t u32ChkSumOld = RT_BE2H_U32(vhdFooter.Checksum);
+        vhdFooter.Checksum = 0;
+        uint32_t u32ChkSum = vhdChecksum(&vhdFooter, sizeof(VHDFooter));
+
+        vhdFooter.Checksum = RT_H2BE_U32(u32ChkSum);
+
+        if (u32ChkSumOld != u32ChkSum)
+        {
+            vdIfErrorMessage(pIfError, "Checksum is invalid (should be %u got %u), repairing\n",
+                             u32ChkSum, u32ChkSumOld);
+            fRepairFooter = true;
+            break;
+        }
+
+        switch (RT_BE2H_U32(vhdFooter.DiskType))
+        {
+            case VHD_FOOTER_DISK_TYPE_FIXED:
+                fDynamic = false;
+                break;
+            case VHD_FOOTER_DISK_TYPE_DYNAMIC:
+                fDynamic = true;
+                break;
+            case VHD_FOOTER_DISK_TYPE_DIFFERENCING:
+                fDynamic = true;
+                break;
+            default:
+            {
+                rc = vdIfError(pIfError, VERR_VD_IMAGE_REPAIR_IMPOSSIBLE, RT_SRC_POS,
+                               "VHD image type %u is not supported",
+                               RT_BE2H_U32(vhdFooter.DiskType));
+                break;
+            }
+        }
+
+        /* Load and check dynamic disk header if required. */
+        if (fDynamic)
+        {
+            size_t cbBlock;
+
+            offDynamicDiskHeader = RT_BE2H_U64(vhdFooter.DataOffset);
+            if (offDynamicDiskHeader + sizeof(VHDDynamicDiskHeader) > cbFile)
+            {
+                rc = vdIfError(pIfError, VERR_VD_IMAGE_REPAIR_IMPOSSIBLE, RT_SRC_POS,
+                               "VHD image type is not supported");
+                break;
+            }
+
+            rc = vdIfIoIntFileReadSync(pIfIo, pStorage, offDynamicDiskHeader,
+                                       &dynamicDiskHeader, sizeof(VHDDynamicDiskHeader));
+            if (RT_FAILURE(rc))
+            {
+                rc = vdIfError(pIfError, VERR_VD_IMAGE_REPAIR_IMPOSSIBLE, RT_SRC_POS,
+                               "Failed to read dynamic disk header (at %llu), %Rrc",
+                               offDynamicDiskHeader, rc);
+                break;
+            }
+
+            /* Verify that checksums match. */
+            u32ChkSumOld = RT_BE2H_U32(dynamicDiskHeader.Checksum);
+            dynamicDiskHeader.Checksum = 0;
+            u32ChkSum = vhdChecksum(&dynamicDiskHeader, sizeof(VHDDynamicDiskHeader));
+
+            dynamicDiskHeader.Checksum = RT_H2BE_U32(u32ChkSum);
+
+            if (u32ChkSumOld != u32ChkSum)
+            {
+                vdIfErrorMessage(pIfError, "Checksum of dynamic disk header is invalid (should be %u got %u), repairing\n",
+                                 u32ChkSum, u32ChkSumOld);
+                fRepairDynHeader = true;
+                break;
+            }
+
+            /* Read the block allocation table and fix any inconsistencies. */
+            offBat = RT_BE2H_U64(dynamicDiskHeader.TableOffset);
+            cBatEntries = RT_BE2H_U32(dynamicDiskHeader.MaxTableEntries);
+            cbBlock = RT_BE2H_U32(dynamicDiskHeader.BlockSize);
+            cbBlock += cbBlock / VHD_SECTOR_SIZE / 8;
+
+            if (offBat + cBatEntries * sizeof(uint32_t) > cbFile)
+            {
+                rc = vdIfError(pIfError, VERR_VD_IMAGE_REPAIR_IMPOSSIBLE, RT_SRC_POS,
+                               "Block allocation table is not inside the image");
+                break;
+            }
+
+            paBat = (uint32_t *)RTMemAllocZ(cBatEntries * sizeof(uint32_t));
+            if (!paBat)
+            {
+                rc = vdIfError(pIfError, VERR_VD_IMAGE_REPAIR_IMPOSSIBLE, RT_SRC_POS,
+                               "Could not allocate memory for the block allocation table (%u bytes)",
+                               cBatEntries * sizeof(uint32_t));
+                break;
+            }
+
+            rc = vdIfIoIntFileReadSync(pIfIo, pStorage, offBat, paBat,
+                                       cBatEntries * sizeof(uint32_t));
+            if (RT_FAILURE(rc))
+            {
+                rc = vdIfError(pIfError, VERR_VD_IMAGE_REPAIR_IMPOSSIBLE, RT_SRC_POS,
+                               "Could not read block allocation table (at %llu), %Rrc",
+                               offBat, rc);
+                break;
+            }
+
+            pu32BlockBitmap = (uint32_t *)RTMemAllocZ(RT_ALIGN_Z(cBatEntries / 8, 4));
+            if (!pu32BlockBitmap)
+            {
+                rc = vdIfError(pIfError, VERR_NO_MEMORY, RT_SRC_POS,
+                               "Failed to allocate memory for block bitmap");
+                break;
+            }
+
+            uint32_t idxMinBlock = UINT32_C(0xffffffff);
+            for (uint32_t i = 0; i < cBatEntries; i++)
+            {
+                paBat[i] = RT_BE2H_U32(paBat[i]);
+                if (paBat[i] < idxMinBlock)
+                    idxMinBlock = paBat[i];
+            }
+
+            vdIfErrorMessage(pIfError, "First data block at sector %u\n", idxMinBlock);
+
+            for (uint32_t i = 0; i < cBatEntries; i++)
+            {
+                if (paBat[i] != UINT32_C(0xffffffff))
+                {
+                    uint64_t offBlock =(uint64_t)paBat[i] * VHD_SECTOR_SIZE;
+
+                    /*
+                     * Check that the offsets are valid (inside of the image) and
+                     * that there are no double references.
+                     */
+                    if (offBlock + cbBlock > cbFile)
+                    {
+                        vdIfErrorMessage(pIfError, "Entry %u points to invalid offset %llu, clearing\n",
+                                         i, offBlock);
+                        paBat[i] = UINT32_C(0xffffffff);
+                        fRepairBat = true;
+                    }
+                    else if (offBlock + cbBlock > offFooter)
+                    {
+                        vdIfErrorMessage(pIfError, "Entry %u intersects with footer, aligning footer\n",
+                                         i);
+                        offFooter = offBlock + cbBlock;
+                        fRepairBat = true;
+                    }
+
+                    if (   paBat[i] != UINT32_C(0xffffffff)
+                        && ASMBitTestAndSet(pu32BlockBitmap, (uint32_t)((paBat[i] - idxMinBlock) / (cbBlock / VHD_SECTOR_SIZE))))
+                    {
+                        vdIfErrorMessage(pIfError, "Entry %u points to an already referenced data block, clearing\n",
+                                         i);
+                        paBat[i] = UINT32_C(0xffffffff);
+                        fRepairBat = true;
+                    }
+                }
+            }
+        }
+
+        /* Write repaired structures now. */
+        if (!(fRepairBat || fRepairDynHeader || fRepairFooter))
+            vdIfErrorMessage(pIfError, "VHD image is in a consistent state, no repair required\n");
+        else if (!(fFlags & VD_REPAIR_DRY_RUN))
+        {
+            if (fRepairBat)
+            {
+                for (uint32_t i = 0; i < cBatEntries; i++)
+                    paBat[i] = RT_H2BE_U32(paBat[i]);
+
+                vdIfErrorMessage(pIfError, "Writing repaired block allocation table...\n");
+
+                rc = vdIfIoIntFileWriteSync(pIfIo, pStorage, offBat, paBat,
+                                            cBatEntries * sizeof(uint32_t));
+                if (RT_FAILURE(rc))
+                {
+                    rc = vdIfError(pIfError, VERR_VD_IMAGE_REPAIR_IMPOSSIBLE, RT_SRC_POS,
+                                   "Could not write repaired block allocation table (at %llu), %Rrc",
+                                   offBat, rc);
+                    break;
+                }
+            }
+
+            if (fRepairDynHeader)
+            {
+                Assert(fDynamic);
+
+                vdIfErrorMessage(pIfError, "Writing repaired dynamic disk header...\n");
+                rc = vdIfIoIntFileWriteSync(pIfIo, pStorage, offDynamicDiskHeader, &dynamicDiskHeader,
+                                            sizeof(VHDDynamicDiskHeader));
+                if (RT_FAILURE(rc))
+                {
+                    rc = vdIfError(pIfError, VERR_VD_IMAGE_REPAIR_IMPOSSIBLE, RT_SRC_POS,
+                                   "Could not write repaired dynamic disk header (at %llu), %Rrc",
+                                   offDynamicDiskHeader, rc);
+                    break;
+                }
+            }
+
+            if (fRepairFooter)
+            {
+                vdIfErrorMessage(pIfError, "Writing repaired Footer...\n");
+
+                if (fDynamic)
+                {
+                    /* Write backup at image beginning. */
+                    rc = vdIfIoIntFileWriteSync(pIfIo, pStorage, 0, &vhdFooter,
+                                                sizeof(VHDFooter));
+                    if (RT_FAILURE(rc))
+                    {
+                        rc = vdIfError(pIfError, VERR_VD_IMAGE_REPAIR_IMPOSSIBLE, RT_SRC_POS,
+                                       "Could not write repaired backup footer (at %llu), %Rrc",
+                                       0, rc);
+                        break;
+                    }
+                }
+
+                rc = vdIfIoIntFileWriteSync(pIfIo, pStorage, offFooter, &vhdFooter,
+                                            sizeof(VHDFooter));
+                if (RT_FAILURE(rc))
+                {
+                    rc = vdIfError(pIfError, VERR_VD_IMAGE_REPAIR_IMPOSSIBLE, RT_SRC_POS,
+                                   "Could not write repaired footer (at %llu), %Rrc",
+                                   cbFile - sizeof(VHDFooter), rc);
+                    break;
+                }
+            }
+
+            vdIfErrorMessage(pIfError, "Corrupted VHD image repaired successfully\n");
+        }
+    } while(0);
+
+    if (paBat)
+        RTMemFree(paBat);
+
+    if (pu32BlockBitmap)
+        RTMemFree(pu32BlockBitmap);
+
+    if (pStorage)
+    {
+        int rc2 = vdIfIoIntFileClose(pIfIo, pStorage);
+        if (RT_SUCCESS(rc))
+            rc = rc2; /* Propagate status code only when repairing the image was successful. */
+    }
+
+    LogFlowFunc(("returns %Rrc\n", rc));
+    return rc;
+}
+
+
+const VDIMAGEBACKEND g_VhdBackend =
+{
+    /* u32Version */
+    VD_IMGBACKEND_VERSION,
+    /* pszBackendName */
+    "VHD",
+    /* uBackendCaps */
+    VD_CAP_UUID | VD_CAP_DIFF | VD_CAP_FILE |
+    VD_CAP_CREATE_FIXED | VD_CAP_CREATE_DYNAMIC |
+    VD_CAP_ASYNC | VD_CAP_VFS | VD_CAP_PREFERRED,
+    /* paFileExtensions */
+    s_aVhdFileExtensions,
+    /* paConfigInfo */
+    NULL,
+    /* pfnProbe */
+    vhdProbe,
+    /* pfnOpen */
+    vhdOpen,
+    /* pfnCreate */
+    vhdCreate,
+    /* pfnRename */
+    vhdRename,
+    /* pfnClose */
+    vhdClose,
+    /* pfnRead */
+    vhdRead,
+    /* pfnWrite */
+    vhdWrite,
+    /* pfnFlush */
+    vhdFlush,
+    /* pfnDiscard */
+    NULL,
+    /* pfnGetVersion */
+    vhdGetVersion,
+    /* pfnGetFileSize */
+    vhdGetFileSize,
+    /* pfnGetPCHSGeometry */
+    vhdGetPCHSGeometry,
+    /* pfnSetPCHSGeometry */
+    vhdSetPCHSGeometry,
+    /* pfnGetLCHSGeometry */
+    vhdGetLCHSGeometry,
+    /* pfnSetLCHSGeometry */
+    vhdSetLCHSGeometry,
+    /* pfnQueryRegions */
+    vhdQueryRegions,
+    /* pfnRegionListRelease */
+    vhdRegionListRelease,
+    /* pfnGetImageFlags */
+    vhdGetImageFlags,
+    /* pfnGetOpenFlags */
+    vhdGetOpenFlags,
+    /* pfnSetOpenFlags */
+    vhdSetOpenFlags,
+    /* pfnGetComment */
+    vhdGetComment,
+    /* pfnSetComment */
+    vhdSetComment,
+    /* pfnGetUuid */
+    vhdGetUuid,
+    /* pfnSetUuid */
+    vhdSetUuid,
+    /* pfnGetModificationUuid */
+    vhdGetModificationUuid,
+    /* pfnSetModificationUuid */
+    vhdSetModificationUuid,
+    /* pfnGetParentUuid */
+    vhdGetParentUuid,
+    /* pfnSetParentUuid */
+    vhdSetParentUuid,
+    /* pfnGetParentModificationUuid */
+    vhdGetParentModificationUuid,
+    /* pfnSetParentModificationUuid */
+    vhdSetParentModificationUuid,
+    /* pfnDump */
+    vhdDump,
+    /* pfnGetTimestamp */
+    vhdGetTimestamp,
+    /* pfnGetParentTimestamp */
+    vhdGetParentTimestamp,
+    /* pfnSetParentTimestamp */
+    vhdSetParentTimestamp,
+    /* pfnGetParentFilename */
+    vhdGetParentFilename,
+    /* pfnSetParentFilename */
+    vhdSetParentFilename,
+    /* pfnComposeLocation */
+    genericFileComposeLocation,
+    /* pfnComposeName */
+    genericFileComposeName,
+    /* pfnCompact */
+    vhdCompact,
+    /* pfnResize */
+    vhdResize,
+    /* pfnRepair */
+    vhdRepair,
+    /* pfnTraverseMetadata */
+    NULL,
+    /* u32VersionEnd */
+    VD_IMGBACKEND_VERSION
+};