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Re: [Qemu-devel] [PATCH v10] Support vhd type VHD_DIFFERENCING


From: John Baboval
Subject: Re: [Qemu-devel] [PATCH v10] Support vhd type VHD_DIFFERENCING
Date: Thu, 16 Apr 2015 14:26:13 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

Thank you for this patch.

I found a few places where you have 32-bit integer overflows that result in failure. They are marked inline below.

On 03/04/2015 09:18 AM, Xiaodong Gong wrote:
diff --git a/block/vpc.c b/block/vpc.c
index 46803b1..d9a8d19 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -29,17 +29,29 @@
  #if defined(CONFIG_UUID)
  #include <uuid/uuid.h>
  #endif
+#include <glib.h>
/**************************************************************/ #define HEADER_SIZE 512
+#define DYNAMIC_HEADER_SIZE 1024
+#define PARENT_LOCATOR_NUM 8
+#define TBBATMAP_HEAD_SIZE 28
+
+#define MACX_PREFIX_LEN 7 /* file:// */
+
+#define PLATFORM_MACX 0x4D616358
+#define PLATFORM_W2RU 0x57327275
+#define PLATFORM_W2KU 0x57326B75
+
+#define VHD_VERSION(major, minor)  (((major) << 16) | ((minor) & 0x0000FFFF))
//#define CACHE enum vhd_type {
      VHD_FIXED           = 2,
      VHD_DYNAMIC         = 3,
-    VHD_DIFFERENCING    = 4,
+    VHD_DIFF            = 4,
  };
// Seconds since Jan 1, 2000 0:00:00 (UTC)
@@ -138,6 +150,15 @@ typedef struct BDRVVPCState {
      Error *migration_blocker;
  } BDRVVPCState;
+typedef struct vhd_tdbatmap_header {
+    char magic[8]; /* always "tdbatmap" */
+
+    uint64_t batmap_offset;
+    uint32_t batmap_size;
+    uint32_t batmap_version;
+    uint32_t checksum;
+} QEMU_PACKED VHDTdBatmapHeader;
+
  static uint32_t vpc_checksum(uint8_t* buf, size_t size)
  {
      uint32_t res = 0;
@@ -157,6 +178,224 @@ static int vpc_probe(const uint8_t *buf, int buf_size, 
const char *filename)
      return 0;
  }
+static int vpc_decode_maxc_loc(BlockDriverState *bs, uint32_t data_length)
+{
+    GIConv cd;
+    gchar *inbuf, *outbuf, *buf;
+    gsize inbyteleft, outbyteleft, outbyte;
+    int ret = 0;
+    gsize ret1;
+    gint  ret2;
+
+    if (!bs || !bs->backing_file || !data_length) {
+        return -1;
+    }
+
+    cd = g_iconv_open("ASCII", "UTF8");
+    if (cd == (GIConv) -1) {
+        return -1;
+    }
+
+    inbuf = bs->backing_file;
+    outbuf =  buf = (gchar *) g_malloc(data_length + 1);
+    if (!outbuf) {
+        ret = -1;
+        goto fail2;
+    }
+    inbyteleft = outbyteleft = data_length;
+
+    ret1 = g_iconv(cd, &inbuf, &inbyteleft, &outbuf, &outbyteleft);
+    if (ret1 == (gsize) -1 || inbyteleft) {
+        ret = -1;
+        goto fail1;
+    }
+    outbyte = data_length - outbyteleft;
+    if (outbyte > sizeof(bs->backing_file) - 1) {
+        ret = -1;
+        goto fail1;
+    }
+    if (outbyte < MACX_PREFIX_LEN) {
+        ret = -1;
+        goto fail1;
+    }
+    buf[outbyte] = '\0';
+
+    pstrcpy(bs->backing_file, sizeof(bs->backing_file), buf + MACX_PREFIX_LEN);
+
+fail1:
+    g_free(buf);
+    buf = NULL;
+
+fail2:
+    ret2 = g_iconv_close(cd);
+    if (ret2 == (gint) -1) {
+        ret = -1;
+    }
+
+    return ret;
+}
+
+static int vpc_decode_w2u_loc(BlockDriverState *bs, uint32_t data_length)
+{
+    GIConv cd;
+    gchar *buf, *inbuf, *outbuf;
+    gsize inbyteleft, outbyteleft, outbyte;
+    gchar *ptr;
+    char len = 0;
+    int ret = 0;
+    gsize ret1;
+    gint ret2;
+
+    if (!bs || !bs->backing_file || !data_length) {
+        return -1;
+    }
+
+    cd = g_iconv_open("ASCII", "UTF-16LE");
+    if (cd == (GIConv) -1) {
+        return -1;
+    }
+
+    inbuf = bs->backing_file;
+    outbuf = buf = ptr = (char *) g_malloc(data_length + 1);
+    if (!buf) {
+        ret = -1;
+        goto fail2;
+    }
+    inbyteleft = outbyteleft = data_length;
+
+    ret1 = g_iconv(cd, &inbuf, &inbyteleft, &outbuf, &outbyteleft);
+    if (ret1 == (gsize) -1 || inbyteleft) {
+        ret = -1;
+        goto fail1;
+    }
+    outbyte = data_length - outbyteleft;
+    if (outbyte > sizeof(bs->backing_file) - 1) {
+        ret = -1;
+        goto fail1;
+    }
+    buf[outbyte] = '\0';
+
+    while (ptr != outbuf) {
+        if (*ptr == '\\') {
+            *ptr = '/';
+        }
+        ptr++;
+    }
+    ptr = strstr(buf, ":");
+    if (ptr) {
+        *ptr = '.';
+        len = ptr - buf;
+    }
+
+    pstrcpy(bs->backing_file, sizeof(bs->backing_file), buf + len);
+
+fail1:
+    g_free(buf);
+    buf = NULL;
+
+fail2:
+    ret2 = g_iconv_close(cd);
+    if (ret2 == (gint) -1) {
+        return -1;
+    }
+
+    return ret;
+}
+
+static int vpc_decode_parent_loc(uint32_t platform,
+                                 BlockDriverState *bs,
+                                 uint32_t data_length)
+{
+    int ret;
+
+    switch (platform) {
+    case PLATFORM_MACX:
+        ret = vpc_decode_maxc_loc(bs, data_length);
+        if (ret < 0) {
+            return ret;
+        }
+        break;
+
+    case PLATFORM_W2RU:
+        /* fall through! */
+    case PLATFORM_W2KU:
+        ret = vpc_decode_w2u_loc(bs, data_length);
+        if (ret < 0) {
+            return ret;
+        }
+        break;
+
+    default:
+        return -1;
+    }
+
+    return 0;
+}
+
+static int vpc_read_backing_loc(VHDDynDiskHeader *dyndisk_header,
+                                BlockDriverState *bs,
+                                Error **errp)
+{
+    BDRVVPCState *s = bs->opaque;
+    uint64_t data_offset;
+    uint32_t data_length;
+    uint32_t platform;
+    bool done = false;
+    uint64_t parent_locator_offset = 0;
+    int i;
+    int ret = 0;
+
+    for (i = 0; i < PARENT_LOCATOR_NUM; i++) {
+        platform =
+            be32_to_cpu(dyndisk_header->parent_locator[i].platform);
+        data_offset =
+            be64_to_cpu(dyndisk_header->parent_locator[i].data_offset);
+        data_length =
+            be32_to_cpu(dyndisk_header->parent_locator[i].data_length);
+
+        /* Extend the location offset */
+        if (parent_locator_offset < data_offset) {
+            parent_locator_offset = data_offset;
+        }
+
+        if (done) {
+            continue;
+        }
+
+        /* Read location of backing file */
+        if (data_offset > s->max_table_entries * s->block_size) {

This multiplication overflows resulting in failure...

+            return -1;
+        }
+        if (data_length > sizeof(bs->backing_file) - 1) {
+            return -1;
+        }
+        ret = bdrv_pread(bs->file, data_offset, bs->backing_file,
+            data_length);
+        if (ret < 0) {
+            return ret;
+        }
+
+        bs->backing_file[data_length] = '\0';
+
+        /* Decode the parent location */
+        ret = vpc_decode_parent_loc(platform, bs, data_length);
+        if (ret < 0) {
+            return ret;
+        }
+
+        /* Right parent location is got */
+        if (bs->backing_file[0] != '\0') {
+            done = true;
+        }
+    }
+
+    if (!done) {
+        return -1;
+    }
+
+    return parent_locator_offset;
+}
+
  static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
                      Error **errp)
  {
@@ -164,11 +403,14 @@ static int vpc_open(BlockDriverState *bs, QDict *options, 
int flags,
      int i;
      VHDFooter *footer;
      VHDDynDiskHeader *dyndisk_header;
-    uint8_t buf[HEADER_SIZE];
+    uint8_t buf[DYNAMIC_HEADER_SIZE];
+    uint8_t tdbatmap_header_buf[TBBATMAP_HEAD_SIZE];
      uint32_t checksum;
      uint64_t computed_size;
-    int disk_type = VHD_DYNAMIC;
+    uint32_t disk_type;
      int ret;
+    VHDTdBatmapHeader *tdbatmap_header;
+    uint64_t parent_locator_offset = 0;
ret = bdrv_pread(bs->file, 0, s->footer_buf, HEADER_SIZE);
      if (ret < 0) {
@@ -176,6 +418,8 @@ static int vpc_open(BlockDriverState *bs, QDict *options, 
int flags,
      }
footer = (VHDFooter *) s->footer_buf;
+    disk_type = be32_to_cpu(footer->type);
+
      if (strncmp(footer->creator, "conectix", 8)) {
          int64_t offset = bdrv_getlength(bs->file);
          if (offset < 0) {
@@ -230,9 +474,9 @@ static int vpc_open(BlockDriverState *bs, QDict *options, 
int flags,
          goto fail;
      }
- if (disk_type == VHD_DYNAMIC) {
+    if (disk_type == VHD_DYNAMIC || disk_type == VHD_DIFF) {
          ret = bdrv_pread(bs->file, be64_to_cpu(footer->data_offset), buf,
-                         HEADER_SIZE);
+                         DYNAMIC_HEADER_SIZE);
          if (ret < 0) {
              goto fail;
          }
@@ -286,6 +530,47 @@ static int vpc_open(BlockDriverState *bs, QDict *options, 
int flags,
          s->free_data_block_offset =
              (s->bat_offset + (s->max_table_entries * 4) + 511) & ~511;
+ /* Read tdbatmap header by offset */
+        if (be32_to_cpu(footer->version) >= VHD_VERSION(1, 2)) {
+            ret = bdrv_pread(bs->file, s->free_data_block_offset,
+                tdbatmap_header_buf, TBBATMAP_HEAD_SIZE);
+            if (ret < 0) {
+                goto fail;
+            }
+
+            tdbatmap_header = (VHDTdBatmapHeader *) tdbatmap_header_buf;
+            if (!strncmp(tdbatmap_header->magic, "tdbatmap", 8)) {
+                s->free_data_block_offset =
+                    be32_to_cpu(tdbatmap_header->batmap_size) * 512
+                    + be64_to_cpu(tdbatmap_header->batmap_offset);
+            }
+        }
+
+        if (dyndisk_header->parent_name[0] || dyndisk_header->parent_name[1]) {
+            int len;
+
+            /* Read parent location from dyn header table */
+            ret = parent_locator_offset = vpc_read_backing_loc(dyndisk_header,
+                bs, errp);
+            if (ret < 0) {
+                goto fail;
+            }
+
+            /* Fix me : Set parent format to avoid probing to raw in
+             * format probe framework */
+            len = strlen("vpc");
+            if (sizeof(bs->backing_format) - 1 < len) {
+                goto fail;
+            }
+            pstrcpy(bs->backing_format, sizeof(bs->backing_format), "vpc");
+        }
+
+        if (s->free_data_block_offset < parent_locator_offset
+            + BDRV_SECTOR_SIZE) {
+            s->free_data_block_offset = parent_locator_offset
+                + BDRV_SECTOR_SIZE;
+        }
+
          for (i = 0; i < s->max_table_entries; i++) {
              be32_to_cpus(&s->pagetable[i]);
              if (s->pagetable[i] != 0xFFFFFFFF) {
@@ -340,35 +625,76 @@ static int vpc_reopen_prepare(BDRVReopenState *state,
  }
/*
- * Returns the absolute byte offset of the given sector in the image file.
- * If the sector is not allocated, -1 is returned instead.
+ * Returns the absolute byte offset of the given sector in the differencing
+ * image file.
+ *
+ * If error happened, -1 is returned.
   *
- * The parameter write must be 1 if the offset will be used for a write
- * operation (the block bitmaps is updated then), 0 otherwise.
+ * When write all type or read dynamic, if the sector is not allocated, -2
+ * is returned instead. If the sector is allocated in current file, the block
+ * offset is returned.
+ *
+ * When read diff. If the sector is not allocated, -2 is returned instead.
+ * If the sector is allocated in the backing file, -3 is returned. If the
+ * sector is allocated in current file, the block offset is returned.
   */
  static inline int64_t get_sector_offset(BlockDriverState *bs,
-    int64_t sector_num, int write)
+    int64_t sector_num, bool write, bool diff)
  {
      BDRVVPCState *s = bs->opaque;
-    uint64_t offset = sector_num * 512;
-    uint64_t bitmap_offset, block_offset;
+    uint64_t offset = sector_num << BDRV_SECTOR_BITS;
+    uint64_t bitmap_offset;
      uint32_t pagetable_index, pageentry_index;
+    int64_t block_offset = LONG_MIN;
+    int ret;
pagetable_index = offset / s->block_size;
-    pageentry_index = (offset % s->block_size) / 512;
+    pageentry_index = (offset % s->block_size) >> BDRV_SECTOR_BITS;
- if (pagetable_index >= s->max_table_entries || s->pagetable[pagetable_index] == 0xffffffff)
-        return -1; // not allocated
+    if (pagetable_index >= s->max_table_entries) {
+        return -2;
+    }
+    if (s->pagetable[pagetable_index] == 0xffffffff) {
+        if (!write && diff) {
+            return -3; /* parent allocated */
+        } else {
+            return -2; /* not allocated */
+        }
+    }
- bitmap_offset = 512 * (uint64_t) s->pagetable[pagetable_index];
-    block_offset = bitmap_offset + s->bitmap_size + (512 * pageentry_index);
+    bitmap_offset = (uint64_t) s->pagetable[pagetable_index]
+        << BDRV_SECTOR_BITS;
+
+    if (!diff || write) {
+        block_offset = bitmap_offset + s->bitmap_size
+            + (pageentry_index << BDRV_SECTOR_BITS);
+    } else {
+        uint32_t bitmap_index, bitmapentry_index;
+        uint8_t bitmap[s->bitmap_size];
+ if (bitmap_offset > s->max_table_entries * s->block_size) {

This one too....

+            return -1;
+        }
+        ret = bdrv_pread(bs->file, bitmap_offset, bitmap, s->bitmap_size);
+        if (ret < 0) {
+            return -1;
+        }
+
+        bitmap_index = pageentry_index / 8;
+        bitmapentry_index = 7 - pageentry_index % 8;
+        if (bitmap[bitmap_index] & 0x1 << bitmapentry_index) {
+            block_offset = bitmap_offset + s->bitmap_size
+                + (pageentry_index << BDRV_SECTOR_BITS);
+        } else {
+            return -3;
+        }
+    }
      // We must ensure that we don't write to any sectors which are marked as
      // unused in the bitmap. We get away with setting all bits in the block
      // bitmap each time we write to a new block. This might cause Virtual PC 
to
      // miss sparse read optimization, but it's not a problem in terms of
      // correctness.
-    if (write && (s->last_bitmap_offset != bitmap_offset)) {
+    if (!diff && write && (s->last_bitmap_offset != bitmap_offset)) {
          uint8_t bitmap[s->bitmap_size];
s->last_bitmap_offset = bitmap_offset;
@@ -437,7 +763,8 @@ static int rewrite_footer(BlockDriverState* bs)
   *
   * Returns the sectors' offset in the image file on success and < 0 on error
   */
-static int64_t alloc_block(BlockDriverState* bs, int64_t sector_num)
+static int64_t alloc_block(BlockDriverState *bs, int64_t sector_num,
+    bool diff)
  {
      BDRVVPCState *s = bs->opaque;
      int64_t bat_offset;
@@ -457,7 +784,11 @@ static int64_t alloc_block(BlockDriverState* bs, int64_t 
sector_num)
      s->pagetable[index] = s->free_data_block_offset / 512;
// Initialize the block's bitmap
-    memset(bitmap, 0xff, s->bitmap_size);
+    if (diff) {
+        memset(bitmap, 0x0, s->bitmap_size);
+    } else {
+        memset(bitmap, 0xff, s->bitmap_size);
+    }
      ret = bdrv_pwrite_sync(bs->file, s->free_data_block_offset, bitmap,
          s->bitmap_size);
      if (ret < 0) {
@@ -477,7 +808,7 @@ static int64_t alloc_block(BlockDriverState* bs, int64_t 
sector_num)
      if (ret < 0)
          goto fail;
- return get_sector_offset(bs, sector_num, 0);
+    return get_sector_offset(bs, sector_num, false, diff);
fail:
      s->free_data_block_offset -= (s->block_size + s->bitmap_size);
@@ -501,37 +832,71 @@ static int vpc_read(BlockDriverState *bs, int64_t 
sector_num,
                      uint8_t *buf, int nb_sectors)
  {
      BDRVVPCState *s = bs->opaque;
-    int ret;
-    int64_t offset;
-    int64_t sectors, sectors_per_block;
      VHDFooter *footer = (VHDFooter *) s->footer_buf;
+    int64_t sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
+    int64_t offset, sectors;
+    int ret;
- if (be32_to_cpu(footer->type) == VHD_FIXED) {
+    switch (be32_to_cpu(footer->type)) {
+    case VHD_FIXED:
          return bdrv_read(bs->file, sector_num, buf, nb_sectors);
-    }
-    while (nb_sectors > 0) {
-        offset = get_sector_offset(bs, sector_num, 0);
- sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
-        sectors = sectors_per_block - (sector_num % sectors_per_block);
-        if (sectors > nb_sectors) {
-            sectors = nb_sectors;
+    case VHD_DYNAMIC:
+        while (nb_sectors > 0) {
+            sectors = sectors_per_block - (sector_num % sectors_per_block);
+            if (sectors > nb_sectors) {
+                sectors = nb_sectors;
+            }
+
+            offset = get_sector_offset(bs, sector_num, false, false);
+            if (offset == -1) {
+                return -1;
+            } else if (offset == -2) {
+                memset(buf, 0, sectors * BDRV_SECTOR_SIZE);
+            } else {
+                ret = bdrv_pread(bs->file, offset, buf,
+                    sectors * BDRV_SECTOR_SIZE);
+                if (ret != sectors * BDRV_SECTOR_SIZE) {
+                    return -1;
+                }
+            }
+
+            nb_sectors -= sectors;
+            sector_num += sectors;
+            buf += sectors * BDRV_SECTOR_SIZE;
          }
+        break;
- if (offset == -1) {
-            memset(buf, 0, sectors * BDRV_SECTOR_SIZE);
-        } else {
-            ret = bdrv_pread(bs->file, offset, buf,
-                sectors * BDRV_SECTOR_SIZE);
-            if (ret != sectors * BDRV_SECTOR_SIZE) {
+    case VHD_DIFF:
+        while (nb_sectors > 0) {
+            offset = get_sector_offset(bs, sector_num, false, true);
+            if (offset == -1) {
                  return -1;
+            } else if (offset == -2) {
+                memset(buf, 0, BDRV_SECTOR_SIZE);
+            } else if (offset == -3) {
+                ret = bdrv_pread(bs->backing_hd, sector_num << BDRV_SECTOR_BITS
+                    , buf, BDRV_SECTOR_SIZE);
+                if (ret < 0) {
+                    return -1;
+                }
+            } else {
+                ret = bdrv_pread(bs->file, offset, buf, BDRV_SECTOR_SIZE);
+                if (ret != BDRV_SECTOR_SIZE) {
+                    return -1;
+                }
              }
+
+            nb_sectors--;
+            sector_num++;
+            buf += BDRV_SECTOR_SIZE;
          }
+        break;
- nb_sectors -= sectors;
-        sector_num += sectors;
-        buf += sectors * BDRV_SECTOR_SIZE;
+    default:
+        return -1;
      }
+
      return 0;
  }
@@ -546,41 +911,101 @@ static coroutine_fn int vpc_co_read(BlockDriverState *bs, int64_t sector_num,
      return ret;
  }
+static inline int64_t write_bitmap(BlockDriverState *bs, int64_t sector_num,
+    int64_t sectors)
+{
+    BDRVVPCState *s = bs->opaque;
+    uint64_t offset = sector_num << BDRV_SECTOR_BITS;
+    uint64_t bitmap_offset;
+    uint32_t pagetable_index, pageentry_index;
+    uint8_t bitmap[s->bitmap_size];
+    uint32_t bitmap_index, bitmapbit_index;
+    int i;
+    int ret;
+
+    pagetable_index = offset / s->block_size;
+    pageentry_index = (offset % s->block_size) / 512;
+    bitmap_offset = 512 * (uint64_t) s->pagetable[pagetable_index];
+
+    if (bitmap_offset > s->max_table_entries * s->block_size) {

And here...

+        return -1;
+    }
+    ret = bdrv_pread(bs->file, bitmap_offset, bitmap, s->bitmap_size);
+    if (ret < 0) {
+        return -1;
+    }
+
+    for (i = 0; i < sectors; i++) {
+        bitmap_index = pageentry_index / 8;
+        bitmapbit_index = 7 - pageentry_index % 8;
+        bitmap[bitmap_index] |= (0x1 << bitmapbit_index);
+        pageentry_index++;
+    }
+    ret = bdrv_pwrite(bs->file, bitmap_offset, bitmap, s->bitmap_size);
+    if (ret < 0) {
+        return -1;
+    }
+
+    return 0;
+}
+
  static int vpc_write(BlockDriverState *bs, int64_t sector_num,
      const uint8_t *buf, int nb_sectors)
  {
      BDRVVPCState *s = bs->opaque;
-    int64_t offset;
-    int64_t sectors, sectors_per_block;
-    int ret;
-    VHDFooter *footer =  (VHDFooter *) s->footer_buf;
+    VHDFooter *footer = (VHDFooter *) s->footer_buf;
+    int64_t sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
+    int64_t offset, sectors;
+    bool diff = true;
+    int ret = 0;
- if (be32_to_cpu(footer->type) == VHD_FIXED) {
+    switch (be32_to_cpu(footer->type)) {
+    case VHD_FIXED:
          return bdrv_write(bs->file, sector_num, buf, nb_sectors);
-    }
-    while (nb_sectors > 0) {
-        offset = get_sector_offset(bs, sector_num, 1);
- sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
-        sectors = sectors_per_block - (sector_num % sectors_per_block);
-        if (sectors > nb_sectors) {
-            sectors = nb_sectors;
-        }
+    case VHD_DYNAMIC:
+        diff = false;
+        /* fall-through ! */
+
+    case VHD_DIFF:
+        while (nb_sectors > 0) {
+            sectors = sectors_per_block - (sector_num % sectors_per_block);
+            if (sectors > nb_sectors) {
+                sectors = nb_sectors;
+            }
- if (offset == -1) {
-            offset = alloc_block(bs, sector_num);
-            if (offset < 0)
+            offset = get_sector_offset(bs, sector_num, true, diff);
+            if (offset == -1) {
                  return -1;
-        }
+            } else if (offset == -2) {
+                offset = alloc_block(bs, sector_num, diff);
+                if (offset < 0) {
+                    return -1;
+                }
+            }
- ret = bdrv_pwrite(bs->file, offset, buf, sectors * BDRV_SECTOR_SIZE);
-        if (ret != sectors * BDRV_SECTOR_SIZE) {
-            return -1;
+            ret = bdrv_pwrite(bs->file, offset, buf,
+                sectors * BDRV_SECTOR_SIZE);
+            if (ret != sectors * BDRV_SECTOR_SIZE) {
+                return -1;
+            }
+
+            if (diff) {
+                ret = write_bitmap(bs, sector_num, sectors);
+                if (ret < 0) {
+                    return -1;
+                }
+            }
+
+            nb_sectors -= sectors;
+            sector_num += sectors;
+            buf += sectors * BDRV_SECTOR_SIZE;
          }
- nb_sectors -= sectors;
-        sector_num += sectors;
-        buf += sectors * BDRV_SECTOR_SIZE;
+        break;
+
+    default:
+        return -1;
      }
return 0;
@@ -906,6 +1331,7 @@ static BlockDriver bdrv_vpc = {
      .bdrv_close             = vpc_close,
      .bdrv_reopen_prepare    = vpc_reopen_prepare,
      .bdrv_create            = vpc_create,
+    .supports_backing       = true,
.bdrv_read = vpc_co_read,
      .bdrv_write             = vpc_co_write,




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