Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer patches

# gpg: Signature made Fri 24 Feb 2017 18:08:26 GMT
# gpg:                using RSA key 0x7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>"
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74  56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream:
  tests: Use opened block node for block job tests
  vvfat: Use opened node as backing file
  block: Add bdrv_new_open_driver()
  block: Factor out bdrv_open_driver()
  block: Use BlockBackend for image probing
  block: Factor out bdrv_open_child_bs()
  block: Attach bs->file only during .bdrv_open()
  block: Pass BdrvChild to bdrv_truncate()
  mirror: Resize active commit base in mirror_run()
  qcow2: Use BB for resizing in qcow2_amend_options()
  blockdev: Use BlockBackend to resize in qmp_block_resize()
  iotests: Fix another race in 030
  qemu-img: Improve documentation for PREALLOC_MODE_FALLOC
  qemu-img: Truncate before full preallocation
  qemu-img: Add tests for raw image preallocation
  qemu-img: Do not truncate before preallocation
  qemu-iotests: redirect nbd server stdout to /dev/null
  qemu-iotests: add ability to exclude certain protocols from tests
  qemu-iotests: Test 137 only supports 'file' protocol

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

21 files changed, 170 insertions, 62 deletions

diff --git a/block/blkdebug.c b/block/blkdebug.c
index d8eee1b..6117ce5 100644
--- a/block/blkdebug.c
+++ b/block/blkdebug.c
@@ -663,7 +663,7 @@ static int64_t blkdebug_getlength(BlockDriverState *bs)
 
 static int blkdebug_truncate(BlockDriverState *bs, int64_t offset)
 {
-    return bdrv_truncate(bs->file->bs, offset);
+    return bdrv_truncate(bs->file, offset);
 }
 
 static void blkdebug_refresh_filename(BlockDriverState *bs, QDict *options)
diff --git a/block/block-backend.c b/block/block-backend.c
index 819f272..492e71e 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1605,7 +1605,7 @@ int blk_truncate(BlockBackend *blk, int64_t offset)
         return -ENOMEDIUM;
     }
 
-    return bdrv_truncate(blk_bs(blk), offset);
+    return bdrv_truncate(blk->root, offset);
 }
 
 static void blk_pdiscard_entry(void *opaque)
diff --git a/block/bochs.c b/block/bochs.c
index 8c9652e..7dd2ac4 100644
--- a/block/bochs.c
+++ b/block/bochs.c
@@ -104,6 +104,12 @@ static int bochs_open(BlockDriverState *bs, QDict *options, int flags,
     struct bochs_header bochs;
     int ret;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     bs->read_only = true; /* no write support yet */
 
     ret = bdrv_pread(bs->file, 0, &bochs, sizeof(bochs));
diff --git a/block/cloop.c b/block/cloop.c
index 7b75f7e..877c9b0 100644
--- a/block/cloop.c
+++ b/block/cloop.c
@@ -66,6 +66,12 @@ static int cloop_open(BlockDriverState *bs, QDict *options, int flags,
     uint32_t offsets_size, max_compressed_block_size = 1, i;
     int ret;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     bs->read_only = true;
 
     /* read header */
diff --git a/block/crypto.c b/block/crypto.c
index 7aa7eb5..7cb2ff2 100644
--- a/block/crypto.c
+++ b/block/crypto.c
@@ -300,6 +300,12 @@ static int block_crypto_open_generic(QCryptoBlockFormat format,
     QCryptoBlockOpenOptions *open_opts = NULL;
     unsigned int cflags = 0;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     opts = qemu_opts_create(opts_spec, NULL, 0, &error_abort);
     qemu_opts_absorb_qdict(opts, options, &local_err);
     if (local_err) {
@@ -383,7 +389,7 @@ static int block_crypto_truncate(BlockDriverState *bs, int64_t offset)
 
     offset += payload_offset;
 
-    return bdrv_truncate(bs->file->bs, offset);
+    return bdrv_truncate(bs->file, offset);
 }
 
 static void block_crypto_close(BlockDriverState *bs)
diff --git a/block/dmg.c b/block/dmg.c
index 58a3ae8..8e387cd 100644
--- a/block/dmg.c
+++ b/block/dmg.c
@@ -413,6 +413,12 @@ static int dmg_open(BlockDriverState *bs, QDict *options, int flags,
     int64_t offset;
     int ret;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     block_module_load_one("dmg-bz2");
     bs->read_only = true;
 
diff --git a/block/file-posix.c b/block/file-posix.c
index 2134e0e..4de1abd 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1591,18 +1591,17 @@ static int raw_create(const char *filename, QemuOpts *opts, Error **errp)
 #endif
     }
 
-    if (ftruncate(fd, total_size) != 0) {
-        result = -errno;
-        error_setg_errno(errp, -result, "Could not resize file");
-        goto out_close;
-    }
-
     switch (prealloc) {
 #ifdef CONFIG_POSIX_FALLOCATE
     case PREALLOC_MODE_FALLOC:
-        /* posix_fallocate() doesn't set errno. */
+        /*
+         * Truncating before posix_fallocate() makes it about twice slower on
+         * file systems that do not support fallocate(), trying to check if a
+         * block is allocated before allocating it, so don't do that here.
+         */
         result = -posix_fallocate(fd, 0, total_size);
         if (result != 0) {
+            /* posix_fallocate() doesn't set errno. */
             error_setg_errno(errp, -result,
                              "Could not preallocate data for the new file");
         }
@@ -1610,6 +1609,17 @@ static int raw_create(const char *filename, QemuOpts *opts, Error **errp)
 #endif
     case PREALLOC_MODE_FULL:
     {
+        /*
+         * Knowing the final size from the beginning could allow the file
+         * system driver to do less allocations and possibly avoid
+         * fragmentation of the file.
+         */
+        if (ftruncate(fd, total_size) != 0) {
+            result = -errno;
+            error_setg_errno(errp, -result, "Could not resize file");
+            goto out_close;
+        }
+
         int64_t num = 0, left = total_size;
         buf = g_malloc0(65536);
 
@@ -1636,6 +1646,10 @@ static int raw_create(const char *filename, QemuOpts *opts, Error **errp)
         break;
     }
     case PREALLOC_MODE_OFF:
+        if (ftruncate(fd, total_size) != 0) {
+            result = -errno;
+            error_setg_errno(errp, -result, "Could not resize file");
+        }
         break;
     default:
         result = -EINVAL;
diff --git a/block/mirror.c b/block/mirror.c
index ca8547b..3d50857 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -662,7 +662,28 @@ static void coroutine_fn mirror_run(void *opaque)
     if (s->bdev_length < 0) {
         ret = s->bdev_length;
         goto immediate_exit;
-    } else if (s->bdev_length == 0) {
+    }
+
+    /* Active commit must resize the base image if its size differs from the
+     * active layer. */
+    if (s->base == blk_bs(s->target)) {
+        int64_t base_length;
+
+        base_length = blk_getlength(s->target);
+        if (base_length < 0) {
+            ret = base_length;
+            goto immediate_exit;
+        }
+
+        if (s->bdev_length > base_length) {
+            ret = blk_truncate(s->target, s->bdev_length);
+            if (ret < 0) {
+                goto immediate_exit;
+            }
+        }
+    }
+
+    if (s->bdev_length == 0) {
         /* Report BLOCK_JOB_READY and wait for complete. */
         block_job_event_ready(&s->common);
         s->synced = true;
@@ -1063,9 +1084,7 @@ void commit_active_start(const char *job_id, BlockDriverState *bs,
                          BlockCompletionFunc *cb, void *opaque, Error **errp,
                          bool auto_complete)
 {
-    int64_t length, base_length;
     int orig_base_flags;
-    int ret;
     Error *local_err = NULL;
 
     orig_base_flags = bdrv_get_flags(base);
@@ -1074,31 +1093,6 @@ void commit_active_start(const char *job_id, BlockDriverState *bs,
         return;
     }
 
-    length = bdrv_getlength(bs);
-    if (length < 0) {
-        error_setg_errno(errp, -length,
-                         "Unable to determine length of %s", bs->filename);
-        goto error_restore_flags;
-    }
-
-    base_length = bdrv_getlength(base);
-    if (base_length < 0) {
-        error_setg_errno(errp, -base_length,
-                         "Unable to determine length of %s", base->filename);
-        goto error_restore_flags;
-    }
-
-    if (length > base_length) {
-        ret = bdrv_truncate(base, length);
-        if (ret < 0) {
-            error_setg_errno(errp, -ret,
-                            "Top image %s is larger than base image %s, and "
-                             "resize of base image failed",
-                             bs->filename, base->filename);
-            goto error_restore_flags;
-        }
-    }
-
     mirror_start_job(job_id, bs, creation_flags, base, NULL, speed, 0, 0,
                      MIRROR_LEAVE_BACKING_CHAIN,
                      on_error, on_error, true, cb, opaque, &local_err,
diff --git a/block/parallels.c b/block/parallels.c
index 2ccefa7..b2ec09f 100644
--- a/block/parallels.c
+++ b/block/parallels.c
@@ -215,7 +215,7 @@ static int64_t allocate_clusters(BlockDriverState *bs, int64_t sector_num,
                                      s->data_end << BDRV_SECTOR_BITS,
                                      space << BDRV_SECTOR_BITS, 0);
         } else {
-            ret = bdrv_truncate(bs->file->bs,
+            ret = bdrv_truncate(bs->file,
                                 (s->data_end + space) << BDRV_SECTOR_BITS);
         }
         if (ret < 0) {
@@ -449,7 +449,7 @@ static int parallels_check(BlockDriverState *bs, BdrvCheckResult *res,
                 size - res->image_end_offset);
         res->leaks += count;
         if (fix & BDRV_FIX_LEAKS) {
-            ret = bdrv_truncate(bs->file->bs, res->image_end_offset);
+            ret = bdrv_truncate(bs->file, res->image_end_offset);
             if (ret < 0) {
                 res->check_errors++;
                 return ret;
@@ -581,6 +581,12 @@ static int parallels_open(BlockDriverState *bs, QDict *options, int flags,
     Error *local_err = NULL;
     char *buf;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     ret = bdrv_pread(bs->file, 0, &ph, sizeof(ph));
     if (ret < 0) {
         goto fail;
@@ -681,7 +687,7 @@ static int parallels_open(BlockDriverState *bs, QDict *options, int flags,
         goto fail_options;
     }
     if (!bdrv_has_zero_init(bs->file->bs) ||
-            bdrv_truncate(bs->file->bs, bdrv_getlength(bs->file->bs)) != 0) {
+            bdrv_truncate(bs->file, bdrv_getlength(bs->file->bs)) != 0) {
         s->prealloc_mode = PRL_PREALLOC_MODE_FALLOCATE;
     }
 
@@ -724,7 +730,7 @@ static void parallels_close(BlockDriverState *bs)
     }
 
     if (bs->open_flags & BDRV_O_RDWR) {
-        bdrv_truncate(bs->file->bs, s->data_end << BDRV_SECTOR_BITS);
+        bdrv_truncate(bs->file, s->data_end << BDRV_SECTOR_BITS);
     }
 
     g_free(s->bat_dirty_bmap);
diff --git a/block/qcow.c b/block/qcow.c
index fb738fc..038b05a 100644
--- a/block/qcow.c
+++ b/block/qcow.c
@@ -106,6 +106,12 @@ static int qcow_open(BlockDriverState *bs, QDict *options, int flags,
     QCowHeader header;
     Error *local_err = NULL;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     ret = bdrv_pread(bs->file, 0, &header, sizeof(header));
     if (ret < 0) {
         goto fail;
@@ -467,7 +473,7 @@ static uint64_t get_cluster_offset(BlockDriverState *bs,
                 /* round to cluster size */
                 cluster_offset = (cluster_offset + s->cluster_size - 1) &
                     ~(s->cluster_size - 1);
-                bdrv_truncate(bs->file->bs, cluster_offset + s->cluster_size);
+                bdrv_truncate(bs->file, cluster_offset + s->cluster_size);
                 /* if encrypted, we must initialize the cluster
                    content which won't be written */
                 if (bs->encrypted &&
@@ -909,7 +915,7 @@ static int qcow_make_empty(BlockDriverState *bs)
     if (bdrv_pwrite_sync(bs->file, s->l1_table_offset, s->l1_table,
             l1_length) < 0)
         return -1;
-    ret = bdrv_truncate(bs->file->bs, s->l1_table_offset + l1_length);
+    ret = bdrv_truncate(bs->file, s->l1_table_offset + l1_length);
     if (ret < 0)
         return ret;
 
diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 3dbde18..9e96f64 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -1734,7 +1734,7 @@ static int check_refblocks(BlockDriverState *bs, BdrvCheckResult *res,
                     goto resize_fail;
                 }
 
-                ret = bdrv_truncate(bs->file->bs, offset + s->cluster_size);
+                ret = bdrv_truncate(bs->file, offset + s->cluster_size);
                 if (ret < 0) {
                     goto resize_fail;
                 }
diff --git a/block/qcow2.c b/block/qcow2.c
index 3e274bd..21e6142 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -814,8 +814,8 @@ static int qcow2_update_options(BlockDriverState *bs, QDict *options,
     return ret;
 }
 
-static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
-                      Error **errp)
+static int qcow2_do_open(BlockDriverState *bs, QDict *options, int flags,
+                         Error **errp)
 {
     BDRVQcow2State *s = bs->opaque;
     unsigned int len, i;
@@ -1205,6 +1205,18 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
     return ret;
 }
 
+static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
+                      Error **errp)
+{
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
+    return qcow2_do_open(bs, options, flags, errp);
+}
+
 static void qcow2_refresh_limits(BlockDriverState *bs, Error **errp)
 {
     BDRVQcow2State *s = bs->opaque;
@@ -1785,7 +1797,7 @@ static void qcow2_invalidate_cache(BlockDriverState *bs, Error **errp)
     options = qdict_clone_shallow(bs->options);
 
     flags &= ~BDRV_O_INACTIVE;
-    ret = qcow2_open(bs, options, flags, &local_err);
+    ret = qcow2_do_open(bs, options, flags, &local_err);
     QDECREF(options);
     if (local_err) {
         error_propagate(errp, local_err);
@@ -2570,7 +2582,7 @@ qcow2_co_pwritev_compressed(BlockDriverState *bs, uint64_t offset,
         /* align end of file to a sector boundary to ease reading with
            sector based I/Os */
         cluster_offset = bdrv_getlength(bs->file->bs);
-        return bdrv_truncate(bs->file->bs, cluster_offset);
+        return bdrv_truncate(bs->file, cluster_offset);
     }
 
     buf = qemu_blockalign(bs, s->cluster_size);
@@ -2784,7 +2796,7 @@ static int make_completely_empty(BlockDriverState *bs)
         goto fail;
     }
 
-    ret = bdrv_truncate(bs->file->bs, (3 + l1_clusters) * s->cluster_size);
+    ret = bdrv_truncate(bs->file, (3 + l1_clusters) * s->cluster_size);
     if (ret < 0) {
         goto fail;
     }
@@ -3250,7 +3262,11 @@ static int qcow2_amend_options(BlockDriverState *bs, QemuOpts *opts,
     }
 
     if (new_size) {
-        ret = bdrv_truncate(bs, new_size);
+        BlockBackend *blk = blk_new();
+        blk_insert_bs(blk, bs);
+        ret = blk_truncate(blk, new_size);
+        blk_unref(blk);
+
         if (ret < 0) {
             return ret;
         }
diff --git a/block/qed.c b/block/qed.c
index 0b62c77..62a0a09 100644
--- a/block/qed.c
+++ b/block/qed.c
@@ -415,8 +415,8 @@ static void bdrv_qed_drain(BlockDriverState *bs)
     }
 }
 
-static int bdrv_qed_open(BlockDriverState *bs, QDict *options, int flags,
-                         Error **errp)
+static int bdrv_qed_do_open(BlockDriverState *bs, QDict *options, int flags,
+                            Error **errp)
 {
     BDRVQEDState *s = bs->opaque;
     QEDHeader le_header;
@@ -550,6 +550,18 @@ out:
     return ret;
 }
 
+static int bdrv_qed_open(BlockDriverState *bs, QDict *options, int flags,
+                         Error **errp)
+{
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
+    return bdrv_qed_do_open(bs, options, flags, errp);
+}
+
 static void bdrv_qed_refresh_limits(BlockDriverState *bs, Error **errp)
 {
     BDRVQEDState *s = bs->opaque;
@@ -1629,7 +1641,7 @@ static void bdrv_qed_invalidate_cache(BlockDriverState *bs, Error **errp)
     bdrv_qed_close(bs);
 
     memset(s, 0, sizeof(BDRVQEDState));
-    ret = bdrv_qed_open(bs, NULL, bs->open_flags, &local_err);
+    ret = bdrv_qed_do_open(bs, NULL, bs->open_flags, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
         error_prepend(errp, "Could not reopen qed layer: ");
diff --git a/block/raw-format.c b/block/raw-format.c
index 8404a82..ce34d1b 100644
--- a/block/raw-format.c
+++ b/block/raw-format.c
@@ -341,7 +341,7 @@ static int raw_truncate(BlockDriverState *bs, int64_t offset)
 
     s->size = offset;
     offset += s->offset;
-    return bdrv_truncate(bs->file->bs, offset);
+    return bdrv_truncate(bs->file, offset);
 }
 
 static int raw_media_changed(BlockDriverState *bs)
@@ -384,6 +384,12 @@ static int raw_open(BlockDriverState *bs, QDict *options, int flags,
     BDRVRawState *s = bs->opaque;
     int ret;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     bs->sg = bs->file->bs->sg;
     bs->supported_write_flags = BDRV_REQ_FUA &
         bs->file->bs->supported_write_flags;
diff --git a/block/replication.c b/block/replication.c
index 729dd12..eff85c7 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -86,6 +86,12 @@ static int replication_open(BlockDriverState *bs, QDict *options,
     const char *mode;
     const char *top_id;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     ret = -EINVAL;
     opts = qemu_opts_create(&replication_runtime_opts, NULL, 0, &error_abort);
     qemu_opts_absorb_qdict(opts, options, &local_err);
diff --git a/block/vdi.c b/block/vdi.c
index 0aeb940..18b4773 100644
--- a/block/vdi.c
+++ b/block/vdi.c
@@ -363,6 +363,12 @@ static int vdi_open(BlockDriverState *bs, QDict *options, int flags,
     int ret;
     Error *local_err = NULL;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     logout("\n");
 
     ret = bdrv_read(bs->file, 0, (uint8_t *)&header, 1);
diff --git a/block/vhdx-log.c b/block/vhdx-log.c
index 02eb104..67a91c0 100644
--- a/block/vhdx-log.c
+++ b/block/vhdx-log.c
@@ -548,7 +548,7 @@ static int vhdx_log_flush(BlockDriverState *bs, BDRVVHDXState *s,
             if (new_file_size % (1024*1024)) {
                 /* round up to nearest 1MB boundary */
                 new_file_size = ((new_file_size >> 20) + 1) << 20;
-                bdrv_truncate(bs->file->bs, new_file_size);
+                bdrv_truncate(bs->file, new_file_size);
             }
         }
         qemu_vfree(desc_entries);
diff --git a/block/vhdx.c b/block/vhdx.c
index 68db9e0..9918ee9 100644
--- a/block/vhdx.c
+++ b/block/vhdx.c
@@ -898,6 +898,12 @@ static int vhdx_open(BlockDriverState *bs, QDict *options, int flags,
     uint64_t signature;
     Error *local_err = NULL;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     s->bat = NULL;
     s->first_visible_write = true;
 
@@ -1165,7 +1171,7 @@ static int vhdx_allocate_block(BlockDriverState *bs, BDRVVHDXState *s,
     /* per the spec, the address for a block is in units of 1MB */
     *new_offset = ROUND_UP(*new_offset, 1024 * 1024);
 
-    return bdrv_truncate(bs->file->bs, *new_offset + s->block_size);
+    return bdrv_truncate(bs->file, *new_offset + s->block_size);
 }
 
 /*
diff --git a/block/vmdk.c b/block/vmdk.c
index 393c84d..9d68ec5 100644
--- a/block/vmdk.c
+++ b/block/vmdk.c
@@ -943,6 +943,12 @@ static int vmdk_open(BlockDriverState *bs, QDict *options, int flags,
     uint32_t magic;
     Error *local_err = NULL;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     buf = vmdk_read_desc(bs->file, 0, errp);
     if (!buf) {
         return -EINVAL;
diff --git a/block/vpc.c b/block/vpc.c
index ed6353d..d0df2a1 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -220,6 +220,12 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
     int disk_type = VHD_DYNAMIC;
     int ret;
 
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
     opts = qemu_opts_create(&vpc_runtime_opts, NULL, 0, &error_abort);
     qemu_opts_absorb_qdict(opts, options, &local_err);
     if (local_err) {
diff --git a/block/vvfat.c b/block/vvfat.c
index c6bf67e..7f230be 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -2968,6 +2968,7 @@ static void write_target_close(BlockDriverState *bs) {
 
 static BlockDriver vvfat_write_target = {
     .format_name        = "vvfat_write_target",
+    .instance_size      = sizeof(void*),
     .bdrv_co_pwritev    = write_target_commit,
     .bdrv_close         = write_target_close,
 };
@@ -3036,14 +3037,13 @@ static int enable_write_target(BlockDriverState *bs, Error **errp)
     unlink(s->qcow_filename);
 #endif
 
-    backing = bdrv_new();
+    backing = bdrv_new_open_driver(&vvfat_write_target, NULL, BDRV_O_ALLOW_RDWR,
+                                   &error_abort);
+    *(void**) backing->opaque = s;
+
     bdrv_set_backing_hd(s->bs, backing);
     bdrv_unref(backing);
 
-    s->bs->backing->bs->drv = &vvfat_write_target;
-    s->bs->backing->bs->opaque = g_new(void *, 1);
-    *(void**)s->bs->backing->bs->opaque = s;
-
     return 0;
 
 err: