vmdk: Allow read only open of VMDK version 3

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/vmdk.c

@@ -605,13 +605,20 @@ static int vmdk_open_vmdk4(BlockDriverState *bs,
         header = footer.header;
     }
 
-    if (le32_to_cpu(header.version) >= 3) {
+    if (le32_to_cpu(header.version) > 3) {
         char buf[64];
         snprintf(buf, sizeof(buf), "VMDK version %d",
                  le32_to_cpu(header.version));
         qerror_report(QERR_UNKNOWN_BLOCK_FORMAT_FEATURE,
                 bs->device_name, "vmdk", buf);
         return -ENOTSUP;
+    } else if (le32_to_cpu(header.version) == 3 && (flags & BDRV_O_RDWR)) {
+        /* VMware KB 2064959 explains that version 3 added support for
+         * persistent changed block tracking (CBT), and backup software can
+         * read it as version=1 if it doesn't care about the changed area
+         * information. So we are safe to enable read only. */
+        error_setg(errp, "VMDK version 3 must be read only");
+        return -EINVAL;
     }
 
     if (le32_to_cpu(header.num_gtes_per_gt) > 512) {