block/vpc: check that the image has not been truncated

this adds a check that a dynamic VHD file has not been
accidently truncated (e.g. during transfer or upload).
Signed-off-by: NPeter Lieven <pl@kamp.de>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/vpc.c

@@ -260,6 +260,13 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
             }
         }
 
+        if (s->free_data_block_offset > bdrv_getlength(bs->file)) {
+            error_setg(errp, "block-vpc: free_data_block_offset points after "
+                             "the end of file. The image has been truncated.");
+            ret = -EINVAL;
+            goto fail;
+        }
+
         s->last_bitmap_offset = (int64_t) -1;
 
 #ifdef CACHE