qcow2: No persistent dirty bitmaps for compat=0.10

Persistent dirty bitmaps require a properly functioning
autoclear_features field, or we cannot track when an unsupporting
program might overwrite them.  Therefore, we cannot support them for
compat=0.10 images.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Message-id: 20171123020832.8165-3-mreitz@redhat.com
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block/qcow2-bitmap.c

@@ -1449,6 +1449,16 @@ bool qcow2_can_store_new_dirty_bitmap(BlockDriverState *bs,
     bool found;
     Qcow2BitmapList *bm_list;
 
+    if (s->qcow_version < 3) {
+        /* Without autoclear_features, we would always have to assume
+         * that a program without persistent dirty bitmap support has
+         * accessed this qcow2 file when opening it, and would thus
+         * have to drop all dirty bitmaps (defeating their purpose).
+         */
+        error_setg(errp, "Cannot store dirty bitmaps in qcow2 v2 files");
+        goto fail;
+    }
+
     if (check_constraints_on_bitmap(bs, name, granularity, errp) != 0) {
         goto fail;
     }

block/qcow2.c

@@ -302,9 +302,17 @@ static int qcow2_read_extensions(BlockDriverState *bs, uint64_t start_offset,
             }
 
             if (!(s->autoclear_features & QCOW2_AUTOCLEAR_BITMAPS)) {
-                warn_report("a program lacking bitmap support "
-                            "modified this file, so all bitmaps are now "
-                            "considered inconsistent");
+                if (s->qcow_version < 3) {
+                    /* Let's be a bit more specific */
+                    warn_report("This qcow2 v2 image contains bitmaps, but "
+                                "they may have been modified by a program "
+                                "without persistent bitmap support; so now "
+                                "they must all be considered inconsistent");
+                } else {
+                    warn_report("a program lacking bitmap support "
+                                "modified this file, so all bitmaps are now "
+                                "considered inconsistent");
+                }
                 error_printf("Some clusters may be leaked, "
                              "run 'qemu-img check -r' on the image "
                              "file to fix.");