[Qemu-devel] [PULL 12/29] qcow2: No persistent dirty bitmaps for compat=0.10

[Kevin Wolf]

From: Max Reitz <address@hidden>

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: Max Reitz <address@hidden>
Message-id: address@hidden
Reviewed-by: Eric Blake <address@hidden>
Signed-off-by: Max Reitz <address@hidden>
---
 block/qcow2-bitmap.c | 10 ++++++++++
 block/qcow2.c        | 14 +++++++++++---
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/block/qcow2-bitmap.c b/block/qcow2-bitmap.c
index f45e46cfbd..efa10c6663 100644
--- a/block/qcow2-bitmap.c
+++ b/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;
     }
diff --git a/block/qcow2.c b/block/qcow2.c
index 4348b2c0c5..1f80961e1b 100644
--- a/block/qcow2.c
+++ b/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.");
-- 
2.13.6