qcow2: Flush metadata during read-only reopen

If lazy refcounts are enabled for a backing file, committing to this
backing file may leave it in a dirty state even if the commit succeeds.
The reason is that the bdrv_flush() call in bdrv_commit() doesn't flush
refcount updates with lazy refcounts enabled, and qcow2_reopen_prepare()
doesn't take care to flush metadata.

In order to fix this, this patch also fixes qcow2_mark_clean(), which
contains another ineffective bdrv_flush() call beause lazy refcounts are
disabled only afterwards. All existing callers of qcow2_mark_clean()
either don't modify refcounts or already flush manually, so that this
fixes only a latent, but not yet actually triggerable bug.

Another instance of the same problem is live snapshots. Again, a real
corruption is prevented by an explicit flush for non-read-only images in
external_snapshot_prepare(), but images using lazy refcounts stay dirty.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 21 insertions, 4 deletions

diff --git a/block/qcow2.c b/block/qcow2.c
index 333e26d733..e903d971c3 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -269,12 +269,15 @@ static int qcow2_mark_clean(BlockDriverState *bs)
     BDRVQcowState *s = bs->opaque;
 
     if (s->incompatible_features & QCOW2_INCOMPAT_DIRTY) {
-        int ret = bdrv_flush(bs);
+        int ret;
+
+        s->incompatible_features &= ~QCOW2_INCOMPAT_DIRTY;
+
+        ret = bdrv_flush(bs);
         if (ret < 0) {
             return ret;
         }
 
-        s->incompatible_features &= ~QCOW2_INCOMPAT_DIRTY;
         return qcow2_update_header(bs);
     }
     return 0;
@@ -900,11 +903,25 @@ static int qcow2_set_key(BlockDriverState *bs, const char *key)
     return 0;
 }
 
-/* We have nothing to do for QCOW2 reopen, stubs just return
- * success */
+/* We have no actual commit/abort logic for qcow2, but we need to write out any
+ * unwritten data if we reopen read-only. */
 static int qcow2_reopen_prepare(BDRVReopenState *state,
                                 BlockReopenQueue *queue, Error **errp)
 {
+    int ret;
+
+    if ((state->flags & BDRV_O_RDWR) == 0) {
+        ret = bdrv_flush(state->bs);
+        if (ret < 0) {
+            return ret;
+        }
+
+        ret = qcow2_mark_clean(state->bs);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
     return 0;
 }