[Qemu-devel] [PATCH] qcow2: fix autoclear image header update

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

The autoclear feature bits can be used for qcow2 file format features
that are safe to "drop" by old programs that do not understand the
feature.  Upon opening the image file unknown autoclear feature bits are
cleared and the image file header is rewritten, but this was happening
too early in the code when critical header fields were not yet loaded.

Process autoclear feature bits after all necessary header information
has been loaded.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
 block/qcow2.c |   17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index c2e49cd..79201fc 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -298,14 +298,6 @@ static int qcow2_open(BlockDriverState *bs, int flags)
         goto fail;
     }
 
-    if (!bs->read_only && s->autoclear_features != 0) {
-        s->autoclear_features = 0;
-        ret = qcow2_update_header(bs);
-        if (ret < 0) {
-            goto fail;
-        }
-    }
-
     /* Check support for various header values */
     if (header.refcount_order != 4) {
         report_unsupported(bs, "%d bit reference counts",
@@ -411,6 +403,15 @@ static int qcow2_open(BlockDriverState *bs, int flags)
         goto fail;
     }
 
+    /* Clear unknown autoclear feature bits */
+    if (!bs->read_only && s->autoclear_features != 0) {
+        s->autoclear_features = 0;
+        ret = qcow2_update_header(bs);
+        if (ret < 0) {
+            goto fail;
+        }
+    }
+
     /* Initialise locks */
     qemu_co_mutex_init(&s->lock);
 
-- 
1.7.10