block: Disallow snapshots if the overlay doesn't support backing files

This addresses scenarios like this one:

  { 'execute': 'blockdev-add', 'arguments':
    { 'options': { 'driver': 'qcow2',
                   'node-name': 'new0',
                   'file': { 'driver': 'file',
                             'filename': 'new.qcow2',
                             'node-name': 'file0' } } } }

  { 'execute': 'blockdev-snapshot', 'arguments':
    { 'node': 'virtio0',
      'overlay': 'file0' } }

Signed-off-by: Alberto Garcia <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>

diff --git a/blockdev.c b/blockdev.c
index 3598b014196a9b762852509faa2b49ef2f5d2b5b..3197791c7b387cb744996c174c95e5c1868b328c 100644 (file)
--- a/blockdev.c
+++ b/blockdev.c
@@ -1678,6 +1678,11 @@ static void external_snapshot_prepare(BlkTransactionState *common,
 
     if (state->new_bs->backing != NULL) {
         error_setg(errp, "The snapshot already has a backing image");
+        return;
+    }
+
+    if (!state->new_bs->drv->supports_backing) {
+        error_setg(errp, "The snapshot does not support backing images");
     }
 }