block: Catch backing files assigned to non-COW drivers

Since we parse backing.* options to add a backing file from the command
line when the driver didn't assign one, it has been possible to have a
backing file for e.g. raw images (it just was never accessed).

This is obvious nonsense and should be rejected.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/block.c b/block.c
index 0978e8cd59..2c3c7608ee 100644
--- a/block.c
+++ b/block.c
@@ -1192,6 +1192,13 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
         bdrv_get_full_backing_filename(bs, backing_filename, PATH_MAX);
     }
 
+    if (!bs->drv || !bs->drv->supports_backing) {
+        ret = -EINVAL;
+        error_setg(errp, "Driver doesn't support backing files");
+        QDECREF(options);
+        goto free_exit;
+    }
+
     backing_hd = bdrv_new("", errp);
 
     if (bs->backing_format[0] != '\0') {