block: Allow backing file links in change_parent_backing_link()

Now that the backing file child role implements .attach/.detach
callbacks, nothing prevents us from modifying the graph even if that
involves changing backing file links.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Acked-by: Fam Zheng <famz@redhat.com>

1 files changed, 3 insertions, 4 deletions

diff --git a/block.c b/block.c
index 698a5c7e0b..79d9ad0ad6 100644
--- a/block.c
+++ b/block.c
@@ -2882,9 +2882,9 @@ static void change_parent_backing_link(BlockDriverState *from,
             continue;
         }
         if (c->role == &child_backing) {
-            /* @from is generally not allowed to be a backing file, except for
-             * when @to is the overlay. In that case, @from may not be replaced
-             * by @to as @to's backing node. */
+            /* If @from is a backing file of @to, ignore the child to avoid
+             * creating a loop. We only want to change the pointer of other
+             * parents. */
             QLIST_FOREACH(to_c, &to->children, next) {
                 if (to_c == c) {
                     break;
@@ -2895,7 +2895,6 @@ static void change_parent_backing_link(BlockDriverState *from,
             }
         }
 
-        assert(c->role != &child_backing);
         bdrv_ref(to);
         /* FIXME Are we sure that bdrv_replace_child() can't run into
          * &error_abort because of permissions? */