[Qemu-devel] [PATCH v2 5/7] block: Fix order in bdrv_replace_child()

[Max Reitz <mreitz@redhat.com>]

We have to start by applying the permission restrictions to new_bs
before we can loosen them on old_bs.  See the comment for the
explanation.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
---
 block.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/block.c b/block.c
index be11504940..da7da8cc6c 100644
--- a/block.c
+++ b/block.c
@@ -2205,6 +2205,19 @@ static void bdrv_replace_child(BdrvChild *child, BlockDriverState *new_bs)
 
     bdrv_replace_child_noperm(child, new_bs);
 
+    /*
+     * Start with the new node's permissions.  If @new_bs is a (direct
+     * or indirect) child of @old_bs, we must complete the permission
+     * update on @new_bs before we loosen the restrictions on @old_bs.
+     * Otherwise, bdrv_check_perm() on @old_bs would re-initiate
+     * updating the permissions of @new_bs, and thus not purely loosen
+     * restrictions.
+     */
+    if (new_bs) {
+        bdrv_get_cumulative_perm(new_bs, &perm, &shared_perm);
+        bdrv_set_perm(new_bs, perm, shared_perm);
+    }
+
     if (old_bs) {
         /* Update permissions for old node. This is guaranteed to succeed
          * because we're just taking a parent away, so we're loosening
@@ -2213,11 +2226,6 @@ static void bdrv_replace_child(BdrvChild *child, BlockDriverState *new_bs)
         bdrv_check_perm(old_bs, NULL, perm, shared_perm, NULL, &error_abort);
         bdrv_set_perm(old_bs, perm, shared_perm);
     }
-
-    if (new_bs) {
-        bdrv_get_cumulative_perm(new_bs, &perm, &shared_perm);
-        bdrv_set_perm(new_bs, perm, shared_perm);
-    }
 }
 
 BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
-- 
2.20.1