From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

The logic around finding overlay here is not obvious. Actually it does
two simple things:

1. If new bs is already in backing chain, split from parent bs by
   several implicit filters we are done, do nothing.

2. Otherwise, don't try to replace implicit filter.

Let's rewrite this in more obvious way.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210610120537.196183-6-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block.c | 53 ++++++++++++++++-------------------------------------
 1 file changed, 16 insertions(+), 37 deletions(-)

diff --git a/block.c b/block.c
index 6472866283..d047cae43c 100644
--- a/block.c
+++ b/block.c
@@ -4237,7 +4237,7 @@ static int bdrv_reopen_parse_backing(BDRVReopenState *reopen_state,
                                      Error **errp)
 {
     BlockDriverState *bs = reopen_state->bs;
-    BlockDriverState *overlay_bs, *below_bs, *new_backing_bs;
+    BlockDriverState *new_backing_bs;
     QObject *value;
     const char *str;
 
@@ -4266,6 +4266,18 @@ static int bdrv_reopen_parse_backing(BDRVReopenState *reopen_state,
         g_assert_not_reached();
     }
 
+    if (bs->backing) {
+        if (bdrv_skip_implicit_filters(bs->backing->bs) == new_backing_bs) {
+            return 0;
+        }
+
+        if (bs->backing->bs->implicit) {
+            error_setg(errp, "Cannot change backing link if '%s' has "
+                       "an implicit backing file", bs->node_name);
+            return -EPERM;
+        }
+    }
+
     /*
      * Ensure that @bs can really handle backing files, because we are
      * about to give it one (or swap the existing one)
@@ -4283,42 +4295,9 @@ static int bdrv_reopen_parse_backing(BDRVReopenState *reopen_state,
         return -EINVAL;
     }
 
-    /*
-     * Find the "actual" backing file by skipping all links that point
-     * to an implicit node, if any (e.g. a commit filter node).
-     * We cannot use any of the bdrv_skip_*() functions here because
-     * those return the first explicit node, while we are looking for
-     * its overlay here.
-     */
-    overlay_bs = bs;
-    for (below_bs = bdrv_filter_or_cow_bs(overlay_bs);
-         below_bs && below_bs->implicit;
-         below_bs = bdrv_filter_or_cow_bs(overlay_bs))
-    {
-        overlay_bs = below_bs;
-    }
-
-    /* If we want to replace the backing file we need some extra checks */
-    if (new_backing_bs != bdrv_filter_or_cow_bs(overlay_bs)) {
-        int ret;
-
-        /* Check for implicit nodes between bs and its backing file */
-        if (bs != overlay_bs) {
-            error_setg(errp, "Cannot change backing link if '%s' has "
-                       "an implicit backing file", bs->node_name);
-            return -EPERM;
-        }
-
-        reopen_state->replace_backing_bs = true;
-        reopen_state->old_backing_bs = bs->backing ? bs->backing->bs : NULL;
-        ret = bdrv_set_backing_noperm(bs, new_backing_bs, set_backings_tran,
-                                      errp);
-        if (ret < 0) {
-            return ret;
-        }
-    }
-
-    return 0;
+    reopen_state->replace_backing_bs = true;
+    reopen_state->old_backing_bs = bs->backing ? bs->backing->bs : NULL;
+    return bdrv_set_backing_noperm(bs, new_backing_bs, set_backings_tran, errp);
 }
 
 /*
-- 
2.31.1