[v4,23/34] block: Make backing files child_of_bds children

diff --git a/block.c b/block.c
index 85b4f947ba..131ae20ffc 100644
--- a/block.c
+++ b/block.c
@@ -2941,6 +2941,20 @@  static bool bdrv_inherits_from_recursive(BlockDriverState *child,
     return child != NULL;
 }
 
+/*
+ * Return the BdrvChildRole for @bs's backing child.  bs->backing is
+ * mostly used for COW backing children (role = COW), but also for
+ * filtered children (role = FILTERED | PRIMARY).
+ */
+static BdrvChildRole bdrv_backing_role(BlockDriverState *bs)
+{
+    if (bs->drv && bs->drv->is_filter) {
+        return BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY;
+    } else {
+        return BDRV_CHILD_COW;
+    }
+}
+
 /*
  * Sets the backing file link of a BDS. A new reference is created; callers
  * which don't need their own reference any more must call bdrv_unref().
@@ -2968,8 +2982,8 @@  void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
         goto out;
     }
 
-    bs->backing = bdrv_attach_child(bs, backing_hd, "backing", &child_backing,
-                                    0, errp);
+    bs->backing = bdrv_attach_child(bs, backing_hd, "backing", &child_of_bds,
+                                    bdrv_backing_role(bs), errp);
     /* If backing_hd was already part of bs's backing chain, and
      * inherits_from pointed recursively to bs then let's update it to
      * point directly to bs (else it will become NULL). */
@@ -3066,7 +3080,7 @@  int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
     }
 
     backing_hd = bdrv_open_inherit(backing_filename, reference, options, 0, bs,
-                                   &child_backing, 0, errp);
+                                   &child_of_bds, bdrv_backing_role(bs), errp);
     if (!backing_hd) {
         bs->open_flags |= BDRV_O_NO_BACKING;
         error_prepend(errp, "Could not open backing file: ");
@@ -3895,8 +3909,8 @@  int bdrv_reopen_multiple(BlockReopenQueue *bs_queue, Error **errp)
         if (state->replace_backing_bs && state->new_backing_bs) {
             uint64_t nperm, nshared;
             bdrv_child_perm(state->bs, state->new_backing_bs,
-                            NULL, &child_backing, 0, bs_queue,
-                            state->perm, state->shared_perm,
+                            NULL, &child_of_bds, bdrv_backing_role(state->bs),
+                            bs_queue, state->perm, state->shared_perm,
                             &nperm, &nshared);
             ret = bdrv_check_update_perm(state->new_backing_bs, NULL,
                                          nperm, nshared, NULL, NULL, errp);
@@ -6852,7 +6866,7 @@  void bdrv_refresh_filename(BlockDriverState *bs)
         drv->bdrv_gather_child_options(bs, opts, backing_overridden);
     } else {
         QLIST_FOREACH(child, &bs->children, next) {
-            if (child->klass == &child_backing && !backing_overridden) {
+            if (child == bs->backing && !backing_overridden) {
                 /* We can skip the backing BDS if it has not been overridden */
                 continue;
             }
diff --git a/block/backup-top.c b/block/backup-top.c
index e2b4d2acd3..f059617095 100644
--- a/block/backup-top.c
+++ b/block/backup-top.c
@@ -143,7 +143,7 @@  static void backup_top_child_perm(BlockDriverState *bs, BdrvChild *c,
         return;
     }
 
-    if (child_class == &child_file) {
+    if (!(role & BDRV_CHILD_FILTERED)) {
         /*
          * Target child
          *
diff --git a/block/vvfat.c b/block/vvfat.c
index b4c8417dbd..e8848a0497 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -3219,7 +3219,8 @@  static void vvfat_child_perm(BlockDriverState *bs, BdrvChild *c,
 {
     BDRVVVFATState *s = bs->opaque;
 
-    assert(c == s->qcow || child_class == &child_backing);
+    assert(c == s->qcow ||
+           (child_class == &child_of_bds && (role & BDRV_CHILD_COW)));
 
     if (c == s->qcow) {
         /* This is a private node, nobody should try to attach to it */
diff --git a/tests/test-bdrv-drain.c b/tests/test-bdrv-drain.c
index b3d7960bd0..15393a0140 100644
--- a/tests/test-bdrv-drain.c
+++ b/tests/test-bdrv-drain.c
@@ -96,7 +96,7 @@  static void bdrv_test_child_perm(BlockDriverState *bs, BdrvChild *c,
      * bdrv_format_default_perms() accepts only these two, so disguise
      * detach_by_driver_cb_parent as one of them.
      */
-    if (child_class != &child_file && child_class != &child_backing) {
+    if (child_class != &child_file && child_class != &child_of_bds) {
         child_class = &child_file;
     }
 
@@ -1399,8 +1399,8 @@  static void test_detach_indirect(bool by_parent_cb)
     bdrv_ref(a);
     child_b = bdrv_attach_child(parent_b, b, "PB-B", &child_file, 0,
                                 &error_abort);
-    child_a = bdrv_attach_child(parent_b, a, "PB-A", &child_backing, 0,
-                                &error_abort);
+    child_a = bdrv_attach_child(parent_b, a, "PB-A", &child_of_bds,
+                                BDRV_CHILD_COW, &error_abort);
 
     bdrv_ref(a);
     bdrv_attach_child(parent_a, a, "PA-A",
@@ -1793,7 +1793,7 @@  static void test_drop_intermediate_poll(void)
     int i;
     int ret;
 
-    chain_child_class = child_backing;
+    chain_child_class = child_of_bds;
     chain_child_class.update_filename = drop_intermediate_poll_update_filename;
 
     for (i = 0; i < 3; i++) {
@@ -1816,7 +1816,7 @@  static void test_drop_intermediate_poll(void)
             /* Takes the reference to chain[i - 1] */
             chain[i]->backing = bdrv_attach_child(chain[i], chain[i - 1],
                                                   "chain", &chain_child_class,
-                                                  0, &error_abort);
+                                                  BDRV_CHILD_COW, &error_abort);
         }
     }
 
@@ -2034,7 +2034,8 @@  static void do_test_replace_child_mid_drain(int old_drain_count,
 
     bdrv_ref(old_child_bs);
     parent_bs->backing = bdrv_attach_child(parent_bs, old_child_bs, "child",
-                                           &child_backing, 0, &error_abort);
+                                           &child_of_bds, BDRV_CHILD_COW,
+                                           &error_abort);
 
     for (i = 0; i < old_drain_count; i++) {
         bdrv_drained_begin(old_child_bs);