+static int ovl_encode_maybe_copy_up(struct dentry *dentry)
+{
+ int err;
+
+ if (ovl_dentry_upper(dentry))
+ return 0;
+
+ err = ovl_want_write(dentry);
+ if (!err) {
+ err = ovl_copy_up(dentry);
+ ovl_drop_write(dentry);
+ }
+
+ if (err) {
+ pr_warn_ratelimited("overlayfs: failed to copy up on encode (%pd2, err=%i)\n",
+ dentry, err);
+ }
+
+ return err;
+}
+
+/*
+ * Before encoding a non-upper directory file handle from real layer N, we need
+ * to check if it will be possible to reconnect an overlay dentry from the real
+ * lower decoded dentry. This is done by following the overlay ancestry up to a
+ * "layer N connected" ancestor and verifying that all parents along the way are
+ * "layer N connectable". If an ancestor that is NOT "layer N connectable" is
+ * found, we need to copy up an ancestor, which is "layer N connectable", thus
+ * making that ancestor "layer N connected". For example:
+ *
+ * layer 1: /a
+ * layer 2: /a/b/c
+ *
+ * The overlay dentry /a is NOT "layer 2 connectable", because if dir /a is
+ * copied up and renamed, upper dir /a will be indexed by lower dir /a from
+ * layer 1. The dir /a from layer 2 will never be indexed, so the algorithm (*)
+ * in ovl_lookup_real_ancestor() will not be able to lookup a connected overlay
+ * dentry from the connected lower dentry /a/b/c.
+ *
+ * To avoid this problem on decode time, we need to copy up an ancestor of
+ * /a/b/c, which is "layer 2 connectable", on encode time. That ancestor is
+ * /a/b. After copy up (and index) of /a/b, it will become "layer 2 connected"
+ * and when the time comes to decode the file handle from lower dentry /a/b/c,
+ * ovl_lookup_real_ancestor() will find the indexed ancestor /a/b and decoding
+ * a connected overlay dentry will be accomplished.
+ *
+ * (*) the algorithm in ovl_lookup_real_ancestor() can be improved to lookup an
+ * entry /a in the lower layers above layer N and find the indexed dir /a from
+ * layer 1. If that improvement is made, then the check for "layer N connected"
+ * will need to verify there are no redirects in lower layers above N. In the
+ * example above, /a will be "layer 2 connectable". However, if layer 2 dir /a
+ * is a target of a layer 1 redirect, then /a will NOT be "layer 2 connectable":
+ *
+ * layer 1: /A (redirect = /a)
+ * layer 2: /a/b/c
+ */
+
+/* Return the lowest layer for encoding a connectable file handle */
+static int ovl_connectable_layer(struct dentry *dentry)
+{
+ struct ovl_entry *oe = OVL_E(dentry);
+
+ /* We can get overlay root from root of any layer */
+ if (dentry == dentry->d_sb->s_root)
+ return oe->numlower;
+
+ /*
+ * If it's an unindexed merge dir, then it's not connectable with any
+ * lower layer
+ */
+ if (ovl_dentry_upper(dentry) &&
+ !ovl_test_flag(OVL_INDEX, d_inode(dentry)))
+ return 0;
+
+ /* We can get upper/overlay path from indexed/lower dentry */
+ return oe->lowerstack[0].layer->idx;
+}
+
+/*
+ * @dentry is "connected" if all ancestors up to root or a "connected" ancestor
+ * have the same uppermost lower layer as the origin's layer. We may need to
+ * copy up a "connectable" ancestor to make it "connected". A "connected" dentry
+ * cannot become non "connected", so cache positive result in dentry flags.
+ *
+ * Return the connected origin layer or < 0 on error.
+ */
+static int ovl_connect_layer(struct dentry *dentry)
+{
+ struct dentry *next, *parent = NULL;
+ int origin_layer;
+ int err = 0;
+
+ if (WARN_ON(dentry == dentry->d_sb->s_root) ||
+ WARN_ON(!ovl_dentry_lower(dentry)))
+ return -EIO;
+
+ origin_layer = OVL_E(dentry)->lowerstack[0].layer->idx;
+ if (ovl_dentry_test_flag(OVL_E_CONNECTED, dentry))
+ return origin_layer;
+
+ /* Find the topmost origin layer connectable ancestor of @dentry */
+ next = dget(dentry);
+ for (;;) {
+ parent = dget_parent(next);
+ if (WARN_ON(parent == next)) {
+ err = -EIO;
+ break;
+ }
+
+ /*
+ * If @parent is not origin layer connectable, then copy up
+ * @next which is origin layer connectable and we are done.
+ */
+ if (ovl_connectable_layer(parent) < origin_layer) {
+ err = ovl_encode_maybe_copy_up(next);
+ break;
+ }
+
+ /* If @parent is connected or indexed we are done */
+ if (ovl_dentry_test_flag(OVL_E_CONNECTED, parent) ||
+ ovl_test_flag(OVL_INDEX, d_inode(parent)))
+ break;
+
+ dput(next);
+ next = parent;
+ }
+
+ dput(parent);
+ dput(next);
+
+ if (!err)
+ ovl_dentry_set_flag(OVL_E_CONNECTED, dentry);
+
+ return err ?: origin_layer;
+}
+