statmount: allow for root to be NULL

Author: bsach64

fs/namespace.c

@@ -5365,6 +5365,12 @@ static int statmount_mnt_root(struct kstatmount *s, struct seq_file *seq)
 	return 0;
 }
 
+static int statmount_mnt_point_detached(struct kstatmount *s, struct seq_file *seq)
+{
+	seq_puts(seq, "[detached]");
+	return 0;
+}
+
 static int statmount_mnt_point(struct kstatmount *s, struct seq_file *seq)
 {
 	struct vfsmount *mnt = s->mnt;
@@ -5589,7 +5595,11 @@ static int statmount_string(struct kstatmount *s, u64 flag)
 		break;
 	case STATMOUNT_MNT_POINT:
 		offp = &sm->mnt_point;
-		ret = statmount_mnt_point(s, seq);
+		if (!s->root.mnt && !s->root.dentry)
+			/* detached mount case */
+			ret = statmount_mnt_point_detached(s, seq);
+		else
+			ret = statmount_mnt_point(s, seq);
 		break;
 	case STATMOUNT_MNT_OPTS:
 		offp = &sm->mnt_opts;
@@ -5736,24 +5746,29 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
 	int err;
 
 	/* Has the namespace already been emptied? */
-	if (mnt_ns_id && mnt_ns_empty(ns))
+	if (!is_umount_ns(ns) && mnt_ns_id && mnt_ns_empty(ns))
 		return -ENOENT;
 
 	s->mnt = lookup_mnt_in_ns(mnt_id, ns);
 	if (!s->mnt)
 		return -ENOENT;
 
-	err = grab_requested_root(ns, &root);
-	if (err)
-		return err;
+	if (!is_umount_ns(ns)) {
+		err = grab_requested_root(ns, &root);
+		if (err)
+			return err;
+	}
 
 	/*
 	 * Don't trigger audit denials. We just want to determine what
 	 * mounts to show users.
 	 */
 	m = real_mount(s->mnt);
-	if (!is_path_reachable(m, m->mnt.mnt_root, &root) &&
-	    !ns_capable_noaudit(ns->user_ns, CAP_SYS_ADMIN))
+
+	if (!is_umount_ns(ns) && !is_path_reachable(m, m->mnt.mnt_root, &root))
+		return -EPERM;
+
+	if (!ns_capable_noaudit(ns->user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 
 	err = security_sb_statfs(s->mnt->mnt_root);