[Andriod]how to obtain root privilege by an app(app) running as system-uid

[Andriod]how to obtain root privilege by an app(app) running as system-uid

I've set sharedUserId as "android.ui.system" in manifest and get it run as
system(uid). so now the apk has system privilege,but what should I do if I
want obtain root privilege on Android devices. by the way,the Android
devices haven't been rooted yet,there is no su.
thanks.