Having waited for 2 years, Qnap NAS finally supports folder level permission.
However, after enabling advanced folder permission, my phone rings again and again… non-stop. Many users complaint that they got access denied for some files and some folders randomly. When you login to the web interface, you can see that the owner field of that file/folder changed to the complainted user… For folders, you can still use the web interface to change it one by one.. but for flies, you can do nothing.
Finally… login to ssh and ll the folders… you will discover that the permissions are 070 !!! …. The owner got NO permission! simply do a chmod u+rwx -R * in the problematic folder can solve this problem… but use this method at your own risk.