After I copied some files from one computer (linux system) to another (linux also), the files are not able to be accessed by normal account. As it can be accessed by root, I chown those files to my username, but still cannot be accessed. While I try to ls -l those files, the accession limit, user and group are all displayed in "?", but can be seen normally when accessed as root. I guess it is something about security, but ls -Z provides same context as other normal files. What's the hell over there? Thanks!!