The root user of Linux doesn't have permission to read/write a regular file, unless all users have permission to read/write this file, like below:
drwxrwsrwx ....... file_name
However the owner of the file can still read/write this file.
0) not the problem of execution, just read/write
1) not the problem of device because regular user can read/write it.
2) the result of command
uid=0(root) gid=0(root) groups=0(root)
so I don't think it's the problem of group.
So what other problems could be?