centos 6.2 在 root 已为所有用户设置读取权限时拒绝读取权限

在 root 拥有的 Centos 6.2 上安装 Dropbox。证明 dropbox 文件夹归 root 所有，对所有人具有读取权限

“root”用户授予所有用户读取权限

[root@localhost Dropbox]# ls -la
total 436
drwxr--r--. 6 root root   4096 Jul 17 20:01 .
drwxr-xr-x. 3 root root   4096 Jul 11 20:11 ..
-rw-r--r--. 1 root root     29 Jul 17 20:01 .dropbox
drwxr-xr-x. 4 root root 151552 Jul 17 20:03 .dropbox.cache
drwxr-xr-x. 2 root root   4096 Jul 11 19:28 ePubs to Share
drwxr-xr-x. 4 root root   4096 Jul 11 20:14 ePubs to Share (1)
-rw-r--r--. 1 root root 269894 Jun  7  2010 Getting Started.pdf
drwxr-xr-x. 6 root root   4096 Jul 17 20:03 stagbuild
[root@localhost Dropbox]# cd ..
[root@localhost DropboxRoot]# ls -la
total 12
drwxr-xr-x.  3 root root 4096 Jul 11 20:11 .
drwx------. 48 epub epub 4096 Jul 17 20:01 ..
drwxr--r--.  6 root root 4096 Jul 17 20:01 Dropbox
[root@localhost DropboxRoot]# 

但是“epub”用户无法查看/读取此目录中的任何文件：

cd DropboxRoot
[epub@localhost DropboxRoot]$ ls -la
total 12
drwxr-xr-x.  3 root root 4096 Jul 11 20:11 .
drwx------. 48 epub epub 4096 Jul 17 20:01 ..
drwxr--r--.  6 root root 4096 Jul 17 20:01 Dropbox
[epub@localhost DropboxRoot]$ ls -la Dropbox/
ls: cannot access Dropbox/stagbuild: Permission denied
ls: cannot access Dropbox/Getting Started.pdf: Permission denied
ls: cannot access Dropbox/ePubs to Share (1): Permission denied
ls: cannot access Dropbox/.: Permission denied
ls: cannot access Dropbox/..: Permission denied
ls: cannot access Dropbox/.dropbox.cache: Permission denied
ls: cannot access Dropbox/ePubs to Share: Permission denied
ls: cannot access Dropbox/.dropbox: Permission denied
total 0
d????????? ? ? ? ?            ? .
d????????? ? ? ? ?            ? ..
-????????? ? ? ? ?            ? .dropbox
d????????? ? ? ? ?            ? .dropbox.cache
d????????? ? ? ? ?            ? ePubs to Share
d????????? ? ? ? ?            ? ePubs to Share (1)
-????????? ? ? ? ?            ? Getting Started.pdf
d????????? ? ? ? ?            ? stagbuild
[epub@localhost DropboxRoot]$ 

有任何想法吗？