有兴趣的可以去看一下这篇文档：http://selinuxproject.org/page/NB_SEforAndroid_1

SELinux support for the ext4 file system. Note that the make_ext4fs utility is used to build these file systems and relies on the file_contexts file having all the relevant entries, if not, it will be unable to set the security.selinux xattr on the inode and fail.