vsftpd - 500 OOPS: vsftpd: refusing to run with writable root inside chroot() - /usr/share/empty/ - centos7

问题：centos7系统，用rpm安装的vsftpd，systemctl启动后，ftp localhost 连接会报错：500 OOPS: vsftpd: refusing to run with writable root inside chroot()

尝试：

1. 在vsftpd.conf配置文件里添加 allow_writeable_chroot=YES ，无效
2. 尝试在SELinux里设置权限 setsebool -P allow_ftpd_full_access on ，无效
3. 尝试更换新版本的rpm安装，无效
4. 手动启动vsftpd，在命令行里逐个传递参数测试。当测试到 secure_chroot_dir 指定值时，有效
5. 反查到 /usr/share/empty 目录有全用户写权限（777），取消写权限（755，root属主）。再使用sysmtemctl启动，有效

secure_chroot_dir

This option should be the name of a directory which is empty. Also, the directory should not be writable by the ftp user. This directory is used as a secure chroot() jail at times vsftpd does not require filesystem access.
Default: /usr/share/empty

参考：

• https://security.appspot.com/vsftpd/vsftpd_conf.html
• https://security.appspot.com/vsftpd/FAQ.txt
• https://stackoverflow.com/questions/16102996/error-message-500-oops-vsftpd-refusing-to-run-with-writable-root-inside-chroo