参考：https://android.stackexchange.com/questions/214839/how-to-run-an-android-init-service-with-superuser-selinux-context

手机为小米4C，已rooted。
以dropbear为例

#如不修改权限会出现permission错误

~# chown 0.0 /etc/init/custom.rc
~# chmod 0644 /etc/init/custom.rc
~# chcon u:object_r:system_file:s0 /etc/init/custom.rc

~# chown 0.0 /system/xbin/dropbear
~# chmod 0644 /system/xbin/dropbear
~# chcon u:object_r:system_file:s0 /system/xbin/dropbear

# Service后面用sh脚本启动dropbear会错误127退出，原因不明
libra:/etc/init # cat dropbear.rc
service dropbear /system/xbin/dropbear -F
         class main
         seclabel u:r:su:s0
         user root
         group root
         disabled

 on property:dev.bootcomplete=1
         start dropbear

avc: denied 问题（SELinux）：

~# magiskpolicy --live 'allow init su process transition'
~# sepolicy-inject -s init -t su -c process -p transition -l