Hi,
Android 5 after the authority to control the beginning of the strict, 5.1.1 to 6 seems to be the application of the rights management plus, you have to ask Android official friends, we have no way "
Android 5.1.1 default enabled serial port login (shell user) and adb root privileges, for the application (apk) root, you need to use the third party root tool, you may need to use the third party su to replace the system comes with Su (/ system / xbin / su)
1) serial port is su after root, adb login is root;
2) change Android, modify or replace su;
3) change the kernel, add a back door;
Thanks!