Fix SELinux contexts for a SU binary

I am trying to root my android 6.0.1 phone using the following method: I first booted into a TWRP image using fastboot boot, then using the TWRP terminal I copied the su binary from the SuperSU kit to /system/xbin/ and ran chmod 6755 ./su, chown root:root ./su and chcon u:object_r:system_file:s0 ./su. Now I booted into Android and opened a terminal emulator and goto /system/xbin/ and see that the SUID bit is set for su. But when I run su, nothing happens and the binary exits with a 0 value. Running strace su shows that the setresuid() and setuid() calls are failing. I suspect this is because of SELinux contexts. What should the SELinux contexts be for the binary?