pam_rootok.so  [
        debug
      ]
pam_rootok is a PAM module that authenticates the user if their UID is 0. Applications that are created setuid-root generally retain the UID of the user but run with the authority of an enhanced effective-UID. It is the real UID that is checked.
      In the case of the su(1) application the historical usage is to
      permit the superuser to adopt the identity of a lesser user
      without the use of a password. To obtain this behavior with PAM
      the following pair of lines are needed for the corresponding entry
      in the /etc/pam.d/su configuration file:
      
# su authentication. Root is granted access by default.
auth  sufficient   pam_rootok.so
auth  required     pam_unix.so