Linux

% type -a su                                                                                               81
su is /bin/su

% ltrace su
Password:
--- SIGCHLD (Child exited) ---
su: incorrect password
+++ exited (status 1) +++

authentication failure

$ sudo passwd root

$ sudo passwd root

% cat /etc/pam.d/su
#%PAM-1.0
auth       sufficient   /lib/security/$ISA/pam_rootok.so
# Uncomment the following line to implicitly trust users in the "wheel" group.
#auth       sufficient   /lib/security/$ISA/pam_wheel.so trust use_uid
# Uncomment the following line to require a user to be in the "wheel" group.
#auth       required     /lib/security/$ISA/pam_wheel.so use_uid
auth       required     /lib/security/$ISA/pam_stack.so service=system-auth
account    sufficient   /lib/security/$ISA/pam_succeed_if.so uid=0 use_uid quiet
account    required     /lib/security/$ISA/pam_stack.so service=system-auth
password   required     /lib/security/$ISA/pam_stack.so service=system-auth
# pam_selinux.so close must be first session rule
session    required     /lib/security/$ISA/pam_selinux.so close
session    required     /lib/security/$ISA/pam_stack.so service=system-auth
# pam_selinux.so open and pam_xauth must be last two session rules
session    required     /lib/security/$ISA/pam_selinux.so open
session    optional     /lib/security/$ISA/pam_xauth.so