Usually I start updates on the remote machine with nohup and then I do logout. After the last update I can login with ssh as common user. Then I try to increace priviliges with su and get error "su: Authentication failure" without asking for root password. I repeated this issue on the other machine with local access and I realize that the reason is updating sys-auth/pambase to 20120417. It needs files /etc/pam.d/su /etc/pam.d/login and /etc/pam.d/passwd to present. They are installed by portage with ._cfg0000_ prefix. It causes inaccessable system until etc-update called, but etc-update can be called only after access. It is like moment 22. I belive it is a serious bug. In addition to broken su, It causes failing to login in physical console for any user with error "login incorrect". Also, I can't understand why portage installs these files with prefx, while files with original names is not present in system. For reproduce the behavior in working system with pambase-20120417, you can rename su, login and passwd in /etc/pam.d with ._cfg0000_ or any other prefix.
*** This bug has been marked as a duplicate of bug 412721 ***
