locked out at localhost login

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changed password to long auto generated one, pressed leave in error, lost password permanently, using root admin made no difference as it prompted reiteration of old lost password to allow me new one. got around it by ticking auto login. tripped up by selinux alert prompting password protected localhost login. tried frub but dont know response. this is on an imac, superdrive is bunged up with data disk i cannot eject so cannot reinstall from that i am self taught used various linux for 6 years without problem until this