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Date: Thu, 14 May 2009 20:33:49 +0200
From: Marcus Meissner <>
To: OSS Security List <>
Subject: utmp reliability?


While debugging a problem for a customer of us I found that UTMP entries
are not generated reliably and utmp entry creation starts to fail on
higher loads/more users or more utmp activity.

Reason is that glibc locks the UTMP file on write/read with a 1 second
timeout, and if that 1 second timeout triggers no entry is written.

Sadly without any indication of failure if you use login(3) or pututline(3). :(

I am wondering if handling UTMP should be more reliable or if at least error
reporting should happen.

Making it reliable will make it possible to DoS any kind of logins / terminal
opens of course, but make it easier on app writers using utmp.

(Yes, I know the really reliable way is Audit, but this is not user readable.)

Ciao, Marcus

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