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Re: locking utmp



On Tue, Mar 19, 2002 at 05:33:13PM +0100, Russell Coker wrote:
> On Tue, 19 Mar 2002 03:34, Steve Langasek wrote:
> > It's a read lock, isn't it?  In which case, does it really matter how
> > many bytes it's locking?  (Unless, perhaps, requesting a read lock on

> If it keeps the read lock for an extended period of time and locks out other 
> processes then it could allow a local DOS attack.

AFAIK, read locks are not exclusive.

> > If the question is really "why does it want a read lock at all?", I
> > don't think I have an answer for that one.

> Getting a read lock on the utmp file before reading it is reasonable.  Not 
> sure why it needs to read it at all though.  Must be something in PAM.

Probably a 'session .* pam_unix.so' line in the PAM config for this
module.  Consensus among PAM developers is that utmp is the closest 
possible definition for a 'unix session', so calling the session
management functions of the pam_unix functions writes to utmp.

Steve Langasek
postmodern programmer

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