Re: pw keeps setting /etc/group to 0600

On Wed, Nov 21, 2012 at 05:45:43PM +0200, Jaakko Heinonen wrote:
> On 2012-11-19, Mateusz Guzik wrote:
> > First, pw should not fail if other instance is running, it should wait
> > instead (think of parallel batch scripts adding some users/groups).
> > 
> > Second, current code has a race:
> > lockfd = open(group_file, O_RDONLY, 0);
> > if (lockfd < 0 || fcntl(lockfd, F_SETFD, 1) == -1)
> > 	err(1, "%s", group_file);
> > if (flock(lockfd, LOCK_EX|LOCK_NB) == -1) {
> > [..]
> > gr_copy(pfd, tfd, gr, old_gr); /* copy from groupfile to tempfile */
> > [..]
> > rename(tempfile,groupfile);
> 
> Hmm, could using the O_EXLOCK flag for open() instead of flock() help here?
> 

Yes, this would fix the race.

But the problem of pw exiting due to other process holding the lock
remains. And I think that fixing it will require holding a lock over
whole time pw is running so that we have stable snapshot of user base at
least in regard of local files.

One could create one lock, say /etc/.pw.lock, that would be used to
synchronize any changes to /etc/master.passwd, /etc/group and whatnot.

And then there is this API issue (but maybe this is just me
nitpicking).

-- 
Mateusz Guzik <mjguzik gmail.com>
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