Date: Wed, 21 Nov 2012 17:45:43 +0200 From: Jaakko Heinonen <jh@FreeBSD.org> To: Mateusz Guzik <mjguzik@gmail.com>, Ryan Stone <rysto32@gmail.com>, FreeBSD Current <freebsd-current@freebsd.org>, bapt@freebsd.org Subject: Re: pw keeps setting /etc/group to 0600 Message-ID: <20121121154542.GA1849@a91-153-116-96.elisa-laajakaista.fi> In-Reply-To: <20121119222843.GB22292@dft-labs.eu> References: <CAFMmRNxDr=%2BpsiazVrJ8e=T4fogiiPv5nEAo%2BnfoD=tPYMehCw@mail.gmail.com> <CAFMmRNxW0FzupbC9w4U5pPZUoqOE%2B3rqFNRUrnsMRGFnO7qPWA@mail.gmail.com> <20121119222843.GB22292@dft-labs.eu>
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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? > Now let's consider threads A and B: > > A: open() > A: lock(); > A: gr_copy > B: open() > > Now B has file descriptor to /etc/group that is about to be removed. > > A: rename() > A: unlock() > B: lock() > > Now B has a lock on unlinked file. > > B: gr_copy() > B: rename() > > ... and stores new content losing modifications done by A -- Jaakko
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