Date: Wed, 26 Aug 2020 17:40:13 +0300 From: Konstantin Belousov <kib@freebsd.org> To: J David <j.david.lists@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: pidfile_open() usage in "mount" Message-ID: <20200826144013.GV2551@kib.kiev.ua> In-Reply-To: <CABXB=RRM7YusQL1gGVGD4s9xi9AB4F5AR5-Jqbp4G1WTZWWA%2BQ@mail.gmail.com> References: <CABXB=RRM7YusQL1gGVGD4s9xi9AB4F5AR5-Jqbp4G1WTZWWA%2BQ@mail.gmail.com>
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On Tue, Aug 25, 2020 at 10:06:22PM -0400, J David wrote:
> It looks like the "mount" program creates /var/run/mountd.pid every
> time it runs, if mountd is not itself running.
>
> This code appears in sbin/mount/mount.c:
>
> static void
> restart_mountd(void)
> {
> struct pidfh *pfh;
> pid_t mountdpid;
>
> mountdpid = 0;
> pfh = pidfile_open(_PATH_MOUNTDPID, 0600, &mountdpid);
> if (pfh != NULL) {
> /* Mountd is not running. */
> pidfile_remove(pfh);
> return;
> }
>
> pidfile_open(3) *creates* /var/run/mountd.pid if it doesn't already
> exist, hence the need to delete it if the call actually succeeds.
> This leads to a race condition when multiple mounts occur at the same
> time. That case is handled later in the code:
>
> /*
> * Refuse to send broadcast or group signals, this has
> * happened due to the bugs in pidfile(3).
> */
> if (mountdpid <= 0) {
> warnx("mountd pid %d, refusing to send SIGHUP", mountdpid);
> return;
> }
>
> "mount" is not "mountd." It seems inappropriate for it to, under any
> circumstances, create mountd's pid file. The multiple workarounds for
> the problems that causes don't seem like the optimal approach.
>
> This is something I'd be willing to open a bug and submit a patch for,
> but so as not to do work that stands no chance of being accepted, I'd
> like to understand first if the preferred approach would be to change
> mount.c not to use pidfile library calls at all, or if it would be
> better to add a function to the pidfile library similar to
> pidfile_open() designed for "consumer" use that would never create the
> file, leaving pidfile_open() for "producer" use?
I think that a new libutil/pidfile.c function, to open only existing
pid file, is the right approach.
It is possible that both pidfile_open() and the new function would share
some significant amount of code.
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