Re: sndioctl not working with mpd playing music

On Mon, Apr 27, 2020 at 07:38:25AM +0200, Alexandre Ratchov wrote:
> On Sun, Apr 26, 2020 at 08:40:08PM +0100, Stuart Henderson wrote:
> >
> > But....
> >
> > The problem with this is, on a multiuser system where you want to share
> > audio between mpd (or audio/squeezelite, which has the same problem) and
> > more than one user, you have to share a cookie between _mpd, user1 and
> > user2, so sndiod's privacy mechanism is nullified.
> >
>
> If mpd was allowed to bypass checks and play while either user1 or
> user2 are using the device, the privacy mechanism would be affected
> anyway. For instance user1 would be allowed to start music during
> phone-calls of user2.
>
> > As an alternative, perhaps the mechanism sndiod now uses for root could be
> > extended for users specified on sndiod's command line. Stupid hardcoded
> > example for mpd, clearly not committable as-is, but demonstrating the concept.
> >
>
> I'll try implement something like this, it's the safer.
>
> It would be nice if _mpd could start as root, open the device, drop
> privileges then. I just tried it, but this fails because mpd calls
> setuid() too early, probably not designed for this.
>
> The hack below makes mpd open the default device before main() is
> called, then it reuses the same handle all the time. It works for me,
> but if the device fails, mpd needs to be restarted; I don't know if
> this is a problem, I don't use mpd very often.

I like the idea (and this is probably how most openbsd daemons which do
privdrop work) but this really should be brought up w/ upstream :)
(which is friendly)

as its an always-running daemon, having the device failing should be
loud and clear so that the admin/user knows he has to restart, instead
of silently dying or failing.. what could cause the device failing,
sndiod dying ? plugging/unplugging a device ?

Fwiw ive been using the var/spool/mpd hack with the aucat cookie
sharing for many years, and yeah at some point it complained about perms
but moving to 750 worked for me. Just had to copy the cookie once, so i
dont think its a big deal to document it in mpd doc.

drwxr-x--- 4 _mpd _mpd 512 Apr 11 09:24 /var/spool/mpd
-rw------- 1 _mpd _mpd 16 Jan 16 2017 /var/spool/mpd/.aucat_cookie
-rw------- 1 _mpd _mpd 16 Oct 12 2018 /var/spool/mpd/.sndio/cookie

Im not sure we should address the 'many *real* users want to access
sndiod' case. And adding a quirk to 'whitelist' a given user within
sndiod feels a bit like a hidden knob...

Landry