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Re: [gpsd-dev] Time Service HOWTO in nearly final form; please review

From: Eric S. Raymond
Subject: Re: [gpsd-dev] Time Service HOWTO in nearly final form; please review
Date: Sat, 26 Oct 2013 11:07:49 -0400
User-agent: Mutt/1.5.21 (2010-09-15)

Beat Bolli <address@hidden>:
> On 26.10.13 14:23, Eric S. Raymond wrote:
> > Beat Bolli <address@hidden>:
> >> The section that explains the differences between root vs. non-root
> >> operation and devices on the command line vs. hotplug needs to be
> >> adjusted now that the root privileges are dropped after setting up the
> >> SHM segments and raising the PPS thread priority (commit
> >> 247c5825172137f337a79a1ff2f73b4af2899f66).
> > 
> > I'm not seeing how, sorry.  Gary moved the thread interlock to avoid a 
> > race condition, and this does mean time_pps_create() will always be
> > called with root permissions.  But it appears to me that this just
> > removes a bug that prevented the code from behaving as documented.
> But would this not imply that the hot-plugged GPS would also have access
> to the unit 0 and 1 SHM regions?

Alas, no.  I should have said "always be called with root permissions
for a command-line device".  Trace the call chain: the relevant section
of code is in ntpshm_pps(), which is called by gpsd_ppsmonitor(), which
is called by pps_thread_activate(), which is called by the ntpshm.c entry point

That is called by gpsd_add_device(), which can be called under one of
two circumstances. It may be processing a command-line path, in which
case root permissions are in effect.  Or it may be called to open a path
passed down the gpsd control socket as the result of a hotplug attempt,
in which case root privileges will have been dropped.
                <a href="";>Eric S. Raymond</a>

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