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Re: [gpsd-dev] PPS and privilege-dropping

From: Gary E. Miller
Subject: Re: [gpsd-dev] PPS and privilege-dropping
Date: Thu, 17 Oct 2013 14:55:30 -0700

Yo Harlan!

On Thu, 17 Oct 2013 20:11:08 +0000
Harlan Stenn <address@hidden> wrote:

> "Eric S. Raymond" writes:
> > 
> > The JSON PPS messages were designed with a specific purpose in mind
> > - to track the skew between NTP-adjusyed system clock and GPS+PPS
> > time so Dave Taht and I can benchmark NTP.
> And there was much rejoicing.

Can you tell me what you think it is doing, and where in the gpsd
code this is generated.  I see nothing like described anywhere near
the timekeeping code in gpsd.

> >> ALL ntpd prefers root for initialization.  There is serious loss of
> >> functionality when not initialized as root.
> > 
> > The HOWTO should explain this.  There's a reference to it in a
> > comment in ntpshm.c, but it didn't tell me enough to understand
> > *why*.
> I'm not sure what is meant here.  Some of the things ntpd needs root
> for include opening port 123, sometimes updating the time, and there
> may also be an issue for shmget() units 0 and 1 (which are 0600
> perms, while units 2 and 3 are 0666).  There may be other cases.

gpsd needs root for several things during timekeeping initialization:
        to give itself a high priority for minimal latency timekeeping.
        to access KPPS
        to access PPS on the GPS port if the port permissions are odd
        to select and access/create the proper ntpd shmget() units
        to select and access/create the proper chronyd socket file

At the moment proper timekeeping initialization can only happen when
gpsd is just started (ATM that is slightly broken).  If a device
is hotplugged into a runing gpsd initialization of KPPS is not
possible, the PPS device may not be accesible and the SHMs and sockets
will be different or unavailable.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

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