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

From: Beat Bolli
Subject: Re: [gpsd-dev] Time Service HOWTO in nearly final form; please review
Date: Sat, 26 Oct 2013 18:06:25 +0200
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On 26.10.13 17:07, Eric S. Raymond wrote:
> 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
> ntpd_link_activate().  
> 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.

Oh, I see now. Thanks for the explanation.

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