Re: [gpsd-users] gpsd/udev behavior

[Paul Fox]

eric s. raymond wrote:
 > Paul Fox <address@hidden>:
 > > manually changing the device permissions from 660 to 666 makes
 > > everything work.
 > > 
 > > the machine is an OLPC XO laptop, so it's entirely possible that it's
 > > a problem with our version of F17, but we don't really touch udev
 > > much.  (i say "we", because my work address is @laptop.org.)  any tips
 > > how this _should_ be working?
 > 
 > This is probably a distro-specific issue brought about by the choice
 > of default ownership or default permissions on the TTY devices.  Or it
 > could be the result of a minor packaging error. See the code near the
 > the following comment in gpsd.c
 > 
 >      /*
 >       * Drop privileges.  Up to now we've been running as root.  Instead,
 >       * set the user ID to 'nobody' (or whatever the --enable-gpsd-user
 >       * is) and the group ID to the owning group of a prototypical TTY
 >       * device. This limits the scope of any compromises in the code.
 >       * It requires that all GPS devices have their group read/write
 >       * permissions set.
 >       */
 > 
 > I'd explain in detail, but I think the comment and code are clear enough.

yes, thank you.  exactly the nudge i needed, and it explains
everything.

in the case of invocation via udev, there is no prototypical TTY
device, so the code falls back on /dev/ttyS0 for the group id.  that
was certainly a reasonable fallback for a legacy PC especially in the
pre-udev world, but gpsd might need more flexibility now. 

on the current XO ARM-based laptop, for instance, there is no ttyS0 at
all.  (there's a ttyS2, and sometimes a ttyS3, but no S0 or S1 for
reasons i won't go into right now.) so gpsd can't open the
prototypical tty, and ends up running as nobody:root.  it therefore
can't open /dev/ttyUSB0 which is owned (on the XO) by root:dialout.

as a stopgap, since all gpsd does is stat() the prototypical device,
this is a useable workaround:
    touch /dev/ttyS0; chown root:dialout /dev/ttyS0

the answer might be a command-line argument (i.e., "-g gid") as a hook
that could be set via /etc/{default,sysconfig}/gpsd.  i'd be happy
to contribute that patch if you thought it was a good idea.  (i
understand the dangers of option creep.)

another (likely unacceptable :-) option would be a commandline flag
telling gpsd not to drop privileges.

(on the XO, we actually have a bigger issue concerning USB device
permissions generally, and the solution will likely end up making this
problem with gpsd moot.  but it won't be a generally useable solution
for others:  http://dev.laptop.org/ticket/11877 )

paul

=---------------------
 paul fox, address@hidden (arlington, ma, where it's 63.0 degrees)