libGL error (r600_dri.so) - Permission denied

 For some time I've noticed an error with some screensavers (recent
versions of xscreensaver) on an x86_64 linux machine with an r600.
Instead of the saver running, I get a blank screen with small text
telling me libGL error: failed to load driver: r600

 At the moment I'm running xorg packages that were current a couple
of weeks ago, and I've eventually moved to xscreensaver-5.22 and
mesa
from the 9.2 branch as at 24th July, plus llvm-3.3.  This system is
gcc-4.8.0 but I've seen the problem on gcc-4.7 systems too, and I
was using llvm-3.2 with mesa-9.1.* until yesterday.

 I've finally got some explanation for this - invoking xscreensaver
with LIBGL_DEBUG=verbose and then running xscreensaver-demo with
e.g. moebius or glhanoi (need to click 'preview', the tiny examples
in the demo window work ok) gave me a little more information [
copied by hand, without the lines where it tries to load these
drivers from a different directory ]

libGL: OpenDriver: trying to use /usr/lib/dri/r600_dri.so
libGL error: failed to open drm device: Permission denied
libGL: OpenDriver: trying to use /usr/lib/dri/swrast_dri.so

 And then it sits there.

 The strange thing is that udev has correectly set up /dev/dri/card0
for root:video with mode 660, user ken is in the video group, and
the xscreensaver daemon has dropped privileges to run as ken.

 If I chmod /dev/dri/card0 to mode 666 then the preview works.

 I've no idea where the problem is happening, but I'd like to work
around it.  It seems to me that the most likely way is to alter the
udev default rule for SUBSYSTEM "drm" to add MODE="0660".  AFAICS
that will probably also affect /dev/dri/controlD64.  Does that sound
like a sensible workaround ?

 Anything I can do to help narrow down where the problem occurs ?

ĸen
-- 
das eine Mal als Tragödie, das andere Mal als Farce
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