[Intel-gfx] What to do with xf86-video-intel backlight control when running Xorg as non root

[Chris Wilson]

On Thu, Feb 13, 2014 at 08:37:47PM +0100, Hans de Goede wrote:
> Hi,
> 
> On 02/13/2014 05:40 PM, Chris Wilson wrote:
> > Otherwise, it seems like we need the
> > proxy in order to keep the xrandr property available to users and
> > prevent those who rely upon it in scripts from seeing regressions.
> 
> Right, that is what I was thinking too, so the question then becomes how
> hard you will scream at me if I add something like this to xf86-video-intel
> linux specific backlight code:
> 
>     if (geteuid() == 0) {
>         /* Old write directly to /sys/class/backlight/... code */
>     {
>     else {
>         /* The & is to avoid the xserver blocking */
>         snprintf(command, sizeof(command), "pkexec %s/libexec/xf86-video-intel-backlight-helper %s %d&",
>                  PREFIX, sna_output->backlight_iface, level);
>         r = system(command);
>         if (r) {
>             /* complain */
>         }
>     }
> 
> If you won't scream too much, and more importantly, if you will accept such
> a patch (including code for the helper), then I'll try to cook up something
> like this tomorrow.

I feel comfortable enough with that approach, and have nothing better to
suggest. (I would just employ the fallback case if open(O_RDWR) fails.)
And limiting the helper to only write its value into
BACKLIGHT_CLASS/%{iface}/brightness should help reduce its attack surface.
Thinking about it, it would be useful if we verified that BACKLIGHT_CLASS
(both the ddx and its helper)was a sysfs directory in the first place.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre