udev rules not working on Arch

[StaticRocket]

OS: Arch Linux x86_64
Host: UX390UAK 1.0
Kernel: 4.18.5-arch1-1-ARCH

Udev rules

ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chgrp video /sys/class/backlight/%k/brightness"
ACTION=="add", SUBSYSTEM=="backlight", RUN+="/bin/chmod g+w /sys/class/backlight/%k/brightness"
ACTION=="add", SUBSYSTEM=="leds", RUN+="/bin/chgrp input /sys/class/leds/%k/brightness"
ACTION=="add", SUBSYSTEM=="leds", RUN+="/bin/chmod g+w /sys/class/leds/%k/brightness"

/bin/chgrp video /sys/class/backlight/%k/brightness returns
/bin/chgrp: cannot access '/sys/class/backlight/%k/brightness': No such file or directory

/sys/class/backlight is a file, not a directory, and it contains the following (viewed using nvim):

" ============================================================================
" Netrw Directory Listing                                        (netrw v156)
"   /sys/class/backlight
"   Sorted by      name
"   Sort sequence: [\/]$,\<core\%(\.\d\+\)\=\>,\.h$,\.c$,\.cpp$,\~\=\*$,*,\.o$,\.obj$,\.info$,\.swp$,\.bak$,\~$
"   Quick Help: <F1>:help  -:go up dir  D:delete  R:rename  s:sort-by  x:special
" ==============================================================================
../                                                                                                                  
./
intel_backlight@                  --> /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight

---

The same thing is occurring with the leds "directory". The current workaround is adding <username> ALL=(ALL) NOPASSWD: /bin/brightnessctl to the sudoers file as hinted by the readme.

[Hummer12007]

What does ls -l /sys/class/backlight/intel_backlight/ output?

[StaticRocket]

ls -l /sys/class/backlight/intel_backlight/ returns

total 0
-r--r--r-- 1 root root 4096 Sep  4 10:24 actual_brightness
-rw-r--r-- 1 root root 4096 Sep  4 10:24 bl_power
-rw-r--r-- 1 root root 4096 Sep  4 10:54 brightness
lrwxrwxrwx 1 root root    0 Sep  4 10:24 device -> ../../card0-eDP-1
-r--r--r-- 1 root root 4096 Sep  4 10:24 max_brightness
drwxr-xr-x 2 root root    0 Sep  4 10:58 power
lrwxrwxrwx 1 root root    0 Sep  4 10:24 subsystem -> ../../../../../../../class/backlight
-r--r--r-- 1 root root 4096 Sep  4 10:24 type
-rw-r--r-- 1 root root 4096 Sep  4 10:24 uevent

[Hummer12007]

Let me double-check with you: are you executing the /bin/chgrp video /sys/class/backlight/%k/brightness in the shell, as opposed to adding udev rules?

[StaticRocket]

I noticed the udev rule wasn't allowing the script to run without root privileges properly, so I ran the commands it was supposed to in the shell just to get some information for an issue report. (I'm new to troubleshooting udev rules.)

[Hummer12007]

I see, how weird. For the record, udev replaces %k with the device name (intel_backlight in this case).

[Hummer12007]

Can you try it on the latest commit btw? (i.e. reinstall brightnessctl and reboot)

[StaticRocket]

Done. No dice. (All I see is a version bump in the commits, so I don't think that will help much.)

[Hummer12007]

And another question. Is your user account in the video group?

[StaticRocket]

It wasn't, but I just added it and there was no effect.

[nkeor]

It happens to me too, but adding a sudo rule does not work. Arch up-to-date kernel 4.18.6

[ManuelSchmitzberger]

I've used the change action instead of the add and everything works:

    ACTION=="change", SUBSYSTEM=="backlight", KERNEL=="radeon_b10", RUN+="/usr/bin/chgrp video /sys/class/backlight/%k/brightness"
    ACTION=="change", SUBSYSTEM=="backlight", KERNEL=="radeon_b10", RUN+="/usr/bin/chmod g+w /sys/class/backlight/%k/brightness"

[gdamjan]

The proper way is to set the udev rules with uaccess:

ACTION=="add", SUBSYSTEM=="backlight", TAG+="uaccess"
ACTION=="add", SUBSYSTEM=="leds", TAG+="uaccess"

(edit: which (acls) is not supported on sysfs sigh)

[marcusbritanicus]

I have faced this issue. Till date I cannot directly modify /sys/class/backlight/%k/brightness and there seems to be a good reason behind it: /sys/class/backlight/%k is a symlink created and owned by root. So when you do chgrp on it, the link group does not change, but, the group of the link's target changes.

So in my case doing echo 100 > /sys/class/backlight/radeon_bl0/brightness gives permission error, where as echo 100 > /sys/devices/pci0000:00/0000:00:01.0/drm/card0/card0-eDP-1/radeon_bl0/brightness works like a charm!

[Hummer12007]

Is it still broken in brightnessctl in community?

[gdamjan]

systemd-logind 243 (just around the corner now) will provide a dbus api for setting the brightness for the users seat.

[goetzc]

Is it still broken in brightnessctl in community?

@Hummer12007 yes.

[jay-aye-see-kay]

Is it still broken in brightnessctl in community?

community/brightnessctl 0.4-2 is working for me, my issue was not having my user in the video` group

For anyone else who finds this thread: sudo usermod -aG video $USER then logout and back in to add yourself to the video group

[goetzc]

@jay-aye-see-kay right, I missed this step from the README. As the Arch package no longer set the binary as suid, but ships the udev rules, the user has to be in the video group as you mention.

[gdamjan]

since systemd 243 you don't need any special permissions on the system to change the backlight brightness. a user can do it for their seat by dbus calling logind.

this is already supported in brightnessctl, but we are waiting for a 0.5 release (hint hint @Hummer12007)

[gonzalezjo]

I know this is a bit old, and this has seemingly been fixed on systems with a newer version of systemd, but this is a working solution in the meantime: Instead of chgrp, you can use chgrp -h to avoid dereferencing the symlink.