Bug#774617: gdm3: pkexec does not allow Debian-gdm to change the backlight brightness

[Simon McVittie]

Package: gdm3
Version: 3.14.1-3
Severity: normal

https://bugs.debian.org/762676 means that, when docked, my laptop
boots to a state in which the greeter is not visible: the laptop's
backlight is off, and the external monitor is blank (grey background)
because the laptop was selected as the primary display.

Pressing the "backlight up" key (Fn+Home on my Thinkpad X220) brings
the brightness up by 1 step, but does not go beyond that. Additionally,
I get lots of these messages in the journal:

> Jan 05 11:26:57 archetype pkexec[2971]: Debian-gdm: The
> value for the SHELL variable was not found the /etc/shells
> file [USER=root] [TTY=unknown] [CWD=/var/lib/gdm3]
> [COMMAND=/usr/lib/gnome-settings-daemon/gsd-backlight-helper
> --set-brightness 232]
> Jan 05 11:26:57 archetype gnome-session[2335]: The value for the SHELL
> variable was not found the /etc/shells file
> Jan 05 11:26:57 archetype gnome-session[2335]: This incident has been
> reported.

This might be because Debian-gdm has /bin/false as its shell:

> % getent passwd Debian-gdm
> Debian-gdm:x:114:121:Gnome Display Manager:/var/lib/gdm3:/bin/false

Workaround: Ctrl+Alt+F6 (switch to text mode, backlight comes on at full
brightness); Ctrl+Alt+F7 (switch back to gdm, backlight stays on).

-- System Information:
Debian Release: 8.0
  APT prefers proposed-updates
  APT policy: (500, 'proposed-updates'), (500, 'unstable'), (500, 'testing'), (500, 'stable'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 3.16.0-4-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_GB.utf8, LC_CTYPE=en_GB.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages gdm3 depends on:
ii  accountsservice                       0.6.37-3+b1
ii  adduser                               3.113+nmu3
ii  dconf-cli                             0.22.0-1
ii  dconf-gsettings-backend               0.22.0-1
ii  debconf [debconf-2.0]                 1.5.55
ii  gir1.2-gdm3                           3.14.1-3
ii  gnome-session [x-session-manager]     3.14.0-2
ii  gnome-session-bin                     3.14.0-2
ii  gnome-settings-daemon                 3.14.2-2
ii  gnome-shell                           3.14.2-3
ii  gnome-terminal [x-terminal-emulator]  3.14.1-1
ii  gsettings-desktop-schemas             3.14.1-1
ii  libaccountsservice0                   0.6.37-3+b1
ii  libaudit1                             1:2.4-1+b1
ii  libc6                                 2.19-13
ii  libcanberra-gtk3-0                    0.30-2.1
ii  libcanberra0                          0.30-2.1
ii  libgdk-pixbuf2.0-0                    2.31.1-2+b1
ii  libgdm1                               3.14.1-3
ii  libglib2.0-0                          2.42.1-1
ii  libglib2.0-bin                        2.42.1-1
ii  libgtk-3-0                            3.14.5-1
ii  libpam-modules                        1.1.8-3.1
ii  libpam-runtime                        1.1.8-3.1
ii  libpam-systemd                        215-8
ii  libpam0g                              1.1.8-3.1
ii  librsvg2-common                       2.40.5-1
ii  libselinux1                           2.3-2
ii  libsystemd0                           215-8
ii  libwrap0                              7.6.q-25
ii  libx11-6                              2:1.6.2-3
ii  libxau6                               1:1.0.8-1
ii  libxdmcp6                             1:1.1.1-1+b1
ii  libxrandr2                            2:1.4.2-1+b1
ii  lsb-base                              4.1+Debian13+nmu1
ii  metacity [x-window-manager]           1:3.14.3-1
ii  mutter [x-window-manager]             3.14.2-1
ii  openbox [x-window-manager]            3.5.2-8
ii  policykit-1                           0.105-8
ii  ucf                                   3.0030
ii  x11-common                            1:7.7+7
ii  x11-xserver-utils                     7.7+3+b1
ii  xterm [x-terminal-emulator]           312-1

Versions of packages gdm3 recommends:
ii  at-spi2-core               2.14.0-1
ii  desktop-base               8.0.2
ii  gnome-icon-theme           3.12.0-1
ii  gnome-icon-theme-symbolic  3.12.0-1
ii  x11-xkb-utils              7.7+1
ii  xserver-xephyr             2:1.16.2.901-1
ii  xserver-xorg               1:7.7+7
ii  zenity                     3.14.0-1

Versions of packages gdm3 suggests:
ii  gnome-orca            3.14.0-2
ii  libpam-gnome-keyring  3.14.0-1+b1

-- debconf information:
* shared/default-x-display-manager: gdm3
  gdm3/daemon_name: /usr/sbin/gdm3