Elogind User, Seat and Session Manager

Introduction

Elogind is the systemd project’s “logind”, extracted out to be a standalone daemon. It integrates with PAM to know the set of users that are logged in to a system and whether they are logged in graphically, on the console, or remotely. Elogind exposes this information via the standard org.freedesktop.login1 D-Bus interface, as well as through the file system using systemd’s standard /run/systemd layout.

Elogind also provides “libelogind”, which is a subset of the facilities offered by “libsystemd”. There is a “libelogind.pc” pkg-config file as well.

All of the credit for elogind should go to the systemd developers.
All of the blame should go to Andy Wingo, who extracted elogind from systemd. And all complaint should go to Sven Eden, who is maintaining elogind past the v219 series.

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Contributing

Elogind was branched from systemd version 219, and preserves the git history of the systemd project. The version of elogind is the upstream systemd version, followed by the patchlevel of elogind.
For example version 219.12 is the twelfth elogind release, which aims to provide a subset of the interfaces of systemd 219.

Why bother?

Elogind has been developed for use in GuixSD, the OS distribution of GNU Guix.
GuixSD uses a specific init manager (DMD), for reasons that are not relevant here, but still aims to eventually be a full-featured distribution that can run GNOME and other desktop environments.

However, to run GNOME these days means that you need to have support for the login1 D-Bus interface, which is currently only provided by systemd.
That is the origin of this project: to take the excellent logind functionality from systemd and provide it as a standalone package.

You’re welcome to use elogind for whatever purpose you like — as-is, or as a jumping-off point for other things — but please don’t use it as part of some anti-systemd vendetta. We are appreciative of the systemd developers logind effort and think that everyone deserves to run it if they like. No matter what kind of PID1 they use.

Differences relative to systemd

The pkg-config file is called libelogind, not libsystemd or libsystemd-logind.

The headers are in <elogind/…>, so like <elogind/sd-login.h> instead of <systemd/sd-login.h>.

Libelogind just implements login-related functionality. It also provides the sd-bus API.

Unlike systemd, whose logind arranges to manage resources for user sessions via RPC calls to systemd, in elogind there is no systemd so there is no global cgroup-based resource management. This has a few implications:

  • Elogind does not create “slices” for users. Elogind will not record that users are associated with slices.
  • The /run/systemd/slices directory will always be empty.
  • Elogind does not have the concept of a “scope”, internally, as it’s the same as a session. Any API that refers to scopes will always return an error code.

On the other hand, elogind does use a similar strategy to systemd in that it places processes in a private cgroup for organizational purposes, without installing any controllers (see http://0pointer.de/blog/projects/cgroups-vs-cgroups.html). This allows elogind to map arbitrary processes to sessions, even if the process does the usual double-fork to be reparented to PID 1.

Elogind does not manage virtual terminals.

Elogind does monitor power button and the lid switch, like systemd, but instead of doing RPC to systemd to suspend, poweroff, or restart the machine, elogind just does this directly. For suspend, hibernate, and hybrid-sleep, elogind uses the same code as systemd-sleep. Instead of using a separate sleep.conf file to configure the sleep behavior, this is included in the [Sleep] section of /etc/elogind/login.conf. See the example login.conf for more. For shutdown, reboot, and kexec, elogind shells out to “halt”, “reboot”, and “kexec” binaries.

The loginctl command has the poweroff, reboot, sleep, hibernate, and hybrid-sleep commands from systemd, as well as the –ignore-inhibitors flag.

The PAM module is called pam_elogind.so, not pam_systemd.so.

Elogind and the running cgroup controller

While ‘configure’ runs, it will detect which controller is in place. If no controller is in place, configure will determine, that elogind should be its own controller, which will be a very limited one.

This approach should generally work, but if you just have no cgroup controller in place, yet, or if you are currently switching to another one, this approach will fail.

In this case you can do one of the two following things:

  1. Boot your system with the target init system and cgroup controller, before configuring and building elogind, or
  2. Use the –with-cgroup-controller=name option.

Example: If you plan to use openrc, but openrc has not yet booted the machine, you can use –with-cgroup-controller=openrc to let elogind know that openrc will be the controller in charge.

However, if you set the controller at configure time to something different than what is in place, elogind will not start until that controller is actively used as the primary controller.

License

LGPLv2.1+ for all code – except src/basic/siphash24.c which is CC0 Public Domain

Dependencies

  • glibc >= 2.16
  • libcap libmount >= 2.27.1 (from util-linux)
    (util-linux < 2.29 *must* be built with –enable-libmount-force-mountinfo, and later versions without –enable-libmount-support-mtab.)
  • libseccomp >= 2.3.1 (optional)
  • libblkid >= 2.24 (from util-linux) (optional)
  • PAM >= 1.1.2 (optional)
  • libacl (optional)
  • libselinux (optional)
  • libpython (optional)
  • pkg-config
  • gperf >= 3.1
  • docbook-xsl (optional, required for documentation)
  • xsltproc (optional, required for documentation)
  • python-lxml (optional, required to build the indices)
  • python, meson, ninja gcc, awk, sed, grep, m4, and similar tools.

During runtime, you need the following additional dependencies:

  • util-linux >= v2.27.1 required
  • dbus >= 1.4.0 (strictly speaking optional, but recommended)
    NOTE: If using dbus < 1.9.18, you should override the default policy directory (–with-dbuspolicydir=/etc/dbus-1/system.d).
  • PolicyKit (optional)

To build in directory build/:

meson build/ && ninja -C build

Any configuration options can be specified as -Darg=value… arguments to meson. After the build directory is initially configured, the configuration can be changed with:

meson configure -Darg=value… build/

‘meson configure’ without any arguments will print out available options and their current values.

Useful commands:

  • ninja -v some/target
  • ninja test
  • sudo ninja install
  • DESTDIR=… ninja install

A tarball can be created with:

git archive –format=tar –prefix=elogind-238/ v238 | xz > elogind-238.tar.xz