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Introduction

DDM (Deepin Display Manager) is the display and login manager for the Treeland Wayland compositor. It authenticates users with PAM, starts their desktop sessions and keeps the login screen running on the initial virtual terminal.

DDM does not draw the login screen. The greeter is part of Treeland, which DDM starts as the treeland.service systemd unit and which runs as the dde user. DDM only manages the lifecycle of the compositor, the seats and the user sessions.

Components

Component Description
ddm The daemon, started by ddm.service as root.
ddm-helper Session helper, spawned by the daemon for every authentication.
libddm-common Shared library with configuration, session and X authorization helpers.
treeland.service The compositor providing the greeter, run as the dde user.
dde-seatd.service Seat and virtual terminal management.

Daemon

The daemon (src/daemon) is a QCoreApplication that owns:

  • Seat management (SeatManager): enumerates logind seats and creates one Display per graphical seat.
  • Display management (TreelandDisplayServer, XorgDisplayServer): starts and stops the Treeland compositor, or an X server for X11 sessions.
  • Authentication (Auth): runs the PAM transaction through ddm-helper and opens the logind session.
  • Application lifecycle (Display): turns greeter requests into logins, logouts, locks and unlocks.
  • Power management (PowerManager): power off, reboot, suspend, hibernate and hybrid sleep through logind, ConsoleKit2 or UPower, whichever is available.
  • Virtual terminals (DdeSeatdControl): asks dde-seatd to allocate grouped VTs and reports VT changes.

Session helper

ddm-helper (src/helper) is a standalone, single-threaded QCoreApplication. The daemon spawns it with fork()/exec() and talks to it over an inherited socketpair. It runs the full PAM transaction (pam_authenticate() and pam_open_session()) and starts the user's session process.

Running PAM in a separate process keeps DDM safe from PAM modules that fork inside pam_open_session() (for example gnome-keyring): forking a multi-threaded Qt process and letting the child call exit() can deadlock on Qt's inherited atexit handlers.

Treeland

Treeland is the compositor and the greeter. It is started by DDM as treeland.service with the --lockscreen option, running as the dde user. Its greeter implementation lives in src/greeter and the LockScreen QML plugin, and exposes GreeterProxy, SessionModel and UserModel to the login screen.

For sessions whose desktop file sets X-DDE-SINGLE-WAYLAND=true, the user session runs inside the already running Treeland instance instead of starting its own compositor. Otherwise DDM stops Treeland and starts the session's own display server.

dde-seatd

dde-seatd is a fork of seatd. DDM talks to its control socket (/run/dde-seatd-control.sock) to allocate and destroy grouped VTs and to be notified about VT changes. Treeland uses the regular libseat seatd backend against /run/dde-seatd.sock.

Login flow

  1. ddm.service starts the daemon. SeatManager enumerates logind seats and creates a Display for every graphical seat.
  2. The Display picks a virtual terminal, starts treeland.service, starts a local socket server and publishes the socket path over D-Bus as org.deepin.DisplayManager.AuthInfo.
  3. The Treeland greeter connects to that socket and sends Connect with the path of its Wayland socket. The daemon connects to it and binds the private treeland_ddm_v1 protocol. It then sends the host name, the power capabilities, the last user and session, the session directories and ShowGreeter.
  4. When the greeter sends Login, the daemon authenticates the user with ddm-helper. On success it starts the session and records the user and session in its own configuration as lastUser and lastSession.
  5. The session type decides what happens next:
    • Single-mode Treeland (X-DDE-SINGLE-WAYLAND=true): the daemon asks dde-seatd for a grouped VT, opens the logind session and asks Treeland to switch to the user.
    • Wayland: the daemon stops Treeland and runs the session's compositor through the waylandSessionCommand script.
    • X11: the daemon stops Treeland, starts the X server with an authorization file in RUNTIME_DIR (/run/ddm/xauth_XXXXXX), writes the user session's authorization cookie to /tmp/xauth_XXXXXX and runs the session through the x11SessionCommand script.
  6. Logout, Lock and Unlock messages are forwarded to systemd-logind (TerminateSession(), LockSession(), UnlockSession()). Unlocking also activates the session again, either through Treeland or by switching to the session's VT.
  7. When the session process exits, the daemon removes the session and frees its grouped VT. For X11 sessions the stopped X server also tears the display down, which makes the seat recreate itself and start Treeland again for the login screen.

VT switches are driven by dde-seatd: on every VT change the daemon tells Treeland to switch to the greeter or to the user owning the new VT.

Communication

Greeter socket

The greeter talks to the daemon over a local socket named ddm-treeland-XXXXXX. Messages are framed with a quint32 opcode:

  • greeter to daemon: Connect, Login, Logout, Lock, Unlock, PowerOff, Reboot, Suspend, Hibernate, HybridSleep, BackToNormal.
  • daemon to greeter: HostName, Capabilities, LastLogin, SessionDirs, LoginFailed, InformationMessage, UserActivateMessage, SwitchToGreeter, UserLoggedIn, ShowGreeter.

Login carries the user name, the password and the absolute path of the desktop file of the session to log in to; the daemon derives the session type from the session directories it configures.

The opcodes are defined in src/common/Messages.h.

Wayland protocol

The daemon binds the private treeland_ddm_v1 global of the Treeland compositor (see the treeland-protocols repository) and uses it to switch between the greeter and a user. The deprecated switch_to_vt and acquire_vt events are ignored, as VT switching is handled by dde-seatd and wlroots/libseat.

D-Bus

The daemon registers org.freedesktop.DisplayManager at /org/freedesktop/DisplayManager (seats, sessions, LastSession) and the deepin-specific org.deepin.DisplayManager at /org/deepin/DisplayManager (AuthInfo, LastActivatedUser). The greeter uses the latter to find the daemon socket and to know which user was activated last.

Configuration

The daemon reads its configuration through the DConfig framework of the Deepin Desktop Environment, under the application id org.deepin.dde.ddm. The schema names and describes every option and is installed together with the daemon, so there is no separate option list in the documentation.

The greeter does not read DDM configuration: the daemon passes what it needs over the greeter socket.

Which sessions exist, and which of them is offered for a user, is decided by the greeter: the daemon sends it the directories the sessions are installed in (x11SessionDirs and waylandSessionDirs) when it connects, and receives back the absolute path of the desktop file of the session to log in to. The daemon also records the last logged in user and session and sends them to the greeter as LastLogin; the greeter uses them to preselect the login.