[systemd-devel] User sessions, session buses, user buses

David Herrmann dh.herrmann at gmail.com
Tue Feb 3 01:28:56 PST 2015


On Fri, Jan 30, 2015 at 9:30 AM, Simon McVittie
<simon.mcvittie at collabora.co.uk> wrote:
> [For those who are there, I'll be at the system hackfest today and at
> FOSDEM this weekend, so if you are interested in these topics, please
> talk to me about them; I'll try to summarize discussion to these lists.
> For those not there, I'll try to keep up with responses via email and
> raise any interesting points in person.]
> I've recently been looking into D-Bus' interaction with X session
> startup, systemd --user, upstart --user, and the definition of a
> session. Every few months or years there is an attempt to sort out
> whether D-Bus has a user bus or a session bus or what. Here is another,
> with particular reference to systemd and potentially kdbus.
> Terminology
> ===========
> login session
> -------------
> A *login session* is as follows: when you log in to *dm, that starts a
> *graphical login session* which lasts until you log out. When you log in
> on a virtual console or via ssh or something, that starts a
> *non-graphical login session*. Background processes like cron or screen
> might also be non-graphical login sessions. A non-graphical login
> session might become graphical by running startx or similar.

We don't support turning text-sessions into graphical sessions (like
'startx' and friends). It works in some scenarios, but is strongly
frowned upon. Furthermore, VT sessions should really be treated like
graphical sessions. From a technical viewpoint, there is no
difference. And with things like kmscon or systemd-console, you really
*have* to treat it the same as graphical logins.

> In a systemd-logind environment, login sessions have an ID which is
> made available to in-session processes in the XDG_SESSION_ID environment
> variable. If the Linux "auditing" feature is enabled, the XDG_SESSION_ID
> equals the audit session ID (/proc/self/sessionid). If not,
> there is no audit session ID, so systemd-logind makes up a sequential
> XDG_SESSION_ID and uses that.
> seat
> ----
> A seat is a set of displays, input devices etc. attached to a machine.
> Most machines have one seat "seat0", which occupies all the devices.

'all the devices' is not exactly right. We still distinguish between
'seated' devices and 'non-seated' devices. That is, some devices are
always global and never assigned to a seat (like power-buttons, though
that particular example might even be broken right now).

But yes, all 'seated' devices are assigned to 'seat0' implicitly, if
not set otherwise.

> By attaching additional devices via something like
> http://plugable.com/products/ud-160-m you can construct a
> multi-seat machine.
> user-session
> ------------
> I don't think there is a standard term for this so I'm making one up.
> The XDG_RUNTIME_DIR specification says that there is at most one
> XDG_RUNTIME_DIR per uid per machine, that it is created at the beginning
> of the user's first login session, and that it is removed at the end of
> a login session if no other login sessions remain for that uid.
> systemd-logind implements those semantics, and also runs a `systemd --user`
> for the lifetime of the user-session. Ubuntu previously used
> libpam-xdg-runtime to provide compatible semantics without systemd.
> The resulting situation looks something like this (assume all the
> sessions shown are under the same uid):
>     --------\             ========\\
>     login   |             user-   ||
>     session |  --------\  session || <- overlapping login session c42
>     23      |  login   |          ||    shares the user-session with 23
>     --------/  session |          || <- 23 ends but c42 keeps u.-session
>                c42     |          ||    alive
>     --------\          |          || <- 57 begins, shares u.-sess.
>     login   |  --------/          || <- c42 ends, 57 keeps user-session
>     session |                     ||    alive
>     57      |                     ||
>     --------/             ========// no more l.sessions, u.-session ends
>     --------\             ========\\ new l.session, u.-session starts
>     login   |             user-   ||
>     session |             session ||
>     ...     .             ...     ..
> user-sessions do not have any meaningful identifier apart from their
> owner's uid (strictly speaking, the owner's "loginuid", i.e. the user
> who initially logged in, even if they su). They do not need another
> identifier, because by definition there is only one per uid at a time.
> Impact on D-Bus
> ===============
> There are two models for how the D-Bus session bus, which is designed to
> be per "high-level session" (whatever that means), can work in a world
> with user-sessions.
> Last time this was discussed, Havoc had the useful insight that this is
> not really a question about D-Bus, it's a question about how you build
> OSs, and in particular how you model sessions.
> (Up to) one high-level session per login
> ----------------------------------------
> The first model, which is how it traditionally worked (before there was
> such a thing as a user-session), is that each graphical login session is
> declared to be a "session", and non-graphical login sessions are pretty
> much swept under the carpet and forgotten about.
> In graphical sessions, vaguely modern Unix OSs typically know how to
> start up a dbus-daemon during the creation of a graphical session (e.g.
> in Debian and derivatives it's started by /etc/X11/Xsession.d, and
> Fedora derivatives have a similar setup under a different name). If they
> don't, modern desktop environments also know how to start a dbus-daemon
> if they need one (e.g. gnome-session does this for GNOME), and if *that*
> doesn't start one (the "I use Firefox under fvwm" use-case), we have a
> slightly shaky but functional "autolaunch" mechanism based on X11
> properties.
> In principle, a PAM module or something could ensure that we have a
> dbus-daemon per login session, even tty/ssh/cron login sessions
> (which all go through PAM). In practice, nobody has ever cared enough to
> implement this, so we're left with D-Bus autolaunch, which can't
> actually work for tty sessions and had bad side-effects from its
> attempts to do so, so I disabled it 3 years ago in favour of
> recommending that users requiring a D-Bus session should start their own
> and manage its lifetime themselves e.g. with dbus-run-session(1).
> If people want to put work into this model, we could do a lot better
> than we do now; for instance, the bus socket could be
> unix:path=${XDG_RUNTIME_DIR}/sessions/${XDG_SESSION_ID}/bus (but with
> the necessary escaping) on systems where those variables are set, rather
> than messing about with $TMPDIR.
> However, the people doing the work have a somewhat strong correlation
> with the people who want a different model, for which read on...
> Digression: (at least some) users and developers don't want that
> ----------------------------------------------------------------
> One major issue with one high-level session (and hence one session bus)
> per login session is that it places an artifical barrier between login
> sessions. It's difficult for cron jobs, ssh logins, tty logins to share
> non-GUI backend services such as dconf, Rygel, Telepathy: on my NAS box,
> which runs Rygel with no GUI to export my music in a way the
> notoriously picky PS3 system software can understand, I had to
> construct a D-Bus session to let Rygel and Tracker components talk
> to each other. Happily, I had already written dbus-run-session(1),
> but I shouldn't need to do this rubbish to let non-GUI daemons talk
> to each other - it should just work.
> Similarly, if you leave a screen/tmux session detached and running
> in the background, systemd is fine with that: its view of the world
> is that there are processes left over from your previous login session,
> keeping the login session alive in "closing" state, with the consequence
> that the user-session remains alive too (unless you have configured
> systemd-logind to kill leftover processes, as required by the sysadmins
> of computer labs and other massively multi-user machines, in which case
> you don't get to run screen sessions and that's by design).
> However, the login-session-centric model can't cope with that:
> the dbus-daemon dies with the X server, the D-Bus services die with
> the dbus-daemon, and now the duplicity(1) process in your screen session
> can't use gvfs to upload your backups to a server (or whatever).
> Another issue is that it is an extremely poor match for how
> `systemd --user` works (although apparently a better match for
> how Ubuntu uses `upstart --user`, which I hope to discuss with Ubuntu
> people at the systemd hackfest today / at FOSDEM) - you can't use
> systemd to manage your login-session-bound D-Bus services, because they
> all share a systemd, which doesn't know which login session wants which
> service.
> I think it's telling that, in recent discussion with Ubuntu developers
> about their use of one `upstart --user` per login session, they
> clarified that they only actually do this for *graphical* login
> sessions, and they provide tools to let a developer copy the environment
> variables from what, conceptually, should be another session,
> effectively "joining" that session for a subtree of processes. I think
> this proves the point that Robert McQueen made on the subject back in 2006:
>   "It's already been discussed multiple times on the list and
>   apparently a user bus is too confusing or not really useful unless
>   you make it a user bus shared across a network cluster. I don't
>   agree, and I think that the consequence of this stance will be a
>   proliferation of hacks like the one I just sketched out (not to
>   mention even grosser hacks like Ubuntu's ACPI scripts grepping the
>   environment of the user's processes in /proc to find their session
>   bus to tell their screensaver to lock)."
> One high-level session per user-session
> ---------------------------------------
> As a result of the problems noted above, various people, most vocally
> the systemd and GNOME developers, have advocated a different model
> for how to model sessions in the OS - what I called "user-sessions"
> above. A typical implementation of that idea forbids multiple graphical
> login sessions altogether: if you type your username and password into
> gdm while already logged in on another virtual console, it will just
> vt-switch to your existing graphical login session and unlock it.
> Under X11, that might well be the best we can do, because typical
> X11 applications can't cope with being asked to put windows on more
> than one $DISPLAY (although I've heard rumours that Emacs can, which
> would make Emacs an ideal candidate to be a user-session service).
> Under Wayland or similar future cleverness, hopefully there'll be
> some way for a new login on a new seat to "steal" all the windows
> from the inactive seat, or some way to merge both seats into
> one big virtual display if the same person is using both (AIUI this
> is what GNOME designers want to do), or some other clever solution.
> What this means for D-Bus
> -------------------------
> This leaves the problem that, if you have chosen the
> user-session-centric model, current D-Bus doesn't actually work very
> well: we have dbus-launch and dbus-run-session for login-session-centric
> D-Bus, but if you want to make D-Bus user-session-centric, you need to
> resort to third-party user bus units like user-session-units, which
> float around Github and somehow never make it upstream. I'm now on my
> third unrelated work project that contains a copypaste of dbus.socket
> and dbus.service, and I'm sure my colleagues have others - this is not
> the flying car future I signed up for. So I would like to get this stuff
> upstream into dbus.git.
> So that the people who are happy with the complexities of the current
> arrangement can remain happy, here is how I intend it to work:
> * ./configure --disable-user-bus: you get a login-session-centric world
> * ./configure --enable-user-bus: you get a user-session-centric world
> * configure either way and selectively install/delete files: you can
>   either have a login-session-centric or user-session-centric world,
>   depending what you install. (This is so I can have dbus-user-bus and
>   dbus-x11 binary packages in Debian, where the answer to "do A or do
>   B?" is always "both!", without having to compile everything twice;
>   the GNOME metapackage could eventually depend on dbus-user-bus.)
> I think in practice that design will mean that by default, libdbus tries
> to connect to both user-session- and login-session-centric locations for
> the bus (but the "wrong one" for this OS will fail and the "right one"
> will succeed), while dbus-user-bus and dbus-x11 would arrange for the
> bus to listen in the appropriate place for one option or the other so
> that the appropriate version succeeds.
> Conveniently, this is what I've already implemented (in two
> different ways): "try the user-session-centric path first, and if that
> fails, fall back to login-session-centric autolaunch". There is the
> slight nit that the server side of user-session-centric operation
> requires systemd, although that could be argued to be a feature: the
> other reasonable implementation is libpam-xdg-runtime, which AIUI is
> unmaintained now that Ubuntu has switched to systemd-logind, and the
> people who are avoiding systemd on general principles (hello Devuan!)
> are probably the sort of people who are sufficiently change-averse to
> prefer D-Bus to work the way it worked in 2005 (if they install it at
> all). I'd be willing to consider implementations that work with other
> service managers if their authors are willing to help maintain them.
> The glorious(?) kdbus future
> ============================
> kdbus is a proposed Linux kernel module providing
> somewhat-D-Bus-compatible message-passing semantics (I'm being vague
> because it's been 4 months since I last looked at it in detail - I hope
> to return to it soon). To provide complete dbus-daemon-like
> functionality, you also need a user-space process to create the bus and
> manage activations; the
> reference implementation, and so far the only implementation, is in
> systemd.
> The systemd developers who are working on the user-space counterpart
> for kdbus have indicated that they have zero interest in supporting a
> login-session-centric setup for session kdbus.  In practice, I think
> this means that the proponents of a login-session-centric world either
> can't use kdbus in their sessions, or need to convince the relevant
> developers that the added complexity of their model is worth it, or need
> to write their own user-space for kdbus sessions and
> disable systemd's. (It's not all bad - they can still use kdbus for
> the system bus, which is already an improvement.)
> Remaining issue: environment variables
> ======================================
> Sadly, not all the issues have been resolved yet. The biggest is
> environment variables: on existing systems there is an expectation
> that environment variables set by *dm, PAM, or /etc/X11/Xsession.d
> (or the non-Debian equivalent) will be propagated into every process
> in the session. In a brief survey of about half the Xsession.d scripts
> in Debian, I identified these environment variables:
> * each variable printed by locale(1)
> plus the obvious ones set by *dm, such as DISPLAY, or by PAM. Similarly,
> a user's ~/.xsession can set arbitrary variables - mine sets CCACHE_DIR,
> EDITOR, MPD_HOST and XDG_CONFIG_DIRS, among others.
> The naive implementation would be to run a tool at the end of
> /etc/X11/Xsession.d that uploads all of these into `dbus-daemon
> --session` (if used) and into `systemd --user` (if used). However, not
> all environment variables set in such scripts are suitable for that use:
> notably, XDG_SESSION_ID from the login session should not be copied into
> activated processes that exist outside any login session.
> As a short-term solution, I'm tempted to write that tool, but make it
> only upload a whitelisted set of variables automatically, and say "if
> you install dbus-user-bus, you are expected to run this tool from your
> ~/.xsession if you need it".
> Thoughts?

Other than my two additions, I agree with Lennart.


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