[systemd-devel] What exactly is multi-seat? -- questions about logind

[Kian Kasad]

On 21/01/26 02:06PM, Pekka Paalanen wrote:
> On Tue, 26 Jan 2021 01:43:43 -0800
> Kian Kasad <kian at kasad.com> wrote:
> 
> > Hi all,
> > 
> > After reading the documentation on logind and multi-seat (specifically
> > sd-login(3) and "Multi-Seat on Linux"), I still have some questions.
> > 
> > First of all, what exactly is multi-seat? Does it just mean allowing
> > multiple sessions to be running at once, like for multiple users to be
> > logged into the same desktop, even though only one will be in use at a
> > time?
> > 
> > Second, why is logind needed for this? Is it not possible to do without
> > logind? I've run Xorg perfectly fine on systems without logind/systemd,
> > so is logind only needed for multiple sessions at once? Is it just to
> > handle the graphical device(s)?
> > 
> > Third, is multi-seat possible at all without logind? 
> > 
> > Most of these questions arose because my main OS, Artix Linux, requires
> > logind (in the form of elogind) for Xorg, but I know that Xorg runs just
> > fine on Alpine Linux, which does not use logind at all.
> 
> Hi,
> 
> I've heard good words of https://sr.ht/~kennylevinsen/seatd/ although I
> haven't tried to use it myself. One more way to handle multiseat - to
> bind them all.

I'll write this one down. I'm not looking for alternatives currently, I
was just confused about the purpose.

> IOW, there are many ways to achieve coordination between multiple
> sessions and one or multiple physical seats, like the Linux virtual
> terminal system that requires root permissions, ioctl and signal magic
> to let things cooperate (and if things do not explicitly cooperate
> correctly, it fails horribly) and does not support multiseat. But if
> you want your program to not need root permissions, have a graceful
> failure if it crashes, not accidentally hijack the seat by mistake, or
> just support multiseat, you kind of need to have a system daemon
> orchestrating things.
> 
> Logind is one of those possible system daemons. It tells your program
> when it is active. It grants access to graphics and input devices
> without your program needing root permissions. It also takes graphics
> and input devices away from your program when deactivated, so you can't
> e.g. spy on other programs' input. It takes care of the arcane magic of
> setting up the VT and tty and restoring them. And more.
> 
> "Your program" here is Xorg, for instance. Or one of the Wayland
> compositors.

Thank you! This is exactly the explanation I was looking for. I didn't
get why a daemon was needed if programs (like Xorg) can handle the
devices on their own, but now I see it's to allow multiple Xorg/Wayland
sessions to run, all without needing SUID.

--
Kian Kasad
PGP 0x1715EEAA14DAEC14
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