Interaction multiplexing

Posted Dec 21, 2025 14:05 UTC (Sun) by SLi (subscriber, #53131)
In reply to: run0 by mathstuf
Parent article: Conill: Rethinking sudo with object capabilities

This feels to me something of a specific solution to a generic problem, and my instinct would be to ask if we can solve the class of problems instead. It doesn't apply only to $SSH_ASKPASS but also things like $DISPLAY; and a large part of the problem is that environment variables are frozen at program startup time.

I would frame the generic problem as:

> Given this process, what human-facing I/O endpoints are actually reachable right now, and which one is the user currently attending to, even if that differs from the environment at exec time?

(Now this is still ambiguous to an extent; one can attach to tmux from multiple places.)

Is there a generic way we could approach solving this? I *think* there is.

Terminal managers like tmux are at a position where they can have that information, probably better than anyone else. They just aren't able to modify the environment of the running program to inform it, so there needs to be an out-of-band way to query the terminal emulator for the information.

I first thought of weird query-response OSC escape sequences; but they are tricky, and I think don't have an advantage in this case, because the one thing tmux *can* provide to all programs is an out-of-band mechanism.

Here's where this line of thought evolved. Option A keeps tmux self-contained and maximally useful on its own; option B minimizes tmux-specific policy and makes it easier to share semantics with other multiplexers or brokers.

# Option A ("flat"):

- We have an environment variable. I'm bad at naming, so I call it INTERACTIVE_CONTEXT.

- it contains a capability-ish _pointer_ to "who can route interaction for me", not the answer:

> INTERACTIVE_CONTEXT=unix:/run/user/1000/interactive/ctx-<token>.sock

- It is set initially by PAM or systemd user manager (or something using something like XDG_RUNTIME_DIR on non-systemd contexts)

- On a GUI login, it points to the GUI session broker. On a SSH login, it points to the SSH session broker

- Multiplexers like tmux wrap it. When tmux starts a session/window, it sets it to

> INTERACTIVE_CONTEXT = unix:/run/user/1000/interactive/tmux-ctx-<serverid>.sock

- The tmux context service can select "the most suitable client" (last-active, last-attached, view-of-pane, whatever); then either handle prompting itself, forward to the outer context of the chosen client, or maybe just tell "sorry, interaction is not available".

- To forward, tmux needs to remember each client's "outer" INTERACTIVE_CONTEXT at attach time. So when `tmux attach` runs, it captures the attaching environment's INTERACTIVE_CONTEXT and stores it per-client.

# Option B (delegating pointer chain)

Like option A, but instead of tmux implementing everything, its socket would implement only

- `resolve()` -> returns INTERACTIVE_CONTEXT for best client
- `describe()` -> says "I'm a multiplexer; here's what I'd choose"
- optionally `prompt()` implemented by forwarding (not UI)

---

I think there are some security implications that need thinking. I think the most useful model here is that the terminal multiplexer arbitrates "the most suitable client" and external policy decides "what this endpoint is allowed to do". One useful mental model may be to treat INTERACTIVE_CONTEXT as a bearer capability scoped to a login or multiplexer context, rather than as a global per-user resource (instead of "any same-UID process can pop auth prompts everywhere")

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Interaction multiplexing

Posted Dec 22, 2025 5:31 UTC (Mon) by mathstuf (subscriber, #69389) [Link] (6 responses)

> > Given this process, what human-facing I/O endpoints are actually reachable right now, and which one is the user currently attending to, even if that differs from the environment at exec time?

Is this really answerable in a generic way? Say I am ssh'd into a machine from my laptop. I attach to a tmux session. How is there any indication that the desktop is at my feet with the screen unlocked and keyboard accessible at the desk I'm sitting in front of? I don't want to manage that state by hand and I really have no idea how one would automate it (maybe some BLE presence metrics, but I am not my phone (or even watch), so that isn't really crossing over the "works most of the time, but enough to be frustrating when it doesn't" threshold in my book).

I don't see why it needs all this extra stuff. You run an agent to answer requests (polkit, passwords, etc.) where you interact (the DE, the I-ssh'd-in tmux session, etc.). Any agent can answer a request; any agent's answer is sufficient. Requests come over the standard protocols (DBus, inotify-on-a-directory) and you manage them given the UI you provide. If you want to know you're in tmux and broadcast it with `tmux display-message -c $client` (or `display-popup`) so that you don't even need the agent's session to be active, that sounds a lot simpler to me.

Interaction multiplexing

Posted Dec 22, 2025 9:23 UTC (Mon) by taladar (subscriber, #68407) [Link] (2 responses)

I like the general idea but why do they need to be requests to agents at all, wouldn't it be even simpler if they were just notifications about a new password request so that even seats where the user logged in after the request was created could respond to the request by polling for pending requests after login or when the user asks for them explicitly?

Interaction multiplexing

Posted Dec 22, 2025 14:11 UTC (Mon) by mathstuf (subscriber, #69389) [Link]

Polkit sits on DBus, so you're stuck with a "proper" agent there. The systemd protocol is "drop files in a directory", so you could definitely use notification actions to trigger the actual prompt (systemd-gnome-ask-password-agent does this). It also scans on startup and sends notifications for anything "laying around" when it starts up. Just need something that doesn't rely on a GUI and notification daemon (the TTY agent I mentioned).

Interaction multiplexing

Posted Dec 22, 2025 17:01 UTC (Mon) by SLi (subscriber, #53131) [Link]

Yes, I think this makes sense; essentially, decoupling request creation from request fulfillment. I think this pushes in the direction of making the request durable and discoverable, which I like. I think it's largely orthogonal to the idea of making the routing contextual.

Interaction multiplexing

Posted Dec 22, 2025 16:48 UTC (Mon) by SLi (subscriber, #53131) [Link] (1 responses)

> Is this really answerable in a generic way?

No, not in any foolproof way. You cannot infer presence. But you can do heuristics that are better than defaulting to whatever it was when you started this program by answering questions like which interaction endpoints are currently attached and responsive, and which of those have most recently had user activity.

tmux, for example, does not know if the local desktop seat is unlocked, but it does know whether there is a client attached over SSH that has recently been active.

So, sure, perfect correctness is probably impossible, but that just bounds the abstraction to something that is less likely to be completely wrong.

Interaction multiplexing

Posted Dec 23, 2025 14:40 UTC (Tue) by taladar (subscriber, #68407) [Link]

Being correct all the time is also not necessarily required if you do allow the user to override the heuristic when it guesses wrong by e.g. running a command on a terminal they do have access to to get to the prompt. 100% correctness is hard but it is only made necessary by the lack of remedial options when the heuristic guesses wrong.

Interaction multiplexing

Posted Dec 22, 2025 16:58 UTC (Mon) by SLi (subscriber, #53131) [Link]

> I don't see why it needs all this extra stuff. You run an agent to answer requests (polkit, passwords, etc.) where you interact (the DE, the I-ssh'd-in tmux session, etc.). Any agent can answer a request; any agent's answer is sufficient. Requests come over the standard protocols (DBus, inotify-on-a-directory) and you manage them given the UI you provide. If you want to know you're in tmux and broadcast it with `tmux display-message -c $client` (or `display-popup`) so that you don't even need the agent's session to be active, that sounds a lot simpler to me.

A broadcast model like this would probably work for a password prompt, in a sense. It would fail for other examples of the same problem class ("which DISPLAY to connect to"). Even for the password prompt, I think it does have some downsides.

First, it's not really very symmetric across transports. GUI agents work well with dbus + desktop notifications; TTY agents require explicit discovery and focus; and headless/multiplexed contexts are rather second class.

Second, I think a generally useful idea is "route this request to the same place the user typed the command". I think a broadcast model of, essentially, "route this request somewhere and hope the user notices" is sufficient for things like boot time disk unlocks, and more frustrating for interactive admin commands over ssh+tmux.

So, yes, I think the broadcast model works, but it gives up the correlation. Maybe that's sufficient.