Void Process [void-process]

https://anil.recoil.org/ideas/void-processes/

Void processes provide a structured way of having Unix processes that adhere to the principle of least privilege.

Backlinks

OCaml Roundup: August 2026 [ocaml-roundup-august-2026]

July 12, 2026
Patrick Ferris

Much of August was spent thinking about POSIX shell semantics, and in particular extending notions from smoosh to include side effects (and maybe concurrency). This is still nascent work, but some ideas and flavours are described below.

Merging Shells

July 12, 2026
Patrick Ferris

The work came from wanting to merge two shell sessions into one. Setting aside the practicalities of an implementation, it makes sense to start first theoretically. When can I merge two shell executions, and when can I not?

It would be nice to think of unset DOWNLOAD_DIR as a function. This function takes the shell state and the operating state as arguments and returns a modified shell state and an "unmodified" operating system state (at least for our purposes). Meanwhile, branch-b may have a lot more side effects, but when focusing on the environment only we can see in a sense the two branches are conflict free. Well, sorta. We also need to ensure we do not also read DOWNLOAD_DIR in branch-b if we intend to remove it in their merge.

This work, in part, is borrowing ideas from MRDTs and can reuse void processes to help reason about when side effects can occur, as well as open-trace (which is now becoming os-trace).

I hope to continue this work, see my latest weekly for more on this.

Eio in the browser

July 12, 2026
Patrick Ferris

I spent some time carving out Eio_browser into a standalone library, something I depend on for Merry to work in the browser.

Eio_browser [eio_browser]

September 12, 2026
Patrick Ferris
https://git.sirref.org/eio_browser

Eio_browser is a backend for Eio that works in the browser. It reuses work I started at Tarides that picked up some momentum but has since been a little dormant.

Crucially, it provides a functional standard environment like Eio_main or any of the OS-specific backends.

module Stdenv : sig
  type base =
    < stdout : Eio_unix.sink_ty r
    ; stderr : Eio_unix.sink_ty r
    ; clock : float Eio.Time.clock_ty r
    ; mono_clock : Eio.Time.Mono.ty r
    ; secure_random : Eio.Flow.source_ty r
    ; debug : Eio.Debug.t
    ; backend_id : string >
end

val run : (Stdenv.base -> unit) -> unit
(** [run fn] runs [fn] passing it the {! Stdenv.base} environment.
    See {! Backend.run} for more information about how effects are handled.*)

Vpnkit, Void Processes, LSP Servers › Void Processes [void-processes]

February 17, 2025

Work continued on implementing (and fully exploring) void processes. A lot of the groundwork already exists in Jake Hillion's master's thesis.

This week I added a feature that I needed to help build the processes we need for the shell I'm building: mount points with modes!

In addition to the root mount (taken care of with pivot_root), we need to be able to add additional mounts into the process' environment. These can now be added. All mount points can be mounted readonly or readwrite.

Here is the "Hello, World!" example (the /say/hey program has been statically compiled using musl-gcc).

let status =
  let void =
    empty 
    |> mount ~mode:R ~src:hey_dir ~tgt:"say"
    |> exec [ "/say/hey" ]
  in
  let t = Void.spawn ~sw void in
  Promise.await (Void.exit_status t)

There really is nothing else in there. Without specifying a root mount, the void process is started with an empty tmpfs root. Next on the list is networking!