sidebus

(it.. runs as a "sidecar" to a VM.. but manages D-Bus.. get it?)

A cross-domain smart D-Bus proxying system that makes (some) XDG Desktop Portals work across virtual machines.

Design Overview

• sidebus-broker host process:
  • to be launched alongside the VMM
  • hosts a "private" bus for VM-instance-specific daemons such as permission-store and document-portal
  • listens on vsock (or on a unix socket that muvm would proxy as vsock) and connects to the VM bus as a client when the agent connects
  • orchestrates the lifecycle of the aforementioned daemons + virtiofsd
    • (we are sharing the directory provided by the document-portal FUSE filesystem!)
  • provides portal front-end interfaces like org.freedesktop.portal.FileChooser on the VM bus
    • by proxying to the host session's xdg-desktop-portal!..
      • (not talking directly to impls: don't want to reimplement per-DE portal selection; also 1:1 mapping is nicer to code)
    • but with extra hooks like exposing files to the guest using our private (per-VM) document-portal!
• sidebus-agent guest process:
  • connects to the broker over vsock and splices the connection into the VM (session) bus
  • can be spawned spawned on-demand by D-Bus
  • uses systemd credentials for config args like vsock port
    • (very convenient to pass via the VMM, e.g. qemu: -smbios type=11,value=io.systemd.credential:sidebus.port=1337)

Development Notes

• the Nix flake provides paths to the daemons we run via environment variables.. conveniently, both in the release build and in the dev shell (so rust-analyzer doesn't complain)!
• the --debug-access flag of sidebus-broker allows for testing on the host, without needing to connect over vsock, e.g.:
  • clear; rm /tmp/b/* ; mkdir -p /tmp/{b,r} ; RUST_LOG=debug cargo run --bin sidebus-broker -- --debug-access /tmp/b --vsock-port 1234 --runtime-dir /tmp/r
  • GTK_A11Y=none DBUS_SESSION_BUS_ADDRESS=unix:path=/tmp/b/vm.sock ashpd-demo

Acknowledgments

Spectrum's work on a similar system has been very helpful during the development process.