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pressure-vessel — tools to put Steam in containers

pressure-vessel is a bit like a simplified version of Flatpak for Steam games.

Use cases

Protecting $HOME

If we change the meaning of /home for a game to be a fake app-specific directory, then the game can't write random files in unknown subdirectories of $HOME, and we can be reasonably sure that uninstalling a game removes all of its files. (As a side benefit, this could make it easier to be sure that all the necessary files have been selected for Steam Cloud Sync, because we could prevent the game writing anywhere that won't be synchronised, other than /tmp or similar.)

Using an alternative runtime

We can get a more predictable library stack than the LD_LIBRARY_PATH-based Steam Runtime by making an alternative runtime available over /usr, /lib*, /bin, /sbin, analogous to Flatpak runtimes, possibly using libcapsule to share the host's graphics driver stack. (This mode is not yet implemented.)

Instructions for testing

  • Install Steam and some games

  • Run Steam in desktop mode (not Big Picture)

  • Configure the launch options for a game in Steam to be:

      /path/to/pressure-vessel-wrap -- %command%
  • For interactive testing, if you ran Steam from a shell, you can use:

      /path/to/pressure-vessel-wrap --interactive -- %command%

    The interactive shell's current working directory matches the game's. Run "$@" in the interactive shell to run the game.

  • To test something manually:

    • cd to the directory that you want to be the current working directory inside the container

    • Run one of:

        /path/to/pressure-vessel-wrap --fake-home=/some/path --interactive -- ./whatever-game
        /path/to/pressure-vessel-wrap --freedesktop-app-id=com.example.Anything --interactive -- ./whatever-game
        /path/to/pressure-vessel-wrap --steam-app-id=70 --interactive -- ./whatever-game

Design constraints

  • Steam games assume that they are executed with the current working directory equal to the game's base directory, typically something like ~/.steam/steam/SteamApps/common/Half-Life.

  • Steam games are delivered via existing code to use the Steam CDN, not as Flatpak packages.

  • Each game is assumed to have one subdirectory of SteamApps (for example ~/.steam/steam/SteamApps/common/Half-Life) and one AppID (for example 70 for Half-Life). These are not correlated in any obvious way.

  • Some games have distinct app IDs but share data: for example, X3: Terran Conflict (ID 2820) and X3: Albion Prelude (ID 201310) share SteamApps/common/X3 Terran Conflict.

  • The Steam client and client library need to be able to communicate via shared memory and SysV semaphores.

    • ⇒ We must share /dev/shm
    • ⇒ We must not unshare the IPC namespace

Assumptions

  • Games may write to their own subdirectories of SteamApps/common.

  • Games should not see each other's saved games and other private data in XDG_CONFIG_HOME etc.

  • X11, Wayland, PulseAudio, D-Bus etc. are assumed to use sockets in /run (including $XDG_RUNTIME_DIR), /var or /tmp.

    • X11 uses /tmp: OK
    • Wayland uses $XDG_RUNTIME_DIR: OK
    • The D-Bus system bus always uses /var/run: OK
    • The D-Bus session bus uses $XDG_RUNTIME_DIR/bus on systems with systemd, systemd-logind and the user bus: OK
    • The D-Bus session bus uses abstract Unix sockets on other systems: OK
    • PulseAudio uses $XDG_RUNTIME_DIR on modern systems with systemd: OK
    • PulseAudio uses /var/run on SteamOS: OK
    • PulseAudio might not work on non-systemd non-SteamOS machines?
  • Games can't see ~/.local/share/fonts, ~/.fonts, ~/Music, ~/Desktop, etc. and this is assumed to be OK.

TODO

  • Flatpak-like runtimes that replace /usr (and also /lib*, /bin, /sbin on systems without merged /usr) are not yet implemented.

  • Games can currently read each other's static data via ~/.steam/steam and ~/.steam/root. This could be prevented, if we can define which parts of ~/.steam are considered to be API and which are to be masked.

  • What paths did PulseAudio historically use? It might not work on non-systemd non-SteamOS machines.

  • Games like AudioSurf that want to read ~/Music aren't going to work. They'll need some new special-case option analogous to Flatpak's --filesystem=xdg-music:ro.

  • To avoid weird behaviour when part of a game respects XDG_CONFIG_HOME and part of it hard-codes ~/.config, maybe we should make $fake_home/config a synonym (bind-mount) for $fake_home/.config, and the same for $fake_home/.local/share with $fake_home/data and $fake_home/.cache with $fake_home/cache?

Design

Each game gets a private home directory in ~/.var/app, the same as Flatpak apps do. The name of the private home directory follows the naming recommendations from the freedesktop.org Desktop Entry Specification. For Steam apps, unless a better name has been specified, the app's freedesktop.org app ID is based on its Steam AppID, for example com.steampowered.App70 for Half-Life.

For the game, XDG_CONFIG_HOME, XDG_DATA_HOME and XDG_CACHE_HOME are set to the ./config, ./data and ./cache directories inside its private home directory, so well-behaved freedesktop-style apps will write there.

  • Example: X3: Terran Conflict writes to $XDG_CONFIG_HOME/EgoSoft/X3TC
  • Example: Mesa writes to $XDG_CACHE_HOME/mesa (assuming $MESA_GLSL_CACHE_DIR is unset)

Anything that hard-codes a path relative to $HOME (including .config, .local/share or .cache) will write to the corresponding directory in ~/.var/app.

  • Example: Avorion writes to ~/.avorion/ in the container, which is ~/.var/app/com.steampowered.App445220/.avorion/ in the real home directory

If more than one game is meant to share a private home directory, then they need to be run with --freedesktop-app-id= or --steam-app-id= to set that up.

  • For example, if X3: Terran Conflict and X3: Albion Prelude are to share data, they would have to both be run with for example --freedesktop-app-id=com.egosoft.X3TC or --steam-app-id=2820.

The app is not currently made available read-only on /app (as it would be in Flatpak) because that would require us to reassemble the root directory.