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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. It is used by Steam's Steam Linux Runtime compatibility tool to run games in individual game-specific containers. For background on pressure-vessel and SteamLinuxRuntime, please see:

Use cases

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, using some or all of the host system's graphics stack.

In particular, this would avoid or mitigate the following problems with the LD_LIBRARY_PATH-based Steam Runtime:

  • The Steam Runtime cannot contain glibc, because the path to ld.so(8) is hard-coded into all executables (it is the ELF interpreter, part of the platform ABI), the version of ld.so is coupled to libdl.so.2, and the version of libdl.so.2 is coupled to the rest of glibc; so everything in the Steam Runtime must be built for a glibc at least as old as the oldest system that Steam supports. This makes it difficult to advance beyond the Ubuntu 12.04-based 'scout' runtime: everything that is updated effectively has to be backported to Ubuntu 12.04.

  • The Steam Runtime was originally designed to prefer its own version of each library even if the version in the host system was newer, but this broke any graphics driver that relied on a newer version of a host library, notably libstdc++.

  • More recent versions of the Steam Runtime prefer the "pinned" host system version of most libraries that appear to be at least as new as the Runtime's own version, but this can also break things if libraries claim to be compatible (by having the same ELF DT_SONAME) but are in fact not compatible, for example:

    • libcurl.so.4 linked to libssl 1.0 is not completely compatible with libcurl.so.4 linked to libssl 1.1

    • The history of libcurl.so.4 in Debian/Ubuntu has involved two incompatible sets of versioned symbols, due to some decisions made in 2005 and 2007 that, with hindsight, were less wise than they appeared at the time

    • Various libraries can be compiled with or without particular features and dependencies; if the version in the Steam Runtime has a feature, and the host system version is newer but does not have that feature, then games cannot rely on the feature

    • In the worst-case, compiling a library without a particular feature or dependency can result in symbols disappearing from its ABI, resulting in games that reference those symbols crashing

    • There is a fairly subtle interaction between libdbus, libdbusmenu-gtk, libdbusmenu-glib and Ubuntu's patched GTK 2 that has resulted in these libraries being forced to be taken from the Steam Runtime, to avoid breaking the Unity dock

  • Preferring the "pinned" host system libraries makes it very easy for a game developer to make their game depend on a newer host-system library without realising that they have done so, resulting in their game not running correctly on older host systems, or on host systems that do not have a library with a matching DT_SONAME.

A future goal is to use libcapsule to avoid the library dependencies of the host system's graphics stack influencing the libraries loaded by games at all, and in particular allow game logic to use the Runtime's libstdc++ while the host system graphics driver uses a different, newer libstdc++. This is not yet implemented.

Protecting $HOME

(This feature is not being actively developed right now.)

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.

Building a local test version of pressure-vessel

pressure-vessel is a reasonably ordinary Meson project. It depends on GLib and libXau.

If you are using it with the under-development Steam Linux Runtime, you will probably want to compile it with an unusual ${prefix}:

prefix="${XDG_DATA_HOME:-"$HOME/.local/share"}/Steam/steamapps/common/SteamLinuxRuntime/pressure-vessel"
libdir="lib/x86_64-linux-gnu"
meson --prefix="$prefix" --libdir="$libdir" _build
ninja -C _build
meson test -v -C _build             # optional
ninja -C _build install

Note that this will give you a non-production version of pressure-vessel that is likely to depend on libraries from your host system, which is good for quick turnaround and debugging, but not suitable for deployment to the public. For that, you'll want a relocatable installation: see below.

If you are developing pressure-vessel, you might well want to alter the bundled libcapsule tools. You can do this by building your own copy of libcapsule. This requires 32- and 64-bit copies of libelf (the version from elfutils) and its development files.

git clone https://gitlab.collabora.com/vivek/libcapsule.git
cd libcapsule
prefix="${XDG_DATA_HOME:-"$HOME/.local/share"}/Steam/steamapps/common/SteamLinuxRuntime/pressure-vessel"
libdir="lib/x86_64-linux-gnu"
NOCONFIGURE=1 ./autogen.sh
mkdir _build _build/i386 _build/x86_64
( cd _build/x86_64; ../../configure
--host=x86_64-linux-gnu --enable-host-prefix=x86_64-linux-gnu- \
--prefix="$prefix" --libdir="\${prefix}/$libdir" \
--disable-gtk-doc --disable-shared --without-glib )
( cd _build/i386; ../../configure
--host=i686-linux-gnu --enable-host-prefix=i386-linux-gnu- \
--prefix="$prefix" --libdir="\${prefix}/$libdir" \
--disable-gtk-doc --disable-shared --without-glib )
make -C _build/x86_64
make -C _build/i386
make -C _build/x86_64 install
make -C _build/i386 install

Use --host=i586-linux-gnu instead of --host=i686-linux-gnu if your distribution uses that tuple for its 32-bit compiler. For the i386 build, if you don't have an i?86-linux-gnu-gcc you might have to add CC="gcc -m32" to the i386 configure command line.

Again, this will give you a non-production version of libcapsule that is likely to depend on libraries from your host system.

Building a relocatable install for deployment

To make the built version compatible with older systems, you will need a SteamRT 1 'scout' environment.

For this, you need prebuilt versions of libcapsule-tools-relocatable:amd64 and libcapsule-tools-relocatable:i386 either in your build environment or available via apt-get. For example, you could do the build in a SteamRT 1 'scout' SDK Docker image that has those packages already. Then you can do:

meson --prefix="$(pwd)/_build/prefix" -Dsrcdir=src _build
ninja -C _build
meson test -v -C _build             # optional
ninja -C _build install
rm -fr _build/relocatable-install
_build/prefix/bin/pressure-vessel-build-relocatable-install \
    --output _build/relocatable-install \
    --archive .
./tests/pressure-vessel/relocatable-install.py \
    _build/relocatable-install      # optional

When building a development version of pressure-vessel using a 'scout' SDK Docker image, if the pressure-vessel-build-relocatable-install script is unable to find some of the required dependencies, it might be necessary to add apt repositories containing development versions of those dependencies to /etc/apt/sources.list, and run apt-get update.

For more convenient use on a development system, if you have a SteamRT 1 'scout' SDK tarball or an unpacked sysroot, you can place a tarball at _build/sysroot.tar.gz or unpack a sysroot into _build/sysroot, and prefix those commands with ./sysroot/run-in-sysroot.py:

./sysroot/run-in-sysroot.py apt-get update
./sysroot/run-in-sysroot.py meson --prefix="$(pwd)/_build/prefix" _build
./sysroot/run-in-sysroot.py ninja -C _build
(etc.)

(Or put them in different locations and pass the --sysroot and --tarball options to ./sysroot/run-in-sysroot.py.)

The relocatable install goes into relocatable-install (or whatever you used as the --output), and a compressed version ends up in the directory passed as an argument to --archive.

Instructions for testing

  • Install Steam and some games. Convenient choices for a reasonably small free-to-play game on various engines include:

    • Floating Point (game ID 302380)
      • very small/quick to install, works OK on weak hardware
      • UnityPlayer 4.3.4f1
    • Life is Strange, episode 1 (319630)
      • Unreal Engine 3
    • Team Fortress 2 (440)
      • Source engine
    • Unturned (304930)
      • UnityPlayer 5.5.3f1
  • Install bubblewrap or Flatpak (in Debian, Ubuntu or SteamOS: sudo apt-get install bubblewrap). pressure-vessel bundles its own fallback copy of bubblewrap, but that one will only work on kernels that are configured to allow unprivileged user namespaces, such as recent Ubuntu and Fedora (but not, for example, Debian, Arch Linux or Red Hat Enterprise Linux unless specially configured).

  • Install Python 3, PyGI and GTK 3 introspection data (in Debian, Ubuntu or SteamOS: sudo apt-get install python3-gi gir1.2-gtk-3.0). This is not necessary for production use but is very convenient when testing interactively.

  • Unpack pressure-vessel-VERSION-bin.tar.gz (or ...-bin+src.tar.gz) in some convenient place, for example ~/.steam/steam/steamapps/common/SteamLinuxRuntime/pressure-vessel:

      $ rm -fr ~/.steam/steam/steamapps/common/SteamLinuxRuntime/pressure-vessel
      $ mkdir -p ~/.steam/steam/steamapps/common/SteamLinuxRuntime/pressure-vessel
      $ tar \
          --strip-components=1 \
          -C ~/.steam/steam/steamapps/common/SteamLinuxRuntime/pressure-vessel \
          -xzvf ~/pressure-vessel-*-bin.tar.gz
  • Put some Flatpak-style runtimes alongside pressure-vessel, for example ~/.steam/steam/steamapps/common/SteamLinuxRuntime/scout, so that you have a ~/.steam/steam/steamapps/common/SteamLinuxRuntime/scout/files/ directory and a ~/.steam/steam/steamapps/common/SteamLinuxRuntime/scout/metadata file. For example, download a com.valvesoftware.SteamRuntime.Platform-amd64,i386-scout-runtime.tar.gz from https://repo.steampowered.com/steamrt-images-scout/snapshots/ and unpack it like this:

      $ rm -fr ~/.steam/steam/steamapps/common/SteamLinuxRuntime/scout
      $ mkdir -p ~/.steam/steam/steamapps/common/SteamLinuxRuntime/scout
      $ tar \
          -C ~/.steam/steam/steamapps/common/SteamLinuxRuntime/scout \
          -xzvf ~/com.valvesoftware.SteamRuntime.Platform-amd64,i386-scout-runtime.tar.gz

    Or to have a SDK environment with more debugging tools, and optionally detached debugging symbols, download a com.valvesoftware.SteamRuntime.Sdk-amd64,i386-scout-runtime.tar.gz and optionally a matching com.valvesoftware.SteamRuntime.Sdk-amd64,i386-scout-debug.tar.gz, and unpack them like this:

      $ rm -fr ~/.steam/steam/steamapps/common/SteamLinuxRuntime/scout_sdk
      $ mkdir -p ~/.steam/steam/steamapps/common/SteamLinuxRuntime/scout_sdk
      $ tar \
          -C ~/.steam/steam/steamapps/common/SteamLinuxRuntime/scout_sdk \
          -xzvf ~/com.valvesoftware.SteamRuntime.Sdk-amd64,i386-scout-runtime.tar.gz
    
      $ tar --strip-components=1 \
          -C ~/.steam/steam/steamapps/common/SteamLinuxRuntime/scout_sdk/files/lib/debug/ \
          -xzvf ~/com.valvesoftware.SteamRuntime.Sdk-amd64,i386-scout-debug.tar.gz

    Or if you have a chroot environment with extra packages installed, such as the /var/chroots/steamrt_scout_amd64 generated by setup_chroot.sh or an unpacked copy of com.valvesoftware.SteamRuntime.Sdk-amd64,i386-scout-sysroot.tar.gz, you can use that by making files a symbolic link to it:

      $ rm -fr ~/.steam/root/common/SteamLinuxRuntime/scout-chroot
      $ mkdir -p ~/.steam/root/common/SteamLinuxRuntime/scout-chroot
      $ ln -s /var/chroots/steamrt_scout_amd64 \
          ~/.steam/root/common/SteamLinuxRuntime/scout-chroot/files
  • Launch a game once without pressure-vessel

  • Configure the launch options for the chosen game. Either use the Steam (non-Big-Picture) UI, for example by logging in as the desktop user on SteamOS, or:

    • be the Steam user, possibly via sudo -u steam -s

    • $ nano ~/.steam/steam/userdata/[0-9]*/config/localconfig.vdf

    • navigate to UserLocalConfigStore/Software/Valve/Steam/Apps/game ID

    • below LastPlayed, add:

        "LaunchOptions" "PRESSURE_VESSEL_WRAP_GUI=1 ~/.steam/steam/steamapps/common/SteamLinuxRuntime/pressure-vessel/bin/pressure-vessel-unruntime -- %command%"
    • restart Steam (on SteamOS use sudo systemctl restart lightdm)

      • TODO: Is there a scriptable way to make Steam edit localconfig.vdf itself, or to make it reload an edited localconfig.vdf?
  • Run the game. If successful, you will get a GUI launcher that lists possible runtimes. Choose one and click Run. You should get a game. pstree -pa will look something like this:

      |-SteamChildMonit,13109
      |   `-sh,13110 -c...
      |       `-bwrap,13132 --new-session --setenv LD_LIBRARY_PATH...
      |           `-Floating Point.,13133
  • The test UI automatically looks for runtimes in the following locations:

    • Next to pressure-vessel's bin and lib directories
    • The directory above pressure-vessel's bin and lib directories

    Runtimes intended for the test UI must have a files subdirectory, like Flatpak runtimes.

    You can also choose to use the host system /usr directly.

  • The default runtime for pressure-vessel-unruntime is the host system, with the LD_LIBRARY_PATH Steam Runtime overlaid onto it. You can specify a runtime with the --runtime option.

    The runtime can be any of these:

    • The files subdirectory of a Flatpak-style runtime such as ~/.steam/steam/steamapps/common/SteamLinuxRuntime/scout or ~/.local/share/flatpak/runtime/com.valvesoftware.SteamRuntime.Platform/x86_64/scout/active, for example produced by [flatdeb][] (this is a special case of a merged /usr). For example, you could use:

        /opt/pressure-vessel/bin/pressure-vessel-unruntime --runtime=$HOME/.steam/steam/steamapps/common/SteamLinuxRuntime/scout -- %command%

      or:

        /opt/pressure-vessel/bin/pressure-vessel-unruntime --runtime=$HOME/.local/share/flatpak/runtime/com.valvesoftware.SteamRuntime.Platform/x86_64/scout/active -- %command%
    • A merged /usr containing bin/bash, lib/ld-linux.so.2, bin/env, share/locale, lib/python2.7 and so on

    • A sysroot containing bin/bash, lib/ld-linux.so.2, usr/bin/env, usr/share/locale, usr/lib/python2.7 and so on, optionally with bin, lib etc. being symlinks into usr. For example, if you have a chroot in /var/chroots/steamrt_scout_amd64 generated by setup_chroot.sh, you could use:

        /opt/pressure-vessel/bin/pressure-vessel-unruntime --runtime=/var/chroots/steamrt_scout_amd64 -- %command%

When using a runtime, graphics drivers come from the host system, while their dependencies (notably glibc and libstdc++) come from either the host system or the runtime, whichever appears to be newer. This is currently hard-coded in pressure-vessel-wrap and cannot be configured.

Debugging

  • To see the filesystem in which the game is executing, get the process ID of some game process from ps and use:

      $ sudo ls -l /proc/$game_pid/root/

    You'll see that the graphics drivers and possibly their dependencies are available in /overrides inside that filesystem, while selected files from the host are visible in /run/host.

  • To test something manually, or to debug startup issues:

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

    • Run:

        /opt/pressure-vessel/bin/pressure-vessel-test-ui -- ./whatever-game

      or for the low-level version:

        /opt/pressure-vessel/bin/pressure-vessel-wrap -- ./whatever-game

      Optionally add more options before the --, such as --runtime, --xterm and --shell-instead.

    In particular, if the xterm can't be launched and you are debugging why, use the --tty option to get a shell in the container.

    If bwrap is failing to start up and so you can't get a shell in the container at all, construct a simpler-but-simpler bwrap command-line (based on what pressure-vessel-wrap logs to stderr) and try that.

  • For interactive testing from Steam, if your runtime (if used) or host system (if no runtime) contains an xterm binary, you can use something like:

      /opt/pressure-vessel/bin/pressure-vessel-unruntime --shell-instead -- %command%

    to run an xterm containing an interactive shell. The interactive shell's current working directory matches the game's, and you can explore the container from there.

    When ready, run "$@" in the interactive shell (with the double quotes included!) to run the game.

    Or, use --shell-after or --shell-fail to try running the game first, and then run a shell afterwards, either unconditionally or only if the command fails.

    Similarly, you can use --xterm to make the game's output appear in an xterm without having an interactive shell.

    When you use any of these options, Steam will treat the xterm as part of the game, so you cannot launch the game again from the Steam UI until you have exited from the xterm.

  • For interactive testing, if you ran Steam from a shell (not normally valid on SteamOS!), you can use something like:

      /opt/pressure-vessel/bin/pressure-vessel-unruntime --tty --shell-instead -- %command%

    This is a simpler version of --xterm, which uses the terminal from which you ran Steam instead of an xterm inside the container.

    The interactive shell's current working directory matches the game's. As with the xterm, run "$@" in the interactive shell to run the game.

  • To get a modifiable copy of the runtime, use something like:

      mkdir -p $HOME/.steam/steam/steamapps/common/SteamLinuxRuntime/var
      /opt/pressure-vessel/bin/pressure-vessel-unruntime \
          --runtime=$HOME/.steam/steam/steamapps/common/SteamLinuxRuntime/scout/files \
          --copy-runtime-into=$HOME/.steam/steam/steamapps/common/SteamLinuxRuntime/var \
          -- %command%

    You will find a temporary copy of the runtime in the directory you chose, in this example something like .../SteamLinuxRuntime/var/tmp-1234567. You can modify it in-place. For example, if you want to replace libdbus-1.so.3 with an instrumented version, you can do something like this:

      cd .../SteamLinuxRuntime/var/tmp-1234567
      rm -f overrides/lib/x86_64-linux-gnu/libdbus-1.so.3*
      rm -f usr/lib/x86_64-linux-gnu/libdbus-1.so.3*
      cp --dereference ~/dbus/dbus/.libs/libdbus-1.so.3 \
          overrides/lib/x86_64-linux-gnu/libdbus-1.so.3

    To avoid temporary copies building up forever, they will be deleted next time you run a game in a container (assuming there is no longer anything running in the previous container), unless you create a file like .../var/tmp-1234567/keep to flag this root directory to be kept for future reference.

More options

Use pressure-vessel-unruntime if you are in a Steam Runtime environment (the Steam Runtime's run.sh or a Steam game), and pressure-vessel-wrap or pressure-vessel-test-ui if you are not ("Add non-Steam game" in Steam, or a non-Steam-related interactive shell).

  • To protect $HOME, add one of the following options before --:

    • --fake-home=/some/path (automatically unshares the home directory)
    • --freedesktop-app-id=com.example.Anything --unshare-home
    • --steam-app-id=70 --unshare-home (when running from Steam you can use --steam-app-id=${SteamAppId}, which is the default)

    This feature is not being actively developed right now.

Steam integration

If a future version of Steam is modified to run certain games using pressure-vessel, it should remove the Steam Runtime environment variables from the environment with which it runs them (like it does for "Add non-Steam game"), and use pressure-vessel-wrap directly. pressure-vessel-unruntime is just a workaround for this not being something that Steam supports yet.

Design constraints

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

    • This is not actually completely true! Apparently some games, like "Estranged: Act I" (ID 261820), start up with the current working directory as a subdirectory of the directory they need. pressure-vessel does not currently receive enough information from Steam to fix this. See steam-runtime#236.
  • 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 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

  • 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.

Design

Setting up the container

With a runtime, the container is basically the same as a Flatpak sandbox. The root directory is a tmpfs. The runtime is bind-mounted on /usr, with symbolic links at /bin, /sbin and /lib* so that commonly-hard-coded paths like /bin/sh, /sbin/ldconfig and /lib/ld-linux.so.2 work as expected.

With a sysroot-style runtime, or when using the host system as the runtime, the setup is the same, except that /bin, /sbin and /lib* might be real directories bind-mounted from the runtime or host if it has not undergone the /usr merge.

Either way, selected paths from the host system are exposed in /run/host, the same as for flatpak run --filesystem=host in Flatpak.

Interactive debugging

--tty, --xterm and the --shell options work by wrapping an increasingly long "adverb" command around the command to be run.

Unsharing the home directory

(This feature is not being actively developed right now.)

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, ./.local/share and ./.cache directories inside its private home directory, so that well-behaved freedesktop-style apps will write there, and badly-behaved freedesktop-style apps that ignore the environment variables and hard-code their default values will also 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)

./config, ./data and ./cache in the private home directory are symbolic links to .config, .local/share and .cache respectively, for better discoverability and compatibility with Flatpak (which uses those directories in ~/.var/app as its usual values for XDG_CONFIG_HOME, XDG_DATA_HOME and XDG_CACHE_HOME).

Anything that hard-codes a path relative to $HOME (including .config, .local/share or .cache) will write to the corresponding directory in ~/.var/app. This is the same as the behaviour of a Flatpak app with --persist=..

  • 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). It could be, if desired.