Tarballs, the ultimate container image format

Ludovic Courtès — maio 16, 2018

A year ago we introduced guix pack, a tool that allows you to create “application bundles” from a set of Guix package definitions. On your Guix machine, you run:

guix pack -S /opt/gnu/bin=bin guile gnutls guile-json

and you get a tarball containing your favorite programming language implementation and a couple of libraries, where /opt/gnu/bin is a symlink to the bin directory containing, in this case, the guile command. Add -f docker and, instead of a tarball, you get an image in the Docker format that you can pass to docker load on any machine where Docker is installed. Overall that’s a relatively easy way to share software stacks with machines that do not run Guix.

The tarball format is plain and simple, it’s the one we know and love, and it’s been there “forever” as its name suggests. The tarball that guix pack produces can be readily extracted on another machine, one that doesn’t run Guix, and you’re done. The problem though, is that you’ll need to either unpack the tarball in the root file system or to play tricks with the unshare command, as we saw in the previous post. Why can’t we just extract such a tarball in our home directory and directly run ./opt/gnu/bin/guile for instance?

Relocatable packages

The main issue is that, except in the uncommon case where developers went to great lengths to make it possible (as with GUB, see the *-reloc*.patch files), packages built for GNU/Linux are not relocatable. ELF files embed things like the absolute file name of the dynamic linker, directories where libraries are to be search for (they can be relative file names with $ORIGIN but usually aren’t), and so on; furthermore, it’s very common to embed things like the name of the directory that contains locale data or other application-specific data. For Guix-built software, all these are absolute file names under /gnu/store so Guix-built binaries won’t run unless those /gnu/store files exist.

On machines where support for “user namespaces” is enabled, we can easily “map” the directory where users unpacked the tarball that guix pack produced to /gnu/store, as shown in the previous post:

$ tar xf /path/to/pack.tar.gz
$ unshare -mrf chroot . /opt/gnu/bin/guile --version
guile (GNU Guile) 2.2.0

It does the job but remains quite tedious. Can’t we automate that?

guix pack --relocatable

The --relocatable (or -R) option of guix pack, which landed a few days ago, produces tarballs with automatically relocatable binaries. Back to our earlier example, let’s say you produce a tarball with this new option:

guix pack --relocatable -S /bin=bin -S /etc=etc guile gnutls guile-json

You can send the resulting tarball to any machine that runs the kernel Linux (it doesn’t even have to be GNU/Linux) with user namespace support—which, unfortunately, is disabled by default on some distros. There, as a regular user, you can run:

$ tar xf /path/to/pack.tar.gz
$ source ./etc/profile    # define ’GUILE_LOAD_PATH’, etc.
$ ./bin/guile
guile: warning: failed to install locale
GNU Guile 2.2.3
Copyright (C) 1995-2017 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> ,use(json)
scheme@(guile-user)> ,use(gnutls)

We were able to run Guile and to use our Guile libraries since sourcing ./etc/profile augmented the GUILE_LOAD_PATH environment variable that tells Guile where to look for libraries. Indeed we can see it by inspecting the value of %load-path at the Guile prompt:

scheme@(guile-user)> %load-path
$1 = ("/gnu/store/w9xd291967cvmdp3m0s7739icjzgs8ns-profile/share/guile/site/2.2" "/gnu/store/b90y3swxlx3vw2yyacs8cz59b8cbpbw5-guile-2.2.3/share/guile/2.2" "/gnu/store/b90y3swxlx3vw2yyacs8cz59b8cbpbw5-guile-2.2.3/share/guile/site/2.2" "/gnu/store/b90y3swxlx3vw2yyacs8cz59b8cbpbw5-guile-2.2.3/share/guile/site" "/gnu/store/b90y3swxlx3vw2yyacs8cz59b8cbpbw5-guile-2.2.3/share/guile")

Wait, it’s all /gnu/store! As it turns out, guix pack --relocatable created a wrapper around guile that populates /gnu/store in the mount namespace of the process. Even though /gnu/store does not exist on that machine, our guile process “sees” our packages under /gnu/store:

scheme@(guile-user)> ,use(ice-9 ftw)
scheme@(guile-user)> (scandir "/gnu/store")
$2 = ("." ".." "0249nw8c7z626fw1fayacm160fpd543k-guile-json-0.6.0R" "05dvazr5wfh7lxx4zi54zfqnx6ha8vxr-bash-static-4.4.12" "0jawbsyafm93nxf4rcmkf1rsk7z03qfa-libltdl-2.4.6" "0z1r7ai6syi2qnf5z8w8n25b1yv8gdr4-info-dir" "1n59wjm6dbvc38b320iiwrxra3dg7yv8-libunistring-0.9.8" "2fg01r58vv9w41kw6drl1wnvqg7rkv9d-libtasn1-4.12" "2ifmksc425qcysl5rkxkbv6yrgc1w9cs-gcc-5.5.0-lib" "2vxvd3vls7c8i9ngs881dy1p5brc7p85-gmp-6.1.2" "4sqaib7c2dfjv62ivrg9b8wa7bh226la-glibc-2.26.105-g0890d5379c" "5kih0kxmipzjw10c53hhckfzkcs7c8mm-gnutls-3.5.13" "8hxm8am4ll05sa8wlwgdq2lj4ddag464-zlib-1.2.11" "90vz0r78bww7dxhpa7vsiynr1rcqhyh4-nettle-3.4" "b90y3swxlx3vw2yyacs8cz59b8cbpbw5-guile-2.2.3" "c4jrwbv7qckvnqa7f3h7bd1hh8rbg72y-libgc-7.6.0" "f5lw5w4nxs6p5gq0c2nb3jsrxc6mmxbi-libgc-7.6.0" "hjxic0k4as384vn2qp0l964isfkb0blb-guile-json-0.6.0" "ksyja5lbwy0mpskvn4rfi5klc00c092d-libidn2-2.0.4" "l15mx9lrwdflyvmb4a05va05v5yqizg5-libffi-3.2.1" "mm0zclrzj3y7rj74hzyd0f224xly04fh-bash-minimal-4.4.12" "vgmln3b639r68vvy75xhcbi7d2w31mx1-pkg-config-0.29.2" "vz3zfmphvv4w4y7nffwr4jkk7k4s0rfs-guile-2.2.3" "w9xd291967cvmdp3m0s7739icjzgs8ns-profile" "x0jf9ckd30k3nhs6bbhkrxsjmqz8phqd-nettle-3.4" "x8z6cr7jggs8vbyh0xzfmxbid63z6y83-guile-2.2.3R" "xbkl3nx0fqgpw2ba8jsjy0bk3nw4q3i4-gnutls-3.5.13R" "xh4k91vl0i8nlyrmvsh01x0mz629w5a9-gmp-6.1.2" "yx12x8v4ny9f6fipk8285jgfzqavii83-manual-database" "zksh1n0p9x903kqbvswgwy2vsk2b7255-libatomic-ops-7.4.8")

The wrapper is a small statically-linked C program. (Scheme would be nice and would allow us to reuse call-with-container, but it would also take up more space.) All it does is create a child process with separate mount and user namespaces, which in turn mounts the tarball’s /gnu/store to /gnu/store, bind-mounts other entries from the host root file system, and chroots into that. The result is a binary that sees everything a “normal” program sees on the host, but with the addition of /gnu/store, with minimal startup overhead.

In a way, it’s a bit of a hack: for example, what gets bind-mounted in the mount namespace of the wrapped program is hard-coded, which is OK, but some flexibility would be welcome (things like Flatpak’s sandbox permissions, for instance). Still, that it Just Works is a pretty cool feature.

Tarballs vs. Snap, Flatpak, Docker, & co.

Come to think of it: if you’re a developer, guix pack is probably one of the easiest ways to create an “application bundle” to share with your users; and as a user, these relocatable tarballs are about the simplest thing you can deal with since you don’t need anything but tar—well, and user namespace support. Plus, since they are bit-reproducible, anyone can rebuild them to ensure they do not contain malware or to check the provenance and licensing of its contents.

Application bundles cannot replace full-blown package management, which allows users to upgrade, get security updates, use storage and memory efficiently, and so on. For the purposes of quickly sharing packages with users or with Guix-less machines, though, you might find Guix packs to be more convenient than Snap, Flatplak, or Docker. Give it a spin and let us know!

About GNU Guix

GNU Guix is a transactional package manager for the GNU system. The Guix System Distribution or GuixSD is an advanced distribution of the GNU system that relies on GNU Guix and respects the user's freedom.

In addition to standard package management features, Guix supports transactional upgrades and roll-backs, unprivileged package management, per-user profiles, and garbage collection. Guix uses low-level mechanisms from the Nix package manager, except that packages are defined as native Guile modules, using extensions to the Scheme language. GuixSD offers a declarative approach to operating system configuration management, and is highly customizable and hackable.

GuixSD can be used on an i686, x86_64 and armv7 machines. It is also possible to use Guix on top of an already installed GNU/Linux system, including on mips64el and aarch64.