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Re: Guix, Nix flakes, and object capabilities

From: Simon Tournier
Subject: Re: Guix, Nix flakes, and object capabilities
Date: Fri, 03 Mar 2023 02:12:48 +0100


On Tue, 28 Feb 2023 at 22:13, Jonathan Frederickson <> 

> I recently had a discussion in #spritely on Libera.Chat about Guix and
> Nix, and in particular a (relatively) new feature of Nix called flakes
> that Guix doesn't currently have an analogue for.

Well, even after several readings of Nix documentation about Flakes, I
am still not sure to understand how the concept would apply for Guix. :-)

> But the package you end up with each time you do that... depends on
> which revision of Guix you're running when you run ~guix shell~! So if
> I point someone to a project with a ~guix.scm~ file, they might not be
> able to use it if their Guix revision is too old. (Or too new, if
> packages have been renamed or removed.) More generally, it means that
> they do not end up with the same dependency graph that I do. This
> makes troubleshooting potentially tricky, because if something breaks
> you have to check the resulting profile to see which versions of your
> package's dependencies (and transitive dependencies) are actually
> installed.

That’s why “guix time-machine” is really cool! :-)

Basically, I have my current Guix revision and I run “guix pull” every…
indecent duration.  However, I run many many “guix time-machine”, well
each time I am working on a project, so daily.

Even, I only run “guix pull” against the default Savannah Guix channel.
Then, depending on the projects, they can require the channels
guix-science, guix-cran or others.

The graph I need for a specific project is controlled by the file
channels.scm and I run, from the directory of that project:

    guix time-machine -C channels.scm -- shell

This channels.scm file can be part of the Git repository of that
project.  And sometimes, I have several files for testing revision

On the top of that, for some cases, I have specific packages for one
project.  Therefore, I have a folder, say guix/extra containing one or
more files which define new or variant packages.

Somehow, my typical line looks like:

  guix time-machine -C state-1.scm -- shell -L guix/extra -m manifest-A.scm
  guix time-machine -C state-2.scm -- shell -L guix/extra -f guix-B.scm


(Although, I should admit that I barely use the option -f. ;-))

> For those who haven't used Nix, it has a solution to this called
> flakes. Flakes let you specify git repositories explicitly as inputs for
> your project[1]. (It also maintains a flake.lock file so you can lock to
> a specific revision automatically while still using a named branch in
> your inputs directly, but I believe you could in theory refer to a
> specific rev in your inputs.) Effectively, the channels you're using for
> dependencies are specified by the project you're building, not whatever
> happens to be configured on your local machine.

>From my understanding, what I describe above seems providing these
Nix flakes, no?

Or could you explain on a concrete example what you can do with Nix
flakes that you cannot do with Guix?

Well, UI and how easy to use can also be considered part of “can
do”. :-) I mean, maybe the current Guix way could be improved on the
light of the Nix flakes.

> I think something like this would be useful for Guix for many of the
> same reasons it's useful in Nix. But there's a bit of a security
> conundrum here. Loading Guix package definitions involves code
> execution, which as far as I can tell isn't currently sandboxed at all!
> And that's a problem. When you load package definitions from a channel
> that you've configured on your system, you've explicitly trusted that
> channel's maintainers. But with a flake-like system... even if you might
> be okay depending on someone else's code, that doesn't necessarily mean
> you fully trust them. You might ultimately choose to sandbox the
> resulting binary, but that's moot if you can't fetch its dependencies
> without running arbitrary code with all of your user's authority.

I am not sure to understand this sandbox part.  For instance,

    guix time-machine -C channels.scm -- shell --container

provides you an isolated environment where you can run untrusted
packages coming from untrusted channels.  Even, the option ’-F,
--emulate-fhs’ combined with the option ’-C, --container’ allow to run
untrusted binaries coming from elsewhere (not built by Guix); you need
some care with the options -E and --expose though.

Do you think something is missing?  What could we improve in this area?

Do you mean evaluate the Guile files provided by the untrusted channels
in a sandbox?

> I think there is a solution to this, though. Right now when you
> evaluate Guix package definitions, you're basically running arbitrary
> Guile code. This of course can do anything you can do. But it doesn't
> have to! If you're familiar with Christine Lemmer-Webber's work on
> Spritely, you'll probably know what I'm getting at here: I think using
> object capabilities[2] would fix this. I recommend reading the linked
> blog post for a good explainer on what object capabilities are, as I
> won't do it justice here, but to perhaps oversimplify: code in a
> capability system only has access to the things you give it, and no
> more. It's like lexical scope, but taken very seriously.

Hum, I should have missed some details – since I am not familiar with
“object capability”.  Thanks for the pointer.

Is it another way to view “contract” [a]?

a: <>

> If you think about what a typical package definition needs to be able to
> do to your system directly, I think it's not actually that much? My
> (admittedly basic, possibly flawed) understanding of how Guix works is
> that most of the heavy lifting is done by ~guix-daemon~, which itself is
> pretty heavily sandboxed, and that most of what the ~guix~ subcommands
> are doing is building derivations which instruct ~guix-daemon~ to
> perform build actions. So while you're building these derivations,
> unless I'm misunderstanding:
> - You don't need network access
> - You don't need (much) filesystem access

>From my understanding, Guix and guix-daemon already works that way, no?

Do you mean evaluate the user-side Guile part of “guix foo -f file.scm”
inside a sandboxed environment?

On Wed, 01 Mar 2023 at 10:56, Josselin Poiret <> wrote:

> Now my personal opinion: you should note that it's a very bad idea in
> general to rely on specific versions of dependencies. Downstream
> consumers of your software should be able to use it with up-to-date
> dependencies, because they can provide security benefits, bugfixes,
> etc. Having version pinning like in go leads to dependency hell, and
> should be frowned upon.

Well, it depends on what you want, IMHO. :-)

Sometimes, you want to reproduce the exact same computational
environment, including all the defects.

>From my personal opinion, channels.scm and manifests.scm, probably
considering also transformations, these features allow – putting more
than less efforts – to achieve a fine control with flexibility.


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