This is part 1 of a 2 part series. This post will define the
problem we're trying to solve, and part 2 will go into some details
on a potential storage mechanism to make this a reality.
Suppose you're working on a highly regulated piece of software.
For example, something on a defense contract, or a medical device,
or the space shuttle. One goal that most regulators will have is
that we can fully determine how the software was built at any point
in time. The gold standard for this is fully reproducible builds,
where you get byte-identical artifacts for rerunning the build
system at different times.
Not all of our build tools support that unfortunately, due to a
variety of reasons which I'm not going to go into. The Debian project has
been making great strides in that direction, as has NixOS. But let's talk about a slightly
weaker guarantee: reproducible build plans.
The idea here is simple: for a given set of source files, I can
deterministically know exactly which versions of its dependencies
will be used. Usually, there's some kind of boundary to how deeply
this determinism goes. For example, which of the following are
- The exact versions of my language-specific (Haskell, Rust,
Python, etc) source files
- The exact versions of the system's libraries
- The version of the kernel I'm building on
- The hardware I'm building on
NOTE Other things, like filesystem state, also apply.
For that matter, in a crazy build system, GPS location could matter
too, if it somehow affected the build. But these are some of the
most common cases.
Building with something like Nix will guarantee determinism in
the first two bullets. Docker can be (ab)used to give the same
guarantee. Virtual machines can give guarantees about the kernel as
The rest of this blog post will talk about just that first
bullet: language-specific source file determinism. That's not
because the other points are unimportant, but because:
- It's the problem I typically have to solve
- Docker and VMs can encapsulate the others very well for most
- In practice, there tends to be the most variability in build
process output from language-specific source files, due to often
large numbers of such dependencies and frequent releases of those
I'll primarily be talking about how this affects the Haskell world, and in
particular the Stack build
tool, but the ideas hopefully generalize well to other
A primary design goal in Stack is reproducible build plans,
usually (but not exclusively) provided via Stackage
Snapshots. These snapshots define a compiler version, a set of
packages and their versions*, and various configuration like build
flags. These snapshots are also immutable. Most users use the Long
Term Support (LTS) flavor of snapshots, and end up with a
stack.yaml configuration file like the following:
Stack knows where to download the
configuration file from (specifically, from a Github repo), and
takes care of that for you automatically. This looks perfectly
reproducible: LTS 10.3 is immutable, fully determines the exact
content of all of its packages, and the flags to provide to build
it. Given the same OS and same executable of the Stack build tool,
you should be able to make a very strong argument to a regulator
that this is a fully reproducible build plan… right?
* And for those familiar: also specifies Hackage revisions of
the cabal file.
How do you know that LTS 10.3 is immutable? Easy: I just told
you! And I am clearly:
- Totally trustworthy
- The only person with the ability to change the
lts-10.3.yaml file. There are clearly no other people
with push access to the repo, or someone at Github with the ability
to override our access controls.
- Going to live forever, and never pass on control of the project
to anyone else.
- Happy to sign a boatload of liability documents that your
regulator demands be signed to determine who will be at fault and
responsible to pay damages when the missile guidance system you're
writing bombs the wrong house due to a faulty version of
leftpad being used.
Obviously, my goal as one of the Stackage Curators is to strive
to deliver on the guarantees we're claiming. We want snapshots to
remain immutable for all time. But we can't ignore the fact that
some things are completely outside of our control. And a good
regulator will notice and challenge this.
Same with packages
OK, let's pretend for just a moment that you could convince your
regulator that snapshots are totally immutable and awesome. Next,
she's going to open up that
lts-10.3.yaml file and see
something along the lines of:
- name: foobar
# And lots and lots and lots more
# Note that our config files in practice look
# nothing like this :)
I imagine a conversation going something like this:
Regulator: Alright, how do you know what
Developer: Well, obviously you go to your package index… which
isn't specified in the snapshot file, of course. It's specified in
Stack's global config. Regulator: Why?
Developer: Well, it allows people to more easily host mirrors.
Regulator: So you mean if you change some other config file, it can
totally change which
foobar-1.2.3 is used?
Developer: Yeah, but that's totally a feature, not a bug. And
anyway, we guarantee in our build process that this doesn't
Regulator: OK. Fine. And how do you know that when you download
foobar-1.2.3 that it contains the exact same content
at all time?
Developer: Oh, remember how I told you that Michael's a real
trustworthy guy and runs Stackage Snapshots? Yeah, same for the
Hackage package index.
Notice the pattern here. Even taken as a given that everyone
wants to work towards immutability, if my job is to make guarantees
to a regulator, everyone's best intentions are irrelevant.
The solution to this is relatively straightforward. Instead of
trusting some arbitrary identifier which gives no guarantees of the
file contents, let's consider this reality instead:
name: lts-10.3 # display purposes only
bytes: 6693345 # byte count, avoid an overflow attack
Now the conversation with the regulator:
Regulator: How do we know what lts-10.3 is?
Developer: We don't, and we don't care.
Regulator: What's it there for?
Developer: Documentation purposes only.
Regulator: OK, and how do we know that we have the right snapshot
Developer: We perform a cryptographic hash on the file contents and
ensure it matches the hash we placed in our config file.
This depends on trusting cryptographic hashes (which most
regulators are willing to do in my experience), and on having some
way of finding the config file based on the cryptographic hash
(more on that in a bit). And for that second bit, we have a
guarantee that the snapshot cannot be changed without detection,
which is not the case with
lts-10.3 as the only
Similarly, we would want to extend the snapshot format itself to
retain this metadata:
- name: foobar
There's no way for an attacker to slip in a nefarious
foobar-1.2.3 without breaking SHA256 security. And
once again, we can use the hash for performing downloads, and then
simply verify the contents actually contain
foobar-1.2.3 by inspecting metadata (in Haskell land:
the cabal package file).
I love typing in
resolver: lts-10.3: it's easy to
remember, quick, and explains exactly what I want. But easy and
quick are not the cornerstones of regulated software. To make this
story more palatable, we could easily add some tooling support,
stack add-hashes, which modifies a
stack.yaml to add the cryptographic hashes to a
--verified mode (or similar) that refuses to
download anything that doesn't have a cryptographic hash to back it
These could even be provided outside of the build tool itself,
there's no necessity for it being in Stack.
Keep build metadata files separately
This may be a specific quirk of Haskell, but I'll spell it out
here anyway. It's common in Haskell build tools to want to analyze
the build metadata files (cabal package files) to determine
dependency trees. Therefore, we'd want to support downloading them
- name: foobar
This allows us to download the metadata without downloading the
entire package. Also, for those familiar with it, this provides a
robust way to handle Hackage file revisions.
In the next post, we'll discuss how to create a storage system
that can provide downloads of packages, package metadata, and
snapshot definitions. Stay tuned!
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