Интервью: тестирование LXC (Linux Containers)

LXC - это проект по разработке технологии контейнерной виртуализации в ядре Linux, преследующий цель обеспечить функциональность контейнеров на уровне OpenVZ в ванильном ядре. Кстати, OpenVZ, который в основном разрабатывают в Parallels, и LXC, поддерживаемый разработчиками Canonical - это не два конкурирующих проекта. OpenVZ можно рассматривать как надстройку над LXC (так как LXC ещё готова к полноценному использованию, многих вещей в ванильном ядре нет, а OpenVZ патч это добавляет) плюс наши утилиты командной строки (vzctl и друзья), плюс всякие шаблоны и т.п. А разработчики OpenVZ – одновременно разработчики LXC! Более подробно о всех хитросплетениях проектов написал Кирилл Колышкин. LXC для управления контейнерами предоставляет свой набор утилит, но если вам знакома утилита vzctl из проекта OpenVZ, то вы можете использовать её, она поддерживает работу с upstream ядром.

Стефан Грабе (Stéphane Graber), разработчик проекта LXC, рассказал как устроены разработка и тестирование в проекте LXC.

Please introduce yourself.


I’m Stéphane Graber. I work for Canonical Ltd. and I’m the project leader for LXC and its other related projects (cgmanager, lxcfs and lxd).

How does development process of LXC look like? Do you use Continuous Integration in development process?

LXC’s development is a bit of an hybrid. We have a bunch of contributors who are used to kernel-style workflow, so mostly communicating on our mailing-list, doing code-reviews through e-mails. And then we have some other contributors who prefer to use github pull-requests.

We accomodate both.

Any change landing in LXC must have been reviewed by one of the committers and committers obviously can’t review their own code.

We have some minimal tests that are run using Travis both on commits in the main repository and for pull requests.

And we have much more comprehensive integration tests covering multiple compilers and multiple architectures which we run on every commit in our Jenkins.

LXC source tree contains directory with a number of tests. How does testing process look? Do you use regular testing?

Testing happens on every commit so git master is usually functional.

We also have daily builds available for Ubuntu which a lot of our developers run on their day to day machine, helping catch anything the testsuite may have missed.

The same testsuite is also used by distributions downstream to make sure LXC didn’t regress in their particular environment.

What tools, tests and testing frameworks do you use? How do often developers create new tests?

LXC’s testsuite is a mix of C binaries and shell scripts. It’s typically targeted at either big features like unprivileged containers doing an end to end test in those cases or at very narrow, specific security features like apparmor where we want to test all the edge cases to make sure nothing can possibly regress.

We usually get new tests with new important features. Regression testing for LXC is made difficult in that most of the bugfixes we incorporate these days is targeted at a very specific environment (typically kernel + kernel configuration + mount table) which we can’t really replicate.

What is the process for testing new features or significant changes?

The developer would typically send an e-mail to our list describing exactly what they plan on implementing, getting feedback there, then branch the code, implement the feature, ideally add a testcase and then send it for review.

At that point, we’ll make sure that the code does match the original description of the feature which everyone agreed on, a few developers will pull the branch and test the feature manually, looking for corner cases, send review feedback and after a few iterations of that, the code gets pushed to master.

What kind of testing do they use (performance, functional, compatibility, stability, unit testing etc)? Do you test manually?

As LXC in itself doesn’t have any runtime overhead by design, performance testing isn’t something we worry about much. Any performance issue with containers is likely to be a kernel bug.

Functional, stability and to some extent unit tests are achieved through our testsuite. It does make sure LXC’s main feature all work as expected. It runs in a perfectly identical environment every time, so it makes detecting any change very easy and because we run it hundreds of times a day, resulting in tens of thousands of containers being created, it’s bound to run into any potential race condition we may have.

The most important thing in virtualization is isolation. How do you test it for containers?

LXC sets up the isolation by using kernel features, it doesn’t itself manage the isolation, so there’s little risk of regression there.

We do still have tests for some specific kernel security features like AppArmor which will try to escape the isolation in order to ensure that our security profile was indeed applied by the kernel and that there wasn’t a big kernel regression (we’ve caught a few over the years).

Do you have a list of known issues for isolation with public available exploits?

We don’t have a list, no.

LXC’s policy is not to consider privileged containers as safe. There has been exploits in the past and there will be in the future because that way of running containers just isn’t secure by design.

LXC does provide apparmor, selinux and seccomp profiles which as far as we know blocks every single exploit out there but resource DoS is still possible and there’s bound to be a few more possible ways of escaping the container.

What we implemented a while back are completely unprivileged containers, those are safe by design as any exploit which would let you escape one such container would equally let any unprivileged user on your system become root. That’s not to say such exploits don’t exist, but they’re a lot more visible and so a lot more likely to have been thought about and solved.

For unprivileged containers, there has been a few exploits of kernel bugs in the past but all of those have been resolved in the current Linux kernel and the fixes have been backported to all stable Linux kernels.

Do you run these exploits as a part of regression testing? Does these tests available for community?

We don’t, though some of our tests probably go through some of the same code paths. Really, any of those security issues would have been kernel bugs and so regression testing belongs in the kernel, not in LXC.

Do you use regular security audit of code?

Not regular. The Ubuntu security team has been kind enough over the years to do code reviews for specific privileged bits of code we wrote.

And again, LXC isn’t a virtual machine, it doesn’t catch and interpret everything that happens in the container. It only does a bunch of kernel calls at the start and at the end of the container and provides a user interface to interact with the container. The isolation and security is entirely done by the kernel.

Also, currently LXC doesn’t expose any interface to the container so there’s no way from within the container to trigger any reaction from the LXC process outside of it. Making the attack surface there, non-existent.

LXC supports a number of architectures (i686, x86_64, ppc, ppc64, S390, armel and armhf). Do you really test all of them regularly?

We do test builds and test runs on i686, x86_64, ppc, ppc64, ppc64el, armel, armhf and arm64 on every commit.

I don’t have a big IBM mainframe in my basement so we don’t do automated S390 testing :)

Do you measure code coverage of LXC? How much of code covered by tests?

We don’t.

Do you use negative testing techniques (like fuzz testing, fault injection etc)?

We’ve received a few patches as a result of people doing some fuzzing, specifically with our configuration parser, but while those are definitely issues we’ll accept patches for. They’re not security issues since you must be the container owner to have access to that, so finding more of those isn’t a priority for us.

Do you use static code analysis tools or services like Coverity? Or maybe another techniques directed to improving quality of code?

Every commit gets uploaded to Coverity and while it often gets confused due to our use of some unusual kernel features, it has helped us find quite a number of issues which we usually fix in batch every so often (look for coverity: in our commit messages).

Do you involve community in testing process of LXC?

Sure, LXC is a community project, we don’t all work for the same company and so everyone is welcome and encouraged to help testing.

I typically act as the release manager, publishing a rough timeline for the release as we get to the point where we have enough changes to warrant a release. Once we start hitting RCs, we encourage all of our contributors and advanced users to make sure their use case still works fine (be that running LXC on Android, running on some obscure embedded system, some odd architectures or making sure their language bindings are up to date with our library).

Let’s imagine someone want to contribute to LXC quality. What will you advice to him?

That we very much welcome help there but to keep in mind that to keep an active community, we need to make contributing easy.

So having extra tools that we can automatically run and that will report real problematic issues is great. Having an overly strict set of rules or requiring everyone add metadata or run their change through some extra tools before contributing, isn’t acceptable to us.

What are the release criterias for new LXC versions?

Testsuite must pass, all commiters must be happy with the state of the release, all templates must work on their respective distribution, all public images (download template) must work in both privileged and unprivileged mode, no existing API call can have been modified or removed within the same major release (1.x), all new options must have been documented in the manpages and typically at least 24h since the last change was commited.

What was the most interesting bug in your practice?

Hard to think about a single particular one. We don’t typically get very difficult frustrating bugs. Getting systemd working properly was probably one of the most frustrating and lengthy experience we had in a while. It required a bunch of upstream systemd fixes which I had to contribute, we had to write a new filesystem for it, rework the way devices work in LXC and add a bunch more configuration options too.

But that one is somewhere between a bug (distro X doesn’t work in LXC) and a feature (get that crazy init system working in a container).

Some of the more frustrating bugs we had to deal with in the past, weren’t LXC bugs at all but kernel bugs instead. Typically the result of some security update done in a rush upstream, pulled by all the distros and breaking LXC and containers in general in some horrifying ways.

Those are very stressful because all of a sudden nothing works anymore. Finding the culprit is pretty simple (kernel changed, things stopped working), but coming up with the right upstream fix is difficult (and a revert is out of the question as that’d re-introduce a now disclosed security flaw).


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