Interview with Matthieu Herrb, X.Org project

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Xorg (the X-server) is a big and complex beast. How does development process of it look like?

It’s not that big compared to many more recent projects (like firefox, chrome or even Gnome/KDE)

The development process is a bit different between actively developed components (the X server itself, a few supporting libraries and the drivers for recent hardware) and legacy components like libXt and all applications based on it.

Also there is a constant interaction with 2 other groups: the Mesa and Linux kernel drm modules developers.

Ideas are discussed during developers meetings (once a year, alternatively in Europe and North-America, with the next one being this September in Bordeaux - France) or on the Xorg-devel mailing list.

Since a few years, we’ve adopted a development model which is close to the Linux kernel development model: patches (git format-patch) are sent to the mailing list for review, get some discussion, and if a consensus is reached, are integrated by the maintainer.

There is one maintainer for the X server (currently Keith Packard is holding that position). For other components, commits are more open and it’s generally the author of a patch that pushes it once it has been reviewed positively.

And to be complete, nowadays development of the active drivers almost completely driven by the companies (Intel, AMD, VMWare), so it’s engineers from those companies who do most of the commits and pushes.

How many developers involved in the development?

If we include the Mesa, Linux kernel graphics stack and X people there are around 50-60 people who commit on a regular basis to one of the repositories.

How does testing process look like? Do you use regular testing (for example for each commit) or it is time from time testing?

We have some tools to do regular, automated testing in place, but they are not as efficient as we’d like them to be.

What tools, tests and testing frameworks do you use? I have found a lot of tests (like XTS, rendercheck, glean, piglit etc) in repository but some of them looks outdated. Do developers create tests on regular basis for new functionality and bugfixes?

In addition to all those existing test suites, which are usually quite cumbersome to set up and run on a regular basis, Peter Hutterer has developed a relatively new, integrated test suite for the X server that is supposed to be run automatically from the X build system (make test) and on our tinderbox server. The script used by many developers also runs this tests by default

But given the huge spectrum of supported systems (even though since the switch from XFree86 to X.Org, this number has been constantly shrinking) only a small subset of them gets actual regular testing.

Most of the tests are done downstream by people who integrate the X.Org suite on their systems and distributions.

This is my case among others. I maintain X.Org on OpenBSD (and have been helping with NetBSD before), so I test configurations that are not handled by the main X developers and often find bugs that slipped through the testing process, either because they are platform specific (for instance OpenBSD is one of the few systems to still run on some exotic architectures like VAX, m88k or even sparc32), or just because our system’s malloc() is able to catch errors that escape other tools used on Linux.

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

The new X server test framework is mostly a unit and functional testing framework to make sure that the X servers components behave the way they are supposed to in a driver independent manner.

In case of using tests do you measure code coverage?

Not really. Since the people who wrote the code and the test are often the same, they have some notion of the coverage of their tests, but there is no formal tool used AFAIK.

How often do you use it: time from time or on regular basis?

Again, the tinderbox platform is supposed to run the tests as often as possible, but much of the other tests are run manually from time to time.

How new features tested?

New features, in X, you’re kidding, right ? More seriously they have been a number of new features mostly at the Mesa (OpenGL) level and at the input driver level. Either new tests for the features are added to the test suite at the same time of the code, or, for the OpenGL case, external compliance tests are used.

Do you use Continuous Integration in development process?

Yes, I’ve mentioned the tinderbox system a number of times already, even if it’s far from perfect.

What tool do you use for tracking open bugs? Who is responsible in tracking of open bugs?

We have a Bugzilla hosted at (, completed by a patch tracking system (patchwork) that tries to ensure that no submitted patch gets forgotten or unhandled.

Sometimes X.Org has security flaws Do you use regular security audit of code?

No and yes :) AFAIK X.Org has no one explicitly dedicated to do code audits on a regular basis. But some distributions (for example Oracle/Solaris in the person of Alan Coopersmith) are using security oriented auditing tools regularly and contributing fixes back to the project. On occasion, when a specific new kind of vulnerability is put into light (like format strings or integer overflow about 10 years ago) we do extensive sweep on the existing code base to try to fix them.

We also get external help from independent security researchers who are always looking for interesting vulnerabilities to exploit, and with the X server still being setuid root on many systems, it’s still a worthwhile target.

Last year Ilja Van Sprundel reported quite a large number of vulnerabilities in both X libraries and in the X server, mainly rotating around the lack of enough validation of the X protocol messages.

Do you use static code analysis?

The answer is similar to my previous one. The tinderbox system is not running any specific static code analyzer beside gcc with -Wall plus a number of extra warnings activated. But some people (including again Alan for Oracle) have access to powerful static analysis tools and run them from time to time.

Coverity has a program to do analysis for free software organizations. X.Org has been part of this program and they helped us to find a fix a number of issues.

X.Org supports all more or less popular operating systems: Linux, FreeBSD, NetBSD, OpenBSD, Solaris, Microsoft Windows. How you provide confidence in stable work for all these OSes?

As I explained above this is handled by volunteers (or paid employees in some cases) from the various projects. The main developers mostly focus on Linux which has become the main development platform for the last 10 years. Personally I’d say that its a bit unfortunate that those (great) people don’t get involved a bit more in the support of the other systems. From my experience there is a lot to learn in developing with more than one system in mind, and from the security point of view diversity has a great value (even if it has some cost on the development process)

Who is responsible for releasing of new version? What are the release criterias?

There is a maintainer for the X server who also is in charge of doing the release. We currently work on a 6 month cycle with a new X server release being released every 6 months. Previous release get a -stable maintainer and are generally maintained for 12 more months.

In addition to the X server releases we still do “katamari” releases with a full consistent set of libraries and utilities in addition to the X server. This is done more or less yearly. (current katamari is 7.7, based on X server 1.14). But the need for katamari releases is often questioned, since system distributors are generally maintaining their own katamaries (with more frequent merges of upstream packages), independently of the official X.Org ones.

The times where the XFree86 project was providing binary distributions for most of its supported systems (from SVR4 to Linux, including NetBSD, OS/2 and a few others) is definitely over.

What was the most interesting bug in your practice? :)

Working with code that was designed and implemented while security didn’t matter is always “interesting”. X was initially a really permissive system (do you remember the “xhost +” times ?). People didn’t care about buffer overflows or other malicious ways to exploit coding errors. Features like the X-SHM extension have been broken from the start. (SHM has been fixed by using a new API, based on file descriptor passing recently).

But the most interesting issue from my point of view is the paper by Loic Dufflot at CanSecWest 2006 where he showed that even with the privilege separation code I had added to OpenBSD, the fact that the X server had direct access to hardware registers made it possible for a “simple” code injection bug to get control of the kernel.

This is something that has always been known (and I even talked about it in a talk at RMLL in 2003) but lacking and actual implementation of a PoC let many developers ignore the issue.

Even if this was of lower impact on Linux than on OpenBSD (because the root/kernel frontier is not as strong on Linux unless you’ve SELinux or similar hardening features active, which wasn’t the default at those times). It came right when other people started to think about how to run X with less privileges. This has lead to the development of Kernel Mode Settings and other techniques that now make it possible to run X server 1.16 without any privilege.

But this can’t always be enough. A small (but which could have had a high impact if it has gone unnoticed for a few more months) example is the CVE-2014-4910 in the Intel driver: a new setuid helper is needed to handle the screen back light now that X doesn’t have the required privileges to change it; and this daemon was written without any care for security, allowing “..” escapes in a path passed directly to open(O_RDWR) as root. The problem was thankfully spotted before an official release of the Intel driver by Adam Sampson by end of June.

Thanks for answers and wish you less bugs in your code!

Thank you.

In conclusion, Yes X.Org is far from perfect from the testing point of view. We are trying to make things better, but since this is not the sexiest thing to contribute on, things are not moving very fast, as most developers still prefer get more attractive things.

Теги: softwareopensourceopenbsdinterviewtestingopenbsd-enfeeden