Modifying sbuild

Posted on July 25, 2016 by Marko Dimjašević

This GSoC week of working on a Debian project was all about sbuild. I made progress with setting up an sbuild session for KLEE and I also reworked a patch for sbuild to support a command-line option not present in the upstream version of sbuild.

With the previous week I managed to install all KLEE dependencies in an sbuild session within Debile. However, having tools and libraries in place is not enough; I also need to tweak quite a few settings in the environment to make KLEE work on a Debian package.

One thing that I needed to tweak are environment variables. For example, WLLVM expects several environment variables set. A problem is in the Python schroot library by default not remembering the environment from previously executed commands. Luckily, the library has a way of using the host’s environment in the container so I configured the variables on the host side and simply shared them with the sbuild container.

Another configuration that needed to be done is to dh_auto_configure, a tool that is part of the debhelper command sequencer, which is used by virtually every Debian source package. In earlier explorations I realized that some packages fail to configure when the WLLVM_CONFIGURE_ONLY variable is not set, i.e. when WLLVM compiler wrappers are used instead of GCC during package configuration. To work around this, I created a wrapper /usr/local/bin/dh_auto_configure for the original dh_auto_configure as such:

    dpkg-buildflags --export
    WLLVM_CONFIGURE_ONLY=1 /usr/bin/dh_auto_configure "$@"

With the PATH environment variable having /usr/local/bin appearing before /usr/bin, the wrapper calls the original dh_auto_configure with the WLLVM_CONFIGURE_ONLY variable set and the configuration proceeds without any troubles. This was a tricky hack as I needed to pass WLLVM_CONFIGURE_ONLY only to dh_auto_configure, and not to other package build commands.

With all above, this should finish setting up the environment needed to run KLEE on a Debian package. However, there will be custom scripts to call after a package is built in order to call KLEE on appropriate LLVM bitcode files.

I also reorganized the code I have in Debile so far as it was getting hard to read.

Finally, I re-based Léo Cavaillé’s earlier patch to a current version of sbuild, i.e. sbuild 0.69.0. His original patch is for 0.64.1. The patch adds a new command-line option to sbuild so it can reuse an existing schroot session. This is important for me as I need to use an schroot session I set up for KLEE with all its customizations, including environment variables. As the older sbuild version is not available in Debian Sid anymore, I had to update Léo’s patch so it works against the latest release of sbuild. At first I was reluctant to do this because sbuild is written in Perl, and Perl looks like a very scary programming language to me. Hey, I don’t even know how to write a hello world program in Perl! Nevertheless, in the #debile IRC channel over at OFTC, other Debile developers suggested it would be better to update the patch than to try to use Debian’s Snapshot Archive to install sbuild 0.64.1, which is what I was trying to do initially. Clément Schreiner, my co-mentor on this project, asked me to write the patch in accordance with the DEP3 format; hopefully, we can get this patch finally merged upstream or at least in the sbuild Debian source package.

On a related note, I fixed a Dockerfile for a Debile slave that was installing the upstream version of sbuild that doesn’t have the command-line option. Now it pulls a couple of binary packages with the patch applied and currently I host both of the packages on my server. That and the patch are waiting in a pull request to the upstream Debile repository.

New KLEE-specific changes are available in the klee branch of my Debile fork.