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Hacking Hurd: setting up the workflow

February 28, 2017

Introduction

Hurd is an interesting piece of software, and digging in may be an interesting way of learning of the ins and outs of operating systems. But you can’t just download the source code, compile it and run it as you would do with most of the programs.

Here I talk about setting up an environment for working on the user space , ie not the micro kernel. In this post I will cover how to install Hurd in a virtual environment, how to use it, and how to set the whole thing up to ease the writing and testing of software on Hurd.

The installation

Hurd in itself is not an operating system, but only a kernel, and can’t be used on its own (like Linux). So what do we install. Well, like there are Linux distribution, there are Hurd distributions, even though less numerous :

  • Debian GNU/Hurd , a porting of the Debian GNU/Linux distribution over Hurd. It is the official distribution.

  • ArchHurd , a porting of the ArchLinux distribution.

  • NixHurd , although it is more experimental and can only run in Qemu.

Since we are only interested in working on Hurd itself, I will propose to install Debian GNU/Hurd, as it is the most stable of the three.

To ease the testing of the system, the installation will be done in a Qemu . To begin, you must install the qemu package (or qemu-kvm) on your system and enable KVM in your Linux Kernel, to speed up the simulation.

You must now setup Qemu. First of all, you must create a virtual disk on which Hurd will installed. The command to do so is :

qemu-img create file.img size

where size is 3G for example (this capacity is indeed enough for what we will do).

Once this is done, you can download the iso with the following command :

wget http://people.debian.org/~sthibault/hurd-i386/installer/cdimage/cd-1.iso

Once this is done, you can launch the system with :

qemu-kvm -m 1G -drive cache=writeback,file=file.img -cdrom cd-1.iso

The virtual machine will then boot the live CD, and you will get a grub where you can choose the installation process. Go for the Pseudo-graphical install which will give you a standard ncurses interface.

GNU/Hurd Live CD
GNU/Hurd Live CD

The installation process is well detailed : if you have ever installed a Linux distribution, it shouldn’t trouble you. Leaving the default option are fine in most cases (the only trouble I got was that it failed when asked to install a graphical environment, so I didn’t install one).

Once this is finished, shutdown the virtual machine.

Installing the software

For now, login as root to setup the environment. The user you created will be used later.

First of all, we will need to upgrade the installed software, but simply running apt update failed for me. The reason was that it first tried to update from the cd. So the first step is to go edit the /etc/apt/sources.list file and comment (or delete) the lines starting by deb cdrom.

Once you’ve done this, run : apt update && apt upgrade

You can now use apt install to install any software you might want. To turn the virtual machine off, first use the shutdown -h 0 command and close the qemu window once you see the running in tight loop message.

Sharing files with host

The workflow advocated by the official page is to develop and compile directly in Hurd. The compiling part is necessary (unless you manage to setup a cross compilation toolchain from Linux to Hurd), by if like me you don’t want to have to setup your whole programming environment in a virtual machine, you may want to at least do the programming part on your host system.

The workflow I will present is thus the following : you work on your host system, send the files to the virtual machine, and compile and run there. Since Qemu does not support sharing a disk with the host, another solution must be found. You could use git to synchronize the files, but it would mean committing every change you want to test. Another solution would to setup a filesystem sharing server on your host and connect to it from Hurd. But Hurd doesn’t seem to have an sshfs implementation and despite having a nfs implementation, it seems to be quite experimental .

We will use the fact that Hurd comes with a working ssh server to use rsync. While qemu automatically setup networking so that the virtual environment has access to internet and Hurd autoconnects, we will need to pass a flag to qemu so that we can connect from the host to the guest. Remember that due to the specific nature of ping, you cannot ping from the guest OS. Thus ping failing does not mean you are not connected.

This flag is -redir tcp:iph::ipg : it will redirect any connection on localhost port iph on the host to ipg port on the guest. For example, if you launch qemu with the command :

qemu-kvm -m 1G -drive cache=writeback,file=file.img -redir tcp:2222::22<br>

you can ssh into your hurd installation with : ssh -p 2222 root@localhost<br>. This may be an easier to work in hurd rather than in qemu.

Since qemu 2.6 complain about -redir begin deprecated, it may be a good idea to replace it. The new option is -net nic -net user,hostfwd=tcp::2222-:22 .

If you only want to work on you virtual machine through ssh, you may want to launch qemu without a graphic interface (for example if you want to launch qemu on a distant headless computer). To do that, append the -nographic option to the previous command. If you want to keep the output on terminal, you can replace -nographic by -curses.

In order for rsync to work, in must also be installed on the guest. Thus run : apt install rsync in Qemu. You can then synchronize any directory with :

rsync -r --rsh='ssh -p 2222' dir user@localhost:/path/<br>

You will get a shitload of errors due to operations not being implemented in Hurd’s version of ext2, but the file will still be synchronized.

Building the sources

All that is left is to install the right software in Hurd to build the translator and run them. Since Hurd allows any translator to run in user space, we will use the user created during the installation.

To build the source, you first need to install some tools and their dependencies. To install the tools, you must run the following command :

apt install build-essential fakeroot git

And for the dependencies : apt build-dep hurd.

Then you must download the sources. They are managed with git, so all you have to do is to clone their repo. The links are given on this page. Once you have all the sources you want, you can copy them to Hurd using rsync as seen above.

To compile the code source, there are a few steps. First of all, you must generate the configure script. To do so, run autoconf in the Hurd directory. Then create a build directory, move into it and call the configure script from it. Then all that’s left is to run make. You can pass to make the name of a submodule to build only it.

Conclusion

You now have a working environment to hack Hurd and contribute. Have fun !