Tutorial: Getting Started
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This tutorial is meant as an up-to-date guide for people coming across rump kernels for the first time, and wanting to understand them and/or to use them. There are two parts. First we list a few items of suggested reading. The second, and bulk, part of this tutorial is about playing around with the code. People interested only in playing around with the code can skip the reading.
Notably, this tutorial will only cover basic rump kernel matters. If you are interested in the Rumprun unikernel, it is not absolutely necessary to follow this tutorial (although you will gain a lot of applicable knowledge). If you want to skip straight to Rumprun, see the links at the bottom of this tutorial.
If you find this tutorial lacking in any way, please suggest improvements on the rump kernel mailing list (see the Community page for more info on the list).
The most succinct description of the architecture, motivation and history, including some use cases, is the ;login: magazine article "Rump Kernels: No OS? No Problem!". You can find a free copy here.
For more in-depth knowledge, reading The Design and Implementation of the Anykernel and Rump Kernels is recommended. Notably, read or skim chapters 2 and 3.
We will go over the practical part in the comfort of userspace. "Why userspace?", you ask. First, it is the most convenient platform to do experimentation in. Second, most concepts map more or less directly to embedded and cloud platforms, so it is even beneficial to get a hang of debugging and building and testing your projects in userspace before potentially moving them to the final target platform or platforms.
The following should work at least on more or less any Linux system and NetBSD systems.
First, we do a full build of the buildrump.sh package. This will give us two things we need for the rest of the tutorial:
- rump kernel components (i.e. the building blocks of rump kernels)
- hypercall implementation for POSIX'y userspace (allows rump kernels to run in userspace)
A kernel alone is slightly boring. We will also need the rumpctrl package, which will give us the ability to interact with rump kernel instances:
- a selection of userland utilities, mostly configuration-oriented (think
- a transport mechanism for the userland utilities to communicate with remote rump kernels
buildrump.sh stage is necessary only if you want to run rump kernels in userspace, but
rumpctrl is useful no matter which platform you run your production rump kernels on (Xen, KVM, ...).
In more concrete terms, for
git clone http://repo.rumpkernel.org/buildrump.sh cd buildrump.sh ./buildrump.sh
git clone http://repo.rumpkernel.org/rumpctrl cd rumpctrl git submodule update --init ./buildnb.sh
(Notably, the above process is slightly wasteful in terms of bandwidth usage and will clone the
src-netbsd repository twice. Advanced users can clone the repository manually and use the
-s argument to the build scripts to specify the location. However, we opted for simplicity in this tutorial.)
Trying it out
So we built things. Now what? Let's assume you've changed back to the top level directory (i.e.
cd .. after the last build command). Look into the
./rumpctrl/bin directory. You will see a bunch of potentially familiar userland utilities, e.g.
ifconfig. What happens if we try to run one?
$ ./rumpctrl/bin/ifconfig error: RUMP_SERVER not set rumpclient init failed
Fail happens. Why? On a regular system, each userland utility communicates implicitly with the host kernel. For example, when you run
ls, the utility makes the appropriate system calls to the host kernel. However, there is no implicit rump kernel. That is the reason why we must explicitly tell the rumpctrl utilities which rump kernel we wish for it to communicate with. That information is conveyed by an URL in the
RUMP_SERVER environment variable. To know the right URL to set, we must first start a rump kernel server.
If you followed the above build process, you will have a rump kernel server available under
./buildrump.sh/rump/bin/rump_server. As a mandatory argument, the server takes an URL which indicates from where the server listens to for requests. The easiest way is to use local domain sockets, which are identified by
unix:// and the remainder of the argument is the pathname. Let's try it out:
$ ./buildrump.sh/rump/bin/rump_server unix:///tmp/rumpctrlsock
Ok, um, where did it go? By default, a
rump_server will background itself, and that is what happened here. You can check that the server is running with
ps. You can even kill it with
kill. However, we will show a more civilized command for the kill in a while.
You can set
$RUMP_SERVER manually to the URL you specified when you ran the server. There is also a bash script, when if sourced, sets the prompt so that the value of
$RUMP_SERVER is visible. That will help keep track of which rump kernel server the commands are being sent to. The script also sets
$PATH so that the
bin directory containing the applications build by rumpctrl is first. Let's give it a spin.
$ . rumpctrl/rumpctrl.sh rumpctrl (NULL)$ export RUMP_SERVER=unix:///tmp/rumpctrlsock rumpctrl (unix:///tmp/rumpctrlsock)$ sysctl -w kern.hostname=look_at_them_run kern.hostname: rump-12224.watou -> look_at_them_run rumpctrl (unix:///tmp/rumpctrlsock)$ sysctl kern.hostname kern.hostname = look_at_them_run
Sweet! Let's use
ls to see what files the rump kernel is hosting.
rumpctrl (unix:///tmp/rumpctrlsock)$ ls -l ls: .: Function not implemented
Sour! What's going on? Rump kernels are component-oriented, which means that we must indicate which components we want the rump kernel to support. When we ran
rump_server earlier, we did not specify that it should support file systems. No file systems, no
ls. So, let's halt the rump kernel and try again.
rumpctrl (unix:///tmp/rumpctrlsock)$ halt rumpctrl (unix:///tmp/rumpctrlsock)$ ./buildrump.sh/rump/bin/rump_server -lrumpvfs unix:///tmp/rumpctrlsock rumpctrl (unix:///tmp/rumpctrlsock)$ ls -l total 2 drwxr-xr-x 2 0 0 512 Jul 28 23:10 dev drwxrwxrwt 2 0 0 512 Jul 28 23:10 tmp rumpctrl (unix:///tmp/rumpctrlsock)$ mount rumpfs on / type rumpfs (local)
Better. We'll get to how you're supposed to know about
-lrumpvfs later in this tutorial.
Now, can we mount tmpfs inside the rump kernel server?
rumpctrl (unix:///tmp/rumpctrlsock)$ mount_tmpfs /swap /tmp mount_tmpfs: tmpfs on /tmp: Operation not supported by device
Bitter! Looks like you'll have to keep reading this tutorial.
rumpctrl extra tricks
Assuming you have sourced
rumpctrl.sh, you can list all of the available commands with
rumpctrl_listcmds. Another way would be to list the contents of the
bin directory. You can test for an individual command with
rumpctrl_hascmd (which may give peace of mind if you e.g. want to execute
rm -rf /). Finally, you can "shell out" with
rumpctrl_hostcmd. Let's try these out.
rumpctrl (unix:///tmp/rumpctrlsock)$ rumpctrl_listcmds arp ed mknod newfs_msdos route [...] rumpctrl (unix:///tmp/rumpctrlsock)$ rumpctrl_hascmd ls #t rumpctrl (unix:///tmp/rumpctrlsock)$ rumpctrl_hascmd top #f rumpctrl (unix:///tmp/rumpctrlsock)$ ls -l / total 2 drwxr-xr-x 2 0 0 512 Jul 28 23:10 dev drwxrwxrwt 2 0 0 512 Jul 28 23:10 tmp rumpctrl (unix:///tmp/rumpctrlsock)$ rumpctrl_hostcmd ls -l / total 110 drwxr-xr-x 2 root root 4096 Jul 23 23:01 bin [...]
In case you are wondering if we should have used
rumpctrl_hostcmd to run
rump_server when we restarted it, you are on the right track. We did not
do that because
rumpctrl_hostcmd had not been introduced yet and because
we are sure that there is no
rump_server rumpctrl command (well, at least
I am sure).
Note: shell redirects or
cd will not work with rumpctrl. The behaviour expected -- the shell is running in host userspace, not guest userspace -- but needs to be taken into account.
rumpctrl (unix:///tmp/rumpctrlsock)$ cd /dev ERROR: cd not available in rumpctrl mode /dev rumpctrl (unix:///tmp/rumpctrlsock)$ echo messages from earth > /file bash: /file: Permission denied rumpctrl (unix:///tmp/rumpctrlsock)$ ls -l /file ls: /file: No such file or directory rumpctrl (unix:///tmp/rumpctrlsock)$ echo messages from earth | dd of=/file 0+1 records in 0+1 records out 20 bytes transferred in 0.001 secs (20000 bytes/sec) rumpctrl (unix:///tmp/rumpctrlsock)$ ls -l /file -rw-r--r-- 1 0 0 20 Jul 29 04:24 /file
Coping with components
Let's assume we want to run some driver in a rump kernel. That driver needs all of its dependencies to work. How to figure out what the dependencies are? For the impatient, there is
rump_allserver, which simply loads all components that were available when
rump_allserver was built. However, it is better to get into the habit of surgically selecting only the necessary components. This will keep footprint of the rump kernel to a minimum and also allow you to include drivers which were built and installed separate from
rump_allserver. We can use the tool
rump_wmd (Where's My Dependency) to resolve dependencies. For example, let's assume we want a rump kernel to support the FFS file system driver.
rumpctrl (unix:///tmp/rumpctrlsock)$ rumpctrl_hostcmd ./buildrump.sh/rump/bin/rump_wmd -L./buildrump.sh/rump/lib -lrumpfs_ffs DEBUG0: Searching component combinations. This may take a while ... DEBUG0: Found a set -lrumpdev -lrumpdev_disk -lrumpvfs -lrumpfs_ffs
Now, we could start a rump kernel server with FFS, but we'll actually start one which supports tmpfs, to not have to magically come up with an FFS image to mount. Before we can start another server, we have to halt the previously running one. Note, you can halt a server only once, as demonstrated below.
rumpctrl (unix:///tmp/rumpctrlsock)$ halt rumpctrl (unix:///tmp/rumpctrlsock)$ halt rumpclient init failed rumpctrl (unix:///tmp/rumpctrlsock)$ rumpctrl_hostcmd ./buildrump.sh/rump/bin/rump_server -lrumpfs_tmpfs -lrumpvfs unix:///tmp/rumpctrlsock rumpctrl (unix:///tmp/rumpctrlsock)$ mount_tmpfs /swap /tmp rumpctrl (unix:///tmp/rumpctrlsock)$ mount rumpfs on / type rumpfs (local) tmpfs on /tmp type tmpfs (local)
If you want, as an exercise create an ISO image and use a
rumpfs_cd9660 driver to access the contents. You will need
to use the
-d argument to
rump_server to map the image to the
rump kernel namespace. Check out the
rump server manpage
if you need more information about the
Building applications for the Rumprun unikernel
The process of building applications for Rumprun is detailed in other Tutorials:
Try out at least some of the commands listed by
rumpctrl_listcmds. Some of them require rather specially configured rump kernels (e.g.
wpa_supplicant), and those are out of the scope of this tutorial. However, you should for example be able to configure a rump server up to a point where
ping 127.0.0.1 works.
The manual pages for all of the commands are available at http://man.netbsd.org/
When you are satisfied with your prowess, move on to for example the kernel development tutorial, which discusses running rump kernels under the debugger.