Separate Kernel Execution: execute a process within user-mode-linux and retrieve its output and status code
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Latest commit 2af6551 Oct 13, 2015 @jpetazzo jpetazzo Add LICENSE (Apache 2.0)
Closes #6

SEparate KErnel eXEcution

SEKEXE uses User Mode Linux to run a Linux process within a "sub kernel", and retrieve its output and exit status (as if it was running as a normal shell command). This is half-way between full-fleged virtual machines and lightweight containers: VMs can run any O/S; containers run the same O/S and kernel as their host; SEKEXE only runs Linux-inside-Linux, but the guest can be a different version.

This is useful if you want to:

  • isolate a process, but your kernel doesn't support containers or other isolation features;
  • run or test specific kernel features that are not supported by your current kernel;
  • run a process as root, but do not have root privileges.

(Of course, in the last scenario, you will not gain any privilege that you didn't have initially; you will be able to create a kind of virtual machine, and you will have root privileges inside that pseudo-VM, but you will not gain privileges outside.)

SEKEXE was developed for one very specific use-case: run the test-suite for Docker within Travis CI. Docker needs root privileges, and a kernel with namespaces, control groups, and AUFS support.

How To Use It

./run "env ; exit 42"
echo $?

This will start a separate kernel, run "env ; exit 42" in it, then display the exit value -- which should be 42 in that case.

If you run ./run by itself (without arguments) you will get a shell inside the environment. This mode is convenient to experiment within the environment.


You need the slirp package. On Debian/Ubuntu, apt-get install slirp is your friend. On other distros, you're on your own, but it shouldn't be very different.

Slirp is used to provide network connectivity without requiring root permissions; if you do not need network access, and cannot install slirp, you can remove the eth0=slirp... parameter from run.


The SEKEXE repository contains a 3.8 Linux kernel with support for LXC and AUFS. It has no support for block devices or real filesystems, though. If you want to replace it with another version of the kernel and need help, let me know.


As mentioned above, SEKEXE uses Slirp to provide network connectivity without root access. This means that TCP and UDP traffic will work, but you won't be able to ping.

An "exchange directory" ($LOGDIR) is created, and the command to run is placed in it. Then the User Mode Linux kernel is started, with a custom init. The new environment shares the filesystem of its host, so you do not have to setup a chroot or something like that. It uses UML's hostfs filesystem for that. The mount is done in read-only; so the environment cannot affect anything in the host system (except within the $LOGDIR directory, which is explicitly mounted read-write).

The custom init takes care of setting up the network, setting up appropriate tmpfs mountpoints, and starting the process. It collects its output and exit status, stores them in $LOGDIR.

The main run process then retrieves those items and exits appropriately.

The normal output of the environment is diverted to /dev/null because it contains spurious kernel messages.