libkrun

libkrun is a dynamic library that allows programs to easily acquire the ability to run processes in a partially isolated environment using KVM Virtualization.

It integrates a VMM (Virtual Machine Monitor, the userspace side of an Hypervisor) with the minimum amount of emulated devices required to its purpose, abstracting most of the complexity that comes from Virtual Machine management, offering users a simple C API.

Possible use cases

• Adding VM-isolation capabilities to an OCI runtime.
• Implementing a lightweight jailer for serverless workloads.
• Bringing additional self-isolation capabilities to conventional services (think of something as simple as chroot, but more powerful).

Goals and non-goals

Goals

• Enable other projects to easily gain KVM-based process isolation capabilities.
• Be self-sufficient (no need for calling to an external VMM) and very simple to use.
• Be as small as possible, implementing only the features required to achieve its goals.
• Have the smallest possible footprint in every aspect (RAM consumption, CPU usage and boot time).
• Be compatible with a reasonable amount of workloads.

Non-goals

• Become a generic VMM.
• Be compatible with all kinds of workloads.
• Provide the best possible performance.

Device support

Virtio devices

• virtio-console
• virtio-fs
• virtio-vsock
• virtio-balloon (only free-page reporting)

Networking

In libkrun, networking is implemented using a novel technique called socket-to-vsock impersonation. This allows the VM to have network connectivity without a virtual interface (hence, virtio-net is not among the list of supported devices).

The current implementation of this technique, found part in this repository and the other part in the kernel patches included with libkrunfw is just a proof-of-concept. It's limited to IPv4 TCP and UNIX connections, only supports recv/send operations, and the implementation itself is still quite hacky. We expect this technique to mature within libkrun, so it can be eventually upstreamed into the Linux kernel and other VMMs.

DNS resolutions issues

As, by default, glibc will use UDP for DNS requests, which is not yet supported by the socket-to-vsock impersonation technique described above, name resolution will fail with the default configuration. To work around this, you need to add the following line to the /etc/resolv.conf of the root filesystem servicing the isolated process:

options use-vc

Building and installing

Linux

Requirements

• libkrunfw
• A working Rust toolchain
• C Library static libraries, as the init binary is statically linked (package glibc-static in Fedora)

Compiling

make

Installing

sudo make install

macOS

Requirements

As part of libkrun building process, it's necessary to produce a Linux ELF binary from init/init.c. The easiest way to do this is by using a binary version of krunvm and its dependencies (libkrunfw, and libkrun itself), such as the one available in the krunvm Homebrew repo, and then executing the build_on_krunvm.sh script found in this repository.

This will create a lightweight Linux VM using krunvm with the current working directory mapped inside it, and produce the Linux ELF binary from init/init.c.

Building the library using krunvm

./build_on_krunvm.sh
make

Using the library

Despite being written in Rust, this library provides a simple C API defined in include/libkrun.h

Examples

chroot_vm

This is a simple example providing chroot-like functionality using libkrun.

Building chroot_vm

cd examples
make

Running chroot_vm

To be able to chroot_vm, you need first a directory to act as the root filesystem for your isolated program.

Use the rootfs target to get a rootfs prepared from the Fedora container image (note: you must have podman installed):

make rootfs

Now you can use chroot_vm to run a process within this new root filesystem:

./chroot_vm ./rootfs_fedora /bin/sh

If the libkrun and/or libkrunfw libraries were installed on a path that's not included in your /etc/ld.so.conf configuration, you may get an error like this one:

./chroot_vm: error while loading shared libraries: libkrun.so: cannot open shared object file: No such file or directory

To avoid this problem, use the LD_LIBRARY_PATH environment variable to point to the location where the libraries were installed. For example, if the libraries were installed in /usr/local/lib64, use something like this:

LD_LIBRARY_PATH=/usr/local/lib64 ./chroot_vm rootfs/ /bin/sh

Status

While functional, libkrun is still in a very early development stage.

Our first priority now is getting feedback from potential users of the library, to build a Community around it that would help us set the priorities and shape it to be useful for them.

Acknowledgments

libkrun incorporates code from Firecracker, rust-vmm and Cloud-Hypervisor.