LKL (Linux Kernel Library) is aiming to allow reusing the Linux kernel code as extensively as possible with minimal effort and reduced maintenance overhead.

Examples of how LKL can be used are: creating userspace applications (running on Linux and other operating systems) that can read or write Linux filesystems or can use the Linux networking stack, creating kernel drivers for other operating systems that can read Linux filesystems, bootloaders support for reading/writing Linux filesystems, etc.

With LKL, the kernel code is compiled into an object file that can be directly linked by applications. The API offered by LKL is based on the Linux system call interface.

LKL is implemented as an architecture port in arch/lkl. It uses host operations defined by the application or a host library (tools/lkl/lib).

The supported hosts for now are POSIX and Windows userspace applications.

$ cd tools/lkl
$ make

will build LKL as a object file, it will install it in tools/lkl/lib together with the headers files in tools/lkl/include then will build the host library, tests and a few of application examples:

• tests/boot - a simple applications that uses LKL and exercises the basic LKL APIs

• fs2tar - a tool that converts a filesystem image to a tar archive

• cptofs/cpfromfs - a tool that copies files to/from a filesystem image

• lklfuse - a tool that can mount a filesystem image in userspace, without root priviledges, using FUSE

$ pkg install binutils gcc49 gnubc

#If you don't have a gcc binary:
$ ln -sf /usr/local/bin/gcc49 /usr/local/bin/gcc

#Prefer ports binutils and GNU bc(1):
$ export PATH=/sbin:/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/usr/lib64/ccache

$ cd tools/lkl
$ gmake

$ sudo apt-get install libfuse-dev libarchive-dev xfsprogs

# Optional, if you would like to be able to run tests
$ sudo apt-get install btrfs-tools

$ cd tools/lkl
$ make

# To check that everything works:
$ cd ../..
$ make test

In order to build LKL for Win32 the mingw cross compiler needs to be installed on the host (e.g. on Ubuntu the following packages are required: binutils-mingw-w64-i686, gcc-mingw-w64-base, gcc-mingw-w64-i686 mingw-w64-common, mingw-w64-i686-dev).

Due to a bug in mingw regarding weak symbols the following patches needs to be applied to mingw-binutils:

https://sourceware.org/ml/binutils/2015-10/msg00234.html

and i686-w64-mingw32-gas, i686-w64-mingw32-ld and i686-w64-mingw32-objcopy need to be recompiled.

With that pre-requisites fullfilled you can now build LKL for Win32 with the following command:

$ cd tools/lkl
$ make CROSS_COMPILE=i686-w64-mingw32-

Q: How is LKL different from UML?

A: UML prodivides a full OS environment (e.g. user/kernel separation, user processes) and also has requirements (a filesystem, processes, etc.) that makes it hard to use it for standalone applications. UML also relies heavily on Linux hosts. On the other hand LKL is designed to be linked directly with the application and hence does not have user/kernel separation which makes it easier to use it in standalone applications.

Q: How is LKL different from LibOS?

A: LibOS re-implements high-level kernel APIs for timers, softirqs, scheduling, sysctl, SLAB/SLUB, etc. LKL behaves like any arch port, implementing the arch level operations requested by the Linux kernel. LKL also offers a host interface so that support for multiple hosts can be implemented.