Microsoft released the Linux kernel source code used in WSL2

Microsoft has released all the changes and additions to the Linux kernel used in the kernel shipped for the subsystem WSL 2 (Windows Subsystem for Linux v2). WSL Second Edition distinguishes itself by delivering a full-fledged Linux kernel, rather than an on-the-fly emulator that translates Linux system calls into Windows system calls.

WSL 2 is a new version of the architecture which allows the Windows subsystem for Linux to run the Linux ELF64 binaries on Windows. This new version of WSL 2 uses Hyper-V features to create a lightweight virtual machine with a minimal Linux kernel.

WSL 2 is part of Windows 10 development version 20H1, which will be released in April 2020.

A GitHub repository of WSL2 source code

The availability from source code allows enthusiasts to build their Linux kernel builds for WSL2, if they wish, taking into account the nuances of this platform. The Linux kernel included in WSL2 is based on version 4.19, running in a Windows environment using a virtual machine that is already used in Azure.

Updates for the Linux kernel are delivered through the Windows update mechanism and are tested on Microsoft's continuous integration infrastructure.

Prepared patches include optimizations to reduce kernel startup time, reduce memory consumption and leave the minimum required set of drivers and subsystems in the kernel.

Microsoft explains in its repository that “the Linux kernel is provided only under the terms of the General Public License of GNU version 2 (GPL-2.0), as stated in the GPL-2.0 Licenses, with an exception of syscall.

The license described in the COPYING file applies to the kernel source code as a whole, although individual source files may have a different license that must be GPL-2.0 compliant.

These include:

• GPL-1.0 +: GNU General Public License v1.0 or later
• GPL-2.0 +: GNU General Public License v2.0 or later
• LGPL-2.0: GNU Library v2 General Public License only
• LGPL-2.0 +: GNU Library General Public License v2 or later
• LGPL-2.1: GNU Limited Public Limited License v2.1 only
• LGPL-2.1 +: GNU Limited General Public License v2.1 or later

In addition, individual files can be provided under a dual license, for example one of the variants compatible with GLP and under a permissive license like BSD, MIT, etc.

User-space API (UAPI) header files, which describe the interface of user-space programs with the kernel, are a special case. According to the note in the kernel COPYING file, the syscall interface is a clear boundary, which extends the requirements of the GPL to any software that uses it to communicate with the kernel.

Because UAPI headers must be included in any source file that creates an executable that runs on the Linux kernel, the exception must be documented by a special license expression.

The usual way to express the license of a source file is to add the corresponding repeating text in the comment above the file. Due to formatting, typographical errors, etc., these "passwords" are difficult to validate for tools used in the context of license compliance.

Also, Microsoft has requested a private mailing list of distros that publishes information about new vulnerabilities at an early stage of detection, allowing distributions to prepare for troubleshooting before the public announcement.

Microsoft requires access to the mailing list for information on new vulnerabilities affecting distribution sets such as Azure Sphere, Windows Subsystem for Linux v2, and Azure HDInsight, which are not based on the practices of existing distributions.

Greg Kroah-Hartman, responsible for maintaining a stable core branch, is ready to act, even though it has not made the decision to grant access.