Bundle and mount Linux kernel headers in WSL2 distros

[benhillis]

Summary

Bundles the Linux kernel UAPI headers shipped in the Microsoft.WSL.Kernel nuget package and mounts them read-only into every WSL2 distro by default. Pattern mirrors the existing kernelModules plumbing.

What it does

• Build: stages linux-headers/ next to the kernel image in the dev binary path (CMakeLists.txt, UserConfig.cmake.sample) and packages them in the MSI (msipackage/package.wix.in).
• Service: new kernelHeaders= setting in .wslconfig (empty = bundled default, or a custom path). Validated alongside kernel/kernelModules in WslCoreVm.
• Guest: mounted via 9p at /usr/src/linux-headers-$(uname -r) with a /lib/modules/$(uname -r)/build symlink so out-of-tree module builds and tools that key off uname -r find them.
• UI: Developer page in wslsettings exposes the path with the same UX as the kernel/kernelModules controls.

Testing

• New KernelHeaders TAEF unit test covers default-mount, custom path, and validation errors.
• Manually verified a fresh distro shows /usr/src/linux-headers-* populated and make headers_check works against it.

[benhillis]

/azp run wsl-github-pr

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[Copilot]

Pull request overview

This PR adds first-class support for shipping Linux kernel UAPI headers with WSL and making them available inside every WSL2 distro by default (mirroring existing kernel modules plumbing). It introduces a new .wslconfig setting (wsl2.kernelHeaders) plus service/guest/UI changes to stage, package, share, mount, and validate the headers.

Changes:

• Build/MSI: stage and package a linux-headers/ directory alongside the kernel and include it in the MSI install under tools/linux-headers.
• Service/Config/UI: add a new wsl2.kernelHeaders setting, expose it in wslsettings, and validate it alongside existing custom kernel/kernelModules settings.
• Guest init: add a new plan9 share and mount flow that places headers under /usr/src/linux-headers-$(uname -r)/include and creates /lib/modules/$(uname -r)/build symlink.

Reviewed changes

Copilot reviewed 25 out of 25 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
UserConfig.cmake.sample	Adds dev-mode symlink staging for linux-headers into WSL_DEV_BINARY_PATH.
CMakeLists.txt	Defines WSL_KERNEL_HEADERS_PATH for dev builds to point at the staged headers directory.
msipackage/package.wix.in	Packages tools/linux-headers into the MSI (non-dev installs) and wires it into the main feature.
src/windows/common/WslCoreConfig.h	Adds wsl2.kernelHeaders config key and includes it in config serialization/reflection.
src/windows/common/WslCoreConfig.cpp	Parses wsl2.kernelHeaders, applies policy override, and validates safe-mode behavior.
src/windows/inc/WslCoreConfigInterface.h	Extends the public config-entry enum with KernelHeadersPath.
src/windows/libwsl/WslCoreConfigInterface.cpp	Adds get/set support for KernelHeadersPath through the libwsl config interface.
src/windows/service/exe/WslCoreVm.h	Adds state to track whether kernel headers should be mounted.
src/windows/service/exe/WslCoreVm.cpp	Resolves/validates headers path, adds a read-only plan9 share, and passes a mount flag to mini-init.
src/shared/inc/lxfsshares.h	Defines the new plan9 share name (kernel_headers).
src/shared/inc/lxinitshared.h	Adds env var names and a new MountKernelHeaders field to the mini-init config message.
src/linux/init/main.cpp	Mounts the kernel headers plan9 share and passes a target mount path to distro init via env vars.
src/linux/init/config.cpp	Moves the temporary headers mount to /usr/src/linux-headers-…/include and creates /lib/modules/.../build symlink.
test/windows/Common.h	Adds kernelHeaders to the test config defaults structure.
test/windows/Common.cpp	Emits kernelHeaders= into generated .wslconfig test content.
test/windows/UnitTests.cpp	Adds a KernelHeaders WSL2 unit test for default mount + error cases.
src/windows/wslsettings/LibWsl.cs	Adds KernelHeadersPath entry to the managed config enum used by the UI.
src/windows/wslsettings/Contracts/Services/IWslConfigService.cs	Marks KernelHeadersPath as a string-valued config setting in the UI layer.
src/windows/wslsettings/ViewModels/Settings/DeveloperViewModel.cs	Adds CustomKernelHeadersPath binding to the Developer page VM.
src/windows/wslsettings/Views/Settings/DeveloperPage.xaml	Adds the Developer page expander for kernel headers path editing/browsing.
src/windows/wslsettings/Views/Settings/DeveloperPage.xaml.cs	Adds a folder picker handler for selecting a headers directory.
src/windows/wslsettings/Helpers/RuntimeHelper.cs	Adds a WinUI folder picker helper (PickSingleFolderAsync).
src/windows/wslsettings/Views/Settings/SettingsApplyHelper.cs	Adds display-name mapping for the new kernel headers setting in pending-changes UI.
localization/strings/en-US/Resources.resw	Adds user-facing strings for validation errors and the new settings UI text.
doc/docs/technical-documentation/boot-process.md	Documents the new “mount bundled kernel headers” guest configuration behavior.

Comments suppressed due to low confidence (2)

src/windows/service/exe/WslCoreVm.cpp:294

• If the bundled headers directory is missing/corrupt, this code will throw MessageCustomKernelHeadersNotFound(), which says the path was specified via 'wsl2.kernelHeaders' in the user’s .wslconfig. In the default-kernel + empty setting case the user didn’t specify anything, so the error is misleading and also blocks WSL startup. Consider distinguishing user-specified vs default-resolved paths (or treating missing bundled headers as a non-fatal warning that disables the mount).

        if (m_vmConfig.KernelHeadersPath.empty())
        {
            if (m_defaultKernel)
            {
#ifdef WSL_KERNEL_HEADERS_PATH

                m_vmConfig.KernelHeadersPath = std::wstring(TEXT(WSL_KERNEL_HEADERS_PATH));

#else

                m_vmConfig.KernelHeadersPath = m_installPath / LXSS_TOOLS_DIRECTORY / L"linux-headers";

#endif
            }
        }
        else if (m_defaultKernel)
        {
            THROW_HR_WITH_USER_ERROR(WSL_E_CUSTOM_KERNEL_NOT_FOUND, Localization::MessageMismatchedKernelHeadersError());
        }

        if (!m_vmConfig.KernelHeadersPath.empty())
        {
            if (!wsl::windows::common::filesystem::FileExists(m_vmConfig.KernelHeadersPath.c_str()))
            {
                THROW_HR_WITH_USER_ERROR(
                    WSL_E_CUSTOM_KERNEL_NOT_FOUND,
                    Localization::MessageCustomKernelHeadersNotFound(
                        wsl::windows::common::helpers::GetWslConfigPath(m_userToken.get()), m_vmConfig.KernelHeadersPath.c_str()));
            }

            m_mountKernelHeaders = true;
        }

src/windows/service/exe/WslCoreVm.cpp:292

• KernelHeadersPath is expected to be a directory tree, but this validation only checks GetFileAttributes != INVALID (via FileExists), so a regular file path would pass and later Plan9 sharing/mounting may fail. Consider validating that the path exists and is a directory (FILE_ATTRIBUTE_DIRECTORY) to fail fast with a clearer user error.

        if (!m_vmConfig.KernelHeadersPath.empty())
        {
            if (!wsl::windows::common::filesystem::FileExists(m_vmConfig.KernelHeadersPath.c_str()))
            {
                THROW_HR_WITH_USER_ERROR(
                    WSL_E_CUSTOM_KERNEL_NOT_FOUND,
                    Localization::MessageCustomKernelHeadersNotFound(
                        wsl::windows::common::helpers::GetWslConfigPath(m_userToken.get()), m_vmConfig.KernelHeadersPath.c_str()));
            }

[ptrivedi]

nit: config.cpp also seems to have a uname syscall being made to get the release. We should try to share the result instead of making another syscall, even though it is a cheaper syscall

[ptrivedi]

why not use the same addShare lambda that is being used above for GPU shares?

[ptrivedi]

A few minor comments, LGTM otherwise

[ptrivedi]

a couple of minor comments, but will approve