Initial commit

This adds a Dockerfile for the kernel build, the WSL2 Linux Kernel as
a git submodule and a GitHub workflow to build and release the kernel
using the Docker image.

The submodule initially points to the WSL2 Linux Kernel tag v5.4.

.github/workflows/build.yaml

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+name: Build
+
+on:
+  push:
+    branches:
+      - '**'
+    paths:
+      - WSL2-Linux-Kernel
+
+jobs:
+
+  Build:
+    runs-on: ubuntu-latest
+
+    env:
+      # Both of these will be set dynamically in a later step
+      GIT_VERSION:
+      KERNEL_VERSION:
+
+    defaults:
+      run:
+        working-directory: WSL2-Linux-Kernel
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+      with:
+        submodules: true
+
+    # Necessary for git describe. This takes some time, but we want to make sure
+    # to have the correct version baked into the kernel image in the end.
+    - name: Fetch kernel tags
+      run: git fetch --tags
+
+    - name: Get kernel config from tag "linux-msft-5.4.91"
+      run: git show linux-msft-5.4.91:Microsoft/config-wsl > .config
+
+    - name: Build kernel
+      uses: addnab/docker-run-action@v3
+      with:
+        image: carlfriedrich/wsl-kernel-build:latest
+        options: |
+          -v ${{ github.workspace }}/WSL2-Linux-Kernel:/src
+          --hostname wsl-kernel-build
+        run: bash -c "make -j 32 CONFIG_LOCALVERSION_AUTO=y KCONFIG_CONFIG=.config"
+
+    - name: Get kernel version
+      run: |
+        KERNEL_VERSION=$(scripts/extract-vmlinux arch/x86/boot/bzImage \
+          | grep -a 'Linux version' | head -1 | cut -d" " -f3)
+        echo "KERNEL_VERSION=${KERNEL_VERSION}" >> $GITHUB_ENV
+
+    - name: Get git version
+      run: echo "GIT_VERSION=$(git describe)" >> $GITHUB_ENV
+
+    - name: Put kernel version into filename
+      run: mv arch/x86/boot/bzImage bzImage-${{env.KERNEL_VERSION}}
+
+    - name: Upload Artifacts
+      uses: actions/upload-artifact@v4
+      with:
+        name: bzImage-${{env.KERNEL_VERSION}}
+        path: |
+          WSL2-Linux-Kernel/bzImage-${{env.KERNEL_VERSION}}
+
+    - name: Push tag
+      uses: mathieudutour/github-tag-action@v6.1
+      with:
+        custom_tag: ${{env.GIT_VERSION}}
+        github_token: ${{secrets.GITHUB_TOKEN}}
+
+    - name: Publish release
+      uses: softprops/action-gh-release@v1
+      with:
+        tag_name: ${{env.GIT_VERSION}}
+        files: bzImage-${{env.KERNEL_VERSION}}
+      env:
+        GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}

.gitmodules

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+[submodule "WSL2-Linux-Kernel"]
+	path = WSL2-Linux-Kernel
+	url = https://github.com/microsoft/WSL2-Linux-Kernel.git

Dockerfile

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+FROM ubuntu:20.04
+
+RUN apt-get update && apt-get install -y \
+    build-essential \
+    flex \
+    bison \
+    dwarves \
+    libssl-dev \
+    libelf-dev \
+    bc \
+    git \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /src

README.md

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+# WSL Kernel Build
+
+This repository has been set up to run automatic kernel builds for the [Windows
+Subsystem for Linux (WSL)][1], specifically aiming to track down its issue
+microsoft/WSL#8696.
+
+We have a [Dockerfile][2] for a container to run the builds in. It has been
+set up according to the [WSL Kernel build documentation][3]. The built image
+is available [on Docker Hub][4].
+
+The [WSL2-Linux-Kernel][5] repository is contained here as a Git submodule. A
+pushed change to its revision triggers a [workflow here][6] building the kernel
+on this specific codebase, which then will be tagged and provided as a
+[release][7]. We're trying to use [`git-bisect`][8] to find the commit
+introducing the beforementioned issue.
+
+Since the issue does not appear instantly, but needs some inconsistent time
+until it pops up, people are invited to help testing the built kernel images in
+their environments. Please follow #1 in this repository for the progress of the
+bisection.
+
+To use a custom kernel, add the following lines to your `.wslconfig` file:
+
+```ini
+[wsl2]
+kernel=C:\\Path\\To\\Kernel\\bzImage-5.4.0-microsoft-standard-WSL2
+```
+
+The `.wslconfig` file is located in your `%UserProfile%` directory (usually
+something like `C:\Users\your_user_name`) and can be created if it does not
+exist. Afterwards, shut down WSL and restart it using either Command Promt or
+PowerShell:
+
+```shell
+wsl --shutdown
+wsl
+```
+
+Verify that the configured kernel is used within WSL:
+
+```shell
+$ uname --kernel-release
+5.4.0-microsoft-standard-WSL2
+```
+
+Please note that the kernel version string is formatted differently than the git
+tag it was created from due to the [implementation of the kernel version][9].
+
+### Manual kernel build
+
+In case you want to manually build a WSL kernel, you can do so as follows:
+
+```shell
+git clone https://github.com/microsoft/WSL2-Linux-Kernel.git
+git checkout v5.4
+git show linux-msft-5.4.91:Microsoft/config-wsl > .config
+docker run --rm --hostname wsl-kernel-build \
+    -u $(id -u ${USER}):$(id -g ${USER}) \
+    -v $(pwd):/src \
+    carlfriedrich/wsl-kernel-build \
+    bash -c "make -j 32 CONFIG_LOCALVERSION_AUTO=y KCONFIG_CONFIG=.config"
+```
+
+If you want to build another version, make sure to clean the build tree before
+checking out a new version:
+
+```shell
+git reset HEAD --hard
+git clean -dfx
+```
+
+[1]: https://github.com/microsoft/WSL
+[2]: Dockerfile
+[3]: https://github.com/microsoft/WSL2-Linux-Kernel/tree/linux-msft-wsl-5.15.57.1?tab=readme-ov-file#build-instructions
+[4]: https://hub.docker.com/repository/docker/carlfriedrich/wsl-kernel-build
+[5]: https://github.com/microsoft/WSL2-Linux-Kernel
+[6]: https://github.com/carlfriedrich/wsl-kernel-build/actions
+[7]: https://github.com/carlfriedrich/wsl-kernel-build/releases
+[8]: https://git-scm.com/docs/git-bisect
+[9]: https://github.com/microsoft/WSL2-Linux-Kernel/blob/linux-msft-wsl-5.15.y/scripts/setlocalversion#L60-L64