mkosi: Optionally build a kernel image from mkosi.kernel/

If mkosi.kernel/ exists, the mkosi script will try to build a kernel
image from it. We use the architecture defconfig as a base and add
our own extra configuration on top.

We also add some extra tooling to the build image required to build
the kernel and include some documentation in HACKING.md on how to
use this new feature.

To avoid the kernel sources from being copied into the build or
final image (which we don't want because it takes a while), we put
the mkosi.kernel/ directory in .gitignore and use
"SourceFileTransfer=mount" so that the sources are still accessible
in the build image.

docs/HACKING.md

@@ -414,3 +414,23 @@ To debug systemd-boot in an IDE such as VSCode we can use a launch configuration
     ]
 }
 ```
+
+## Hacking on the kernel + systemd
+
+If you're hacking on the kernel in tandem with systemd, you can clone a kernel repository in mkosi.kernel/ in
+the systemd repository, and mkosi will automatically build that kernel and install it into the final image.
+To prevent the distribution's kernel from being installed (which isn't necessary since we're building our
+own kernel), you can add the following snippets to mkosi.default.d/20-local.conf:
+
+(This snippet is for Fedora, the list of packages will need to be changed for other distributions)
+
+```
+[Distribution]
+CacheInitrd=no
+
+[Content]
+BasePackages=conditional
+Packages=systemd
+         util-linux
+         dracut
+```

mkosi.build

@@ -259,3 +259,40 @@ fi
 # Make sure services aren't enabled by default on Debian/Ubuntu.
 mkdir -p "$DESTDIR/etc/systemd/system-preset"
 echo "disable *" > "$DESTDIR/etc/systemd/system-preset/99-mkosi.preset"
+
+if [ -d mkosi.kernel/ ]; then
+        cd "$SRCDIR/mkosi.kernel"
+        mkdir -p "$BUILDDIR/mkosi.kernel"
+
+        make O="$BUILDDIR/mkosi.kernel" defconfig
+
+        scripts/config \
+                --file $BUILDDIR/mkosi.kernel/.config \
+                --enable BPF_SYSCALL \
+                --enable BPF_JIT \
+                --enable BPF_JIT_ALWAYS_ON \
+                --enable BPF_JIT_DEFAULT_ON \
+                --enable BPF_UNPRIV_DEFAULT_OFF \
+                --enable USERMODE_DRIVER \
+                --enable BPF_PRELOAD \
+                --enable BPF_PRELOAD_UMD \
+                --enable BPF_LSM \
+                --enable BTRFS_FS \
+                --enable BTRFS_FS_POSIX_ACL \
+                --enable PSI \
+                --enable CGROUPS \
+                --enable CGROUP_BPF \
+                --enable MEMCG \
+                --enable MEMCG_SWAP \
+                --enable MEMCG_KMEM
+
+        # Make sure all unset options are set to their default value.
+        make O="$BUILDDIR/mkosi.kernel" olddefconfig
+
+        make O="$BUILDDIR/mkosi.kernel" -j "$(nproc)"
+
+        KERNEL_RELEASE="$(make O=$BUILDDIR/mkosi.kernel -s kernelrelease)"
+        mkdir -p "$DESTDIR/usr/lib/modules/$KERNEL_RELEASE"
+        make O="$BUILDDIR/mkosi.kernel" INSTALL_MOD_PATH="$DESTDIR/usr" modules_install
+        make O="$BUILDDIR/mkosi.kernel" INSTALL_PATH="$DESTDIR/usr/lib/modules/$KERNEL_RELEASE" install
+fi

mkosi.default.d/10-systemd.conf

@@ -43,17 +43,23 @@ Packages=
         zsh
 
 BuildPackages=
+        bc
+        binutils
+        bison
         clang
+        flex
         gcc
         gettext
         git
         gnu-efi
         gperf
         llvm
+        make
         meson
         pkgconf
         rpm
         rsync
+        tar
         zstd
 
 [Host]

mkosi.default.d/arch/10-mkosi.arch

@@ -31,6 +31,7 @@ BuildPackages=
         docbook-xsl
         libxslt
         linux-api-headers
+        pahole
         perl
         python-jinja
         python-lxml

mkosi.default.d/centos_epel/10-mkosi.centos_epel

@@ -38,6 +38,7 @@ Packages=
 BuildPackages=
         bpftool
         docbook-xsl
+        dwarves
         gnu-efi-devel
         libgcrypt-devel # CentOS Stream 8 libgcrypt-devel doesn't ship a pkg-config file.
         libxslt

mkosi.default.d/debian/10-mkosi.debian

@@ -62,6 +62,7 @@ BuildPackages=
         libssl-dev
         libxkbcommon-dev
         libzstd-dev
+        pahole
         python3-jinja2
         python3-lxml
         xsltproc

mkosi.default.d/fedora/10-mkosi.fedora

@@ -33,6 +33,7 @@ Packages=
 BuildPackages=
         bpftool
         docbook-xsl
+        dwarves
         gnu-efi-devel
         pam-devel
         pkgconfig # pkgconf shim to provide /usr/bin/pkg-config

mkosi.default.d/opensuse/10-mkosi.opensuse

@@ -35,6 +35,7 @@ BuildPackages=
         bpftool
         dbus-1-devel
         docbook-xsl-stylesheets
+        dwarves
         fdupes
         gcc-c++
         glib2-devel

mkosi.default.d/ubuntu/10-mkosi.ubuntu

@@ -63,6 +63,7 @@ BuildPackages=
         libssl-dev
         libxkbcommon-dev
         libzstd-dev
+        pahole
         python3-jinja2
         python3-lxml
         xsltproc