docs/zlevis/implementation.md: add dracut implementation

docs/zlevis/implementation.md

@@ -1,6 +1,6 @@
 ## Mkinitfs
 
-[Mkinitfs](https://gitlab.alpinelinux.org/alpine/mkinitfs) is the `initramfs` generator of [Alpine Linux](https://alpinelinux.org/). Among the usual operations to setup `zlevis`, i.e. writing the encryption key of the ZFS root pool to the TPM after the creation of the ZFS root pool, enabling the `zlevis-hook` in `mkinitfs.conf` and installing `zlevis` on the host system, we require to make an adaptation to the `initramfs-init` script of `mkinitfs`. 
+[Mkinitfs](https://gitlab.alpinelinux.org/alpine/mkinitfs) is the `initramfs` generator of [Alpine Linux](https://alpinelinux.org/). Among the usual operations to setup `zlevis`, i.e. writing the encryption key of the `ZFS` root pool to the TPM after the creation of the `ZFS` root pool, enabling the `zlevis-hook` in `mkinitfs.conf` and installing `zlevis` on the host system, we require to make an adaptation to the `initramfs-init` script of `mkinitfs`. 
 
 Particularly, in the `prepare_zfs_root()` function we need to add the option `zlevis decrypt "$_root_pool" | zfs load-key -L prompt "$_root_pool"`:
 
@@ -50,4 +50,65 @@ kernel/drivers/char/tpm*
 
 ## Dracut
 
-Work in progress.
+[Dracut](https://dracut-ng.github.io/dracut-ng) is the default `initramfs` generator of [Gentoo Linux](https://www.gentoo.org/) and [Void Linux](https://voidlinux.org/). Among the usual operations to setup `zlevis`, i.e. writing the encryption key of the `ZFS` root pool to the TPM after the creation of the `ZFS` root pool, enabling the `zlevis` module in `/etc/dracut.conf.d` and installing `zlevis` on the host system, we require to add the `zlevis` module in `/usr/lib/dracut/modules.d`. 
+
+Create the **executable** module setup of the `zlevis-dracut` module:
+
+``` shell title="/usr/lib/dracut/modules.d/85zlevis/module-setup.sh"
+#!/bin/bash
+
+# Exit immediately if a command exits with a non-zero status
+set -e
+
+# Depend on udev-rules and zfs
+depends() {
+  echo udev-rules zfs
+  return 0
+}
+
+install() {
+  # Install the appropriate binaries and libraries
+  inst_multiple /usr/bin /usr/bin/zlevis /usr/bin/zlevis-decrypt /usr/bin/jose /usr/bin/tpm2*
+  inst_multiple /usr/lib /usr/lib/libtss2-tcti*
+
+  # Run the zlevis decryption hook before the 90zfs hook
+  inst_hook pre-mount 85 "${moddir}/zlevis.sh"
+  inst_simple "${moddir}/zlevis.sh" "/sbin/zlevis.sh"
+}
+
+# Exit with the status of the last command
+exit $?
+```
+
+and create the **executable** module script of the `zlevis-dracut` module:
+
+``` shell title="/usr/lib/dracut/modules.d/85zlevis/zlevis.sh"
+#!/bin/sh
+
+# Exit immediately if a command exits with a non-zero status
+set -e
+
+# Load the ZFS kernel module
+modprobe zfs 2>/dev/null
+udevadm settle
+
+# Search for encrypted pool's by means of the cmdline root atribute
+local _root_vol="${root}"
+local _root_pool="${_root_vol%%/*}"
+
+# Import the root pool
+zpool import -N -d /dev $_root_pool
+
+# If the pool is encrypted run `zlevis decrypt` to obtain the key stored in the TPM and load the key
+if [ $(zpool list -H -o feature@encryption $_root_pool) = "active" ]; then
+	local _encryption_root=$(zfs get -H -o value encryptionroot $_root_vol)
+	if [ "$_encryption_root" != "-" ]; then
+		zlevis decrypt $_root_pool | zfs load-key -L prompt "$_root_pool" || echo "Failed to unlock $_root_pool with TPM"
+	fi
+fi
+
+# Exit with the status of the last command
+exit $?
+```
+
+Now `dracut` will recognise the `zlevis` module, and if enabled will be added to the `initramfs` to be executed before the `90zfs` hook.