factory: create components compatible mount units for kernel

The kernel-modules components require that /lib/{modules,firmware} can
be created by composition of what is shipped with the kernel plus
parts that will be shipped by said component type.

For this, we need to have modinfo files in a writable directory as
these files will need to be regenerated when a new kernel-modules
component is installed. These files need to be readable from
/lib/modules/UTS_RELEASE. Inside this directory, "kernel" and "vdso"
subfolders will be bind-mounted from the kernel snap, while "updates"
will have modules from the components.

In the case of UC, for modules we need an additional bind mount on top
of the directory coming from the base (this is not needed for hybrid
systems).

In the case of firmware, content from the kernel will be mounted to
/lib/firmware as in the past. However, we will rely on the existence
of a /lib/firmware/updates in the kernel to mount firware coming from
components.

The mount units for the active kernel are created at boot time and
stored in /run/systemd/system so they are accessible after the switch
root, and are actually mounted after the switch happens, but with
dependencies set so kernel modules are not loaded before the mounts
are ready. This replaces the fstab/generator that was creating the
mounts for modules/firmware in UC and hybrid respectively, and now
unifies the approach for both.

factory/usr/lib/core/extra-paths

@@ -4,6 +4,4 @@ set -eu
 
 cat <<EOF >>/sysroot/etc/fstab
 /run/mnt/data /writable none bind,x-initrd.mount 0 0
-/run/mnt/kernel/firmware /usr/lib/firmware none bind,x-initrd.mount 0 0
-/run/mnt/kernel/modules /usr/lib/modules none bind,x-initrd.mount 0 0
 EOF

factory/usr/lib/core/kernel-mounts

@@ -0,0 +1,81 @@
+#!/bin/sh
+
+set -eux
+
+# Prepare kernel modules/firmware tree if it does not exist
+
+ksnap_d=/run/mnt/kernel
+core=false
+data_d=/run/mnt/data
+if [ -d /run/mnt/base ]; then
+    core=true
+    data_d=/run/mnt/data/system-data
+fi
+
+kernelVer=
+for f in "$ksnap_d"/System.map-*; do
+    kernelVer=${f##"$ksnap_d"/System.map-}
+done
+mod_root_d="$data_d"/usr/lib/modules
+mod_d="$mod_root_d"/"$kernelVer"
+ksnap_mod_d="$ksnap_d"/modules/"$kernelVer"
+
+if [ ! -d "$mod_d" ]; then
+    for d in initrd kernel vdso updates; do
+        mkdir -p "$mod_d"/"$d"
+    done
+    # Copy modinfo files
+    cp "$ksnap_mod_d"/modules.* "$mod_d"
+fi
+
+fw_root_d="$data_d"/usr/lib/firmware/updates
+mkdir -p "$fw_root_d"
+
+sysinit_wants=/run/systemd/system/sysinit.target.wants
+mkdir -p "$sysinit_wants"
+
+conf_mount_dir() {
+    what=$1
+    where=$2
+    options=$3
+    before_deps=
+    if [ $# -gt 3 ]; then
+        before_deps=$4
+    fi
+
+    unit=$(systemd-escape -p --suffix=mount "$where")
+    unit_path=/run/systemd/system/"$unit"
+    cat <<EOF > "$unit_path"
+[Unit]
+ConditionPathExists=!/etc/initrd-release
+DefaultDependencies=no
+After=systemd-remount-fs.service
+Before=systemd-udev.service systemd-modules-load.service $before_deps
+Before=sysinit.target
+Before=umount.target
+Conflicts=umount.target
+
+[Mount]
+What=$what
+Options=$options
+EOF
+
+    ln -s "$unit_path" "$sysinit_wants"/"$unit"
+}
+
+mod_unit_dep=
+if [ "$core" = true ]; then
+    # For classic we are actually working on the rootfs, for core
+    # we need to bind mount to the base that is the rootfs.
+    mod_where=/usr/lib/modules
+    mod_unit_dep=$(systemd-escape -p --suffix=mount "$mod_where")
+    conf_mount_dir "$mod_root_d" "$mod_where" rbind,ro
+fi
+conf_mount_dir "$ksnap_mod_d"/kernel "$mod_d"/kernel bind,ro "$mod_unit_dep"
+conf_mount_dir "$ksnap_mod_d"/vdso "$mod_d"/vdso bind,ro "$mod_unit_dep"
+
+fw_where=/usr/lib/firmware
+fw_unit=$(systemd-escape -p --suffix=mount "$fw_where")
+conf_mount_dir "$ksnap_d"/firmware "$fw_where" rbind,ro
+# Note that this will fail if the kernel snap does not have a firmware/updates dir
+conf_mount_dir "$fw_root_d" /usr/lib/firmware/updates rbind,ro "$fw_unit"

factory/usr/lib/systemd/system/classic-mounts.service

@@ -11,6 +11,5 @@ Before=initrd-fs.target
 
 [Service]
 Type=oneshot
-# We force re-running kernel-snap-generator
-ExecStart=systemctl daemon-reload
+ExecStart=/usr/lib/core/kernel-mounts
 StandardError=journal+console

factory/usr/lib/systemd/system/detect-classic-sysroot.service

@@ -3,11 +3,11 @@ Description=Detect Ubuntu classic sysroot
 DefaultDependencies=no
 Before=initrd-root-device.target
 After=snap-initramfs-mounts.service
-Wants=classic-mounts.service
 
 ConditionPathIsMountPoint=!/run/mnt/base
 
 [Service]
 Type=oneshot
 RemainAfterExit=yes
 ExecStart=/bin/ln -s data /run/mnt/sysroot
+ExecStart=/usr/lib/core/kernel-mounts

factory/usr/lib/systemd/system/populate-writable.service

@@ -25,3 +25,4 @@ ExecStart=/usr/bin/chroot /sysroot /usr/lib/core/handle-writable-paths / /etc/sy
 # TODO:UC20: move the-modeenv implementation to snap-bootstrap too
 ExecStart=/usr/lib/the-modeenv
 ExecStart=/usr/lib/core/extra-paths
+ExecStart=/usr/lib/core/kernel-mounts