Bug#778849: Support restoring initrd on shutdown and pivoting into it

[intrigeri <intrigeri@debian.org>]

Hi,

intrigeri:
> I might try to come up with a hackish PoC for Tails soon

Here we go! Installing the four following files (slightly adapted to
drop a couple Tails-specific bits) on a Stretch system seems to do the
job. I hope it can allow interested people to validate this approach,
and then if there's enough demand I bet someone will integrate it into
initramfs-tools properly :)

If additional cleanup must be done from inside the initramfs after
returning to it, drop snippets in /usr/share/initramfs-tools/hooks/*
that install the required scripts into /lib/systemd/system-shutdown/
*in the initramfs*. E.g. for Tails I had to do quite more work there
to ensure the aufs stack our root filesystem uses is disassembled
properly (again in order to have the aufs read-write branch, on tmpfs,
cleaned up and its content erased by Linux' memory poisoning); I'll
contribute this code to live-boot if/when this feature is properly
integrated into initramfs-tools.

I don't know if I'll work more on this wrt. initramfs-tools.
It'll depend a lot on the timing of Tails moving to dracut, which is
entirely unclear at this time. Sorry!


/lib/systemd/system/initramfs-shutdown.service
----------------------------------------------

[Unit]
Description=Restore /run/initramfs on shutdown
Documentation=https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface/
After=local-fs.target boot.mount boot.automount
Wants=local-fs.target
Conflicts=shutdown.target umount.target
DefaultDependencies=no
ConditionPathExists=!/run/initramfs/bin/sh

[Service]
RemainAfterExit=yes
Type=oneshot
ExecStart=/bin/true
ExecStop=/usr/share/initramfs-tools/initramfs-restore

[Install]
WantedBy=multi-user.target

/usr/share/initramfs-tools/initramfs-restore
--------------------------------------------

#!/bin/sh

set -e
set -u

WORKDIR=$(mktemp -d)
/usr/bin/unmkinitramfs /initrd.img "$WORKDIR"
mv "$WORKDIR"/main/* /run/initramfs/
rm -rf "$WORKDIR"

/lib/systemd/system-shutdown/initramfs-tools
--------------------------------------------

#!/bin/sh

# Otherwise systemd-shutdown cannot execute /run/initramfs/shutdown
mount -o remount,exec /run

/usr/share/initramfs-tools/hooks/shutdown
-----------------------------------------

#!/bin/sh

set -e

PREREQ=""

prereqs () {
       echo "${PREREQ}"
}

case "${1}" in
       prereqs)
               prereqs
               exit 0
               ;;
esac

. /usr/share/initramfs-tools/hook-functions

# systemd-shutdown itself
mkdir -p $DESTDIR/lib/systemd
copy_exec /lib/systemd/systemd-shutdown /shutdown

# Ensure systemd detects when we're in the initramfs on shutdown
# (see the in_initrd function in the systemd source tree)
touch $DESTDIR/etc/initrd-release

exit 0



Cheers,
-- 
intrigeri