[Bug 1969365] [NEW] focal: backport kexec fallback patch

[Dan Watkins]

Public bug reported:

It would be great if focal's systemd could have
https://github.com/systemd/systemd/commit/71180f8e57f8fbb55978b00a13990c79093ff7b3
backported to it.

[Impact]

We have observed that kexec'ing to another kernel will fail as the drive
containing the `kexec` binary has been unmounted by the time systemd
attempts to do so, indicated in the console:

         Starting Reboot via kexec...
[  163.960938] shutdown[1]: (sd-kexec) failed with exit status 1.
[  163.963463] reboot: Restarting system

[Test Plan]

1) Launch a 20.04 instance
2) `apt-get install kexec-tools`
3) In `/boot`, filling in whatever <cmdline> needed in your environment:

kexec -l vmlinuz --initrd initrd.img --append '<cmdline>'

4) `reboot`

(I have reproduced this in a single-disk VM, so I assume it reproduces
~everywhere: if not, `apt-get remove kexec-tools` before the `reboot`
could be used to emulate the unmounting.)

[Where problems could occur]

Users could inadvertently be relying on the current behaviour: if they
have configured their systems to kexec, they currently will be rebooting
normally, and this patch would cause them to start actually kexec'ing.

[Other info]

We're currently maintaining a systemd tree with only this patch added to
focal's tree: this patch has received a bunch of testing from us in
focal.

This patch landed in v246, so it's already present in supported releases
later than focal.

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1969365

Title:
  focal: backport kexec fallback patch

Status in systemd package in Ubuntu:
  New

Bug description:
  It would be great if focal's systemd could have
  https://github.com/systemd/systemd/commit/71180f8e57f8fbb55978b00a13990c79093ff7b3
  backported to it.

  [Impact]

  We have observed that kexec'ing to another kernel will fail as the
  drive containing the `kexec` binary has been unmounted by the time
  systemd attempts to do so, indicated in the console:

           Starting Reboot via kexec...
  [  163.960938] shutdown[1]: (sd-kexec) failed with exit status 1.
  [  163.963463] reboot: Restarting system

  [Test Plan]

  1) Launch a 20.04 instance
  2) `apt-get install kexec-tools`
  3) In `/boot`, filling in whatever <cmdline> needed in your environment:

  kexec -l vmlinuz --initrd initrd.img --append '<cmdline>'

  4) `reboot`

  (I have reproduced this in a single-disk VM, so I assume it reproduces
  ~everywhere: if not, `apt-get remove kexec-tools` before the `reboot`
  could be used to emulate the unmounting.)

  [Where problems could occur]

  Users could inadvertently be relying on the current behaviour: if they
  have configured their systems to kexec, they currently will be
  rebooting normally, and this patch would cause them to start actually
  kexec'ing.

  [Other info]

  We're currently maintaining a systemd tree with only this patch added
  to focal's tree: this patch has received a bunch of testing from us in
  focal.

  This patch landed in v246, so it's already present in supported
  releases later than focal.

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