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

[Nick Rosbrook]

The patch for this is indeed present in Jammy and newer. I don't
currently see a strong enough reason to SRU this to Focal, but if you or
someone else thinks it's important, feel free to explain here.

** Changed in: systemd (Ubuntu)
       Status: New => Fix Released

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Title:
  focal: backport kexec fallback patch

Status in systemd package in Ubuntu:
  Fix Released

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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