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

[Dan Watkins]

Thanks for the reply, Nick!

I think it's important enough to land because:

* you cannot execute `kexec` correctly on an Ubuntu 20.04 system without this patch (it will fall back to performing a full reboot),
* kexec can be used to reduce downtime for critical systems which take a long time to reboot (e.g. because they have a lot of hardware to initialise), and
* kexec-tools is in main (and has been since at least trusty) which indicates to me that it is expected that kexec will work on Ubuntu

I'd also add that the patch is three lines in a code path which is only
used by people opting into using `kexec`, so the potential downside is
pretty minimal.

(I'll set the bug back to New for now, until you have a chance to
respond.)

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