This bug was fixed in the package linux-base - 4.5ubuntu6

---------------
linux-base (4.5ubuntu6) impish; urgency=medium

  * linux-version: Fix sorting of versions provided via stdin to match
    the output when provided via argv (LP: #1926985).

 -- dann frazier <dann.fraz...@canonical.com>  Mon, 03 May 2021 10:16:43
-0600

** Changed in: linux-base (Ubuntu)
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/1926985

Title:
  linux-version sort: argv and stdin behaviors differ

Status in linux-base package in Ubuntu:
  Fix Released
Status in linux-base package in Debian:
  New

Bug description:
  [Impact]
  Systems that use flash-kernel may end up installing the wrong kernel image.

  `linux-version sort` can provide different answers depending on how it
  is called (see Test Case). The stdin behavior appears to be a bug, and
  this is the mode flash-kernel uses. So, for example, a user with an
  arm64 u-boot system might end up booting 5.8.0-50-generic-64k instead
  of the expected 5.8.0-50-generic if both are installed. There maybe
  additional issues caused by this; for example, I found this with our
  automation that tests kernels on various arm64 platforms in version
  increasing order. Only after consulting logs did we realize that 64k
  kernels were not ever getting booted because the tooling (which uses
  argv mode) disagreed with the kernel installer (flash-kernel using
  stdin mode) about which kernel should be the default.

  [Test Case]
  Using argv:
  $ linux-version sort 5.8.0-50-generic 5.8.0-50-generic-64k
  5.8.0-50-generic
  5.8.0-50-generic-64k

  Using stdin (incorrect):
  $ cat versions.txt
  5.8.0-50-generic
  5.8.0-50-generic-64k
  $ cat versions.txt | linux-version sort
  5.8.0-50-generic-64k
  5.8.0-50-generic

  [Where Problems Could Occur]
  An obvious place where problems could occur is if someone is relying on 
rebooting into the kernel that is incorrectly being sorted greatest. For those 
using only Ubuntu kernel packages, the only case I'm aware of is arm64 generic 
vs. generic-64k case used in the examples above. The generic-64k flavor is 
available in >= 20.10, as well as 20.04-hwe. ARM server users - those most 
likely to want the generic-64k flavor - are very unlikely to be using 
flash-kernel. The standard for ARM servers is UEFI firmware, which use GRUB and 
are unaffected by this change. We found this on an old HP m400 platform which 
is the only Ubuntu certified u-boot-based arm64 server, and is long out of 
support.

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