Bug#993246: clevis: stores wrong path to systemd-reply-password if built on merged-/usr system

[Simon McVittie]

Source: clevis
Version: 16-2
Severity: important
Tags: bookworm sid
User: reproducible-builds at lists.alioth.debian.org
Usertags: usrmerge
X-Debbugs-Cc: reproducible-bugs at lists.alioth.debian.org

If clevis is built on a merged-/usr system (as created by new
installations of Debian >= 10, debootstrap --merged-usr, or
installing the usrmerge package into an existing installation), the
path to systemd-reply-password is recorded in the binary package
as /usr/lib/systemd-reply-password, rather than the canonical
/lib/systemd-reply-password.

This can be seen on the reproducible-builds.org infra:
https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/diffoscope-results/clevis.html

If you have sbuild available, an easy way to reproduce this is to build
twice, once with --add-depends=usrmerge and once without.

I suspect the same thing would happen if clevis was built on a system
where /lib and /usr/lib had instead been unified via a symlink farm.

The problematic situation is if the package is *built* on a unified-/usr
system, but *used* on a non-unified-/usr system. In this situation,
/usr/lib/systemd-reply-password exists on the build system but not on
the system where clevis will be used, resulting in the features that
use this executable not working correctly.

Technical Committee resolution #978636 mandates heading towards a
transition to merged-/usr, and this will become a non-issue at the end of
that transition; but variation between merged-/usr and non-merged-/usr
builds is a problem while that transition is taking place, because it
can lead to partial upgrades behaving incorrectly. It is likely that
this class of bugs will become release-critical later in the bookworm
development cycle.

I see that debian/patches/debian/2021-02-10.build-on-non-usrmerge.patch
tries to resolve this, but it doesn't seem to be having the desired
effect, because /usr is checked before /lib. Since you're applying a
Debian-specific patch for this *anyway*, I think the easiest solution
is likely to be to change this patch so that the patched result just
hard-codes the canonical path:

sd_reply_pass = find_program(
  join_paths('/', 'lib', 'systemd', 'systemd-reply-password'),
  required: false
)

Some developers advocate unifying /lib with /usr/lib via a symlink farm
in /lib instead of merged-/usr, but that strategy would have a similar
practical effect on this particular package, and the same solution would
be required.

A side benefit of fixing this is that this change seems likely to be
sufficient to make the package reproducible (as recommended by Policy
§4.15).

    smcv