[Question] Will the OCP 4.22 MCO support Ignition spec 3.6.0 to match Butane\openshift 4.22.0 ?

[tmkymst]

What we are asking

This is a community clarification question about how the MCO tracks Ignition config spec
versions. For the concrete "OCP 4.22 MCO rejects a 3.6.0 MachineConfig" symptom, see
coreos/butane#715 for reference. We would appreciate the maintainers' confirmation on the
points below.

Background

To keep the questions concrete, here is what we have already confirmed from primary sources and on a
live cluster. Please correct anything inaccurate.

• The MCO release-4.22 (and main) requires github.com/coreos/ignition/v2 v2.20.0 and, in
  pkg/controller/common/helpers.go, imports config/v3_5 (no config/v3_6). It parses incoming
  configs via ign3.ParseCompatibleVersion(...).
• On a live OCP 4.22 cluster, go version -m on the MCO binary reports
  dep github.com/coreos/ignition/v2 v2.20.0.
• Stable Ignition config spec 3.6.0 first exists in the Ignition library v2.26.0 (2026-02-17);
  spec 3.5.0 has existed since v2.20.0 (2024-10-22).
• Upstream Butane v0.28.0 (2026-05-19) stabilized the openshift 4.22.0 variant to emit
  ignition.version: 3.6.0; openshift 4.21.0 (and earlier) emit 3.5.0.
• We read this to mean the OCP 4.22 MCO's accepted-input and rendered-output Ignition spec
  ceiling is 3.5.0, while the matching Butane variant now produces 3.6.0.

Throughout, we deliberately distinguish three separate "versions" to avoid ambiguity:

• (A) the Ignition config spec version (3.5.0, 3.6.0) — the on-disk MachineConfig format;
• (B) the Ignition Go library + parser package the MCO vendors (coreos/ignition/v2 +
  config/v3_N) — this is what sets the MCO's spec ceiling;
• (C) the Ignition binary/utility shipped in RHCOS that consumes the config at firstboot.

Questions

1. Confirm current state. Is it correct that the OCP 4.22 MCO accepts and renders Ignition
   config spec only up to 3.5.0 (because it vendors coreos/ignition/v2 v2.20.0 and imports
   config/v3_5), so a MachineConfig declaring stable ignition.version: 3.6.0 is rejected at
   render time?

2. Version-tracking policy. Butane's
   latest openshift variant tracks the latest spec (3.6.0). Does the MCO intend to track the
   Ignition library/parser (B) up to the level shipped in its own release or is staying on v2.20.0 /
   config/v3_5 for 4.22
   intentional? What policy decides when the MCO's config/v3_N import is bumped?

3. z-stream behavior. Could the accepted/rendered Ignition spec ceiling be
   raised to 3.6.0 within a 4.22.z (patch / z-stream) update or would a config/v3_6 bump only ship in a later minor (4.23+)? Is
   raising the spec ceiling within a z-stream considered supported at all?

4. Backward compatibility (ParseCompatibleVersion). If/when the MCO moves to
   config/v3_6, our understanding is that config/v3_6.ParseCompatibleVersion accepts any known
   v3 spec up to and including 3.6.0 and converts it to the v3_6 internal type — so a 3.5.0
   MachineConfig generated by Butane
   openshift 4.21.0 would continue to be accepted. Is that correct — i.e. a config/v3_6 MCO
   would not break existing 3.5.0 MachineConfigs?

5. Butane ↔ MCO coordination. Where is the cross-release synchronization between the Butane
   openshift variant → Ignition spec mapping and the MCO's supported spec ceiling guaranteed (or
   is it not)? Historically the Butane side was rolled back when the MCO lagged
   (coreos/butane#312).

Why this matters

Following the documented workflow (set the Butane openshift version: to match the cluster, i.e.
4.22.0) currently yields a 3.6.0 MachineConfig that the 4.22 MCO rejects, degrading the target
MachineConfigPool. Clear answers to the above would make it clear how to address this.

Related

• coreos/butane issue (roll back the openshift 4.22.0 variant to 3.5.0, per the
  coreos/butane#312 precedent): coreos/butane#715
• Prior Butane policy / precedents: coreos/butane#312 (standing policy),
  coreos/butane#269 / coreos/butane#307 (prior rollbacks for openshift 4.9 / 4.10).
• MCO PR openshift/machine-config-operator#4903 (MCO-1579 — last Ignition bump, to 3.5; OCP 4.19).

Environment

• OpenShift Container Platform: stable-4.22
• MCO vendored Ignition: github.com/coreos/ignition/v2 v2.20.0 (confirmed via go version -m)
• Butane: 0.28.0