Merge pull request 'fix(cilium): point cgroup.hostRoot at the host cgroup2 root (socket-LB never covered host processes)' (#42) from fix/cilium-cgroup-hostroot into main

Reviewed-on: #42

decisions/2026-10-03-cilium-cgroup-hostroot.md

@@ -0,0 +1,76 @@
+---
+date: 2026-10-03
+branch: fix/cilium-cgroup-hostroot
+title: Cilium socket-LB never covered host processes — cgroup.hostRoot pointed at an unmounted path
+---
+
+## What changed
+
+`cgroup.hostRoot` changes from `/run/cilium/cgroupv2` to `/sys/fs/cgroup` in all three parity
+copies: `platform/cilium/helmrelease.yaml` (authoritative), `platform/cilium/values.yaml`
+(mirror) and `infra/ansible/roles/cilium/templates/cilium-values.yaml.j2` (bootstrap).
+`autoMount` stays `false`.
+
+**The defect.** With `autoMount: false`, the Cilium chart only bind-mounts whatever exists at
+`hostRoot`, and expects the operator to have mounted cgroup2 there (cilium 1.20.1 values.yaml).
+The repo's rationale was "k3s provides the cgroupv2 mount". That is true only for
+`/sys/fs/cgroup`. Nothing mounts cgroup2 at `/run/cilium/cgroupv2` on these nodes; it is an
+empty `DirectoryOrCreate` hostPath. So the agent mounted cgroup2 there itself, from inside its
+own private cgroup namespace, and that mount's root is the agent container's own leaf cgroup.
+The socket-LB programs (`cil_sock4_connect` etc.) were attached to it and covered only the
+agent.
+
+**The consequence.** Every other host-netns process got no Service/LB translation:
+- Service ClusterIPs were unreachable from every node's host.
+- The node holding the Gateway's L2 lease could not reach the Gateway VIP (`no route to host`).
+  Other nodes reached it only because their packets crossed the wire to the lease holder, where
+  the tc datapath handles them.
+- k3s-prod-1 holds both the Gateway lease and the kube-vip API VIP. Its apiserver could not reach
+  the OIDC issuer, logging `oidc: authenticator not initialized` since at least 2026-09-12
+  (899 occurrences by 2026-10-03). kubectl SSO through the VIP therefore failed, and in-cluster
+  OIDC clients failed on whichever calls landed on that apiserver.
+- Webhooks and aggregated APIs were unaffected only because k3s runs
+  `--enable-aggregator-routing=true` (endpoint IPs, not ClusterIPs).
+
+Cilium's own source (v1.20.1 `bpf/bpf_sock.c`, `__sock4_xlate_fwd`) handles exactly this case
+when the hook does run: a host-netns connect to an L7-LB Service (the Gateway) is rewritten to
+`127.0.0.1:<l7_lb_proxy_port>`, the local Envoy listener. The fix is to make the hook cover the
+host.
+
+## What I verified
+
+- `cilium-dbg status --verbose` on k3s-prod-1: `Socket LB: Enabled`, `Coverage: Hostns-only`.
+  The live cilium-config shows `bpf-lb-sock=false`, but socket LB is implied by kube-proxy
+  replacement. The Gateway Service frontends `192.168.2.20:{80,443}` and
+  `10.43.225.79:{80,443}` map to `127.0.0.1:14337`.
+- Host: `findmnt /run/cilium/cgroupv2` is empty on all three nodes, and `/sys/fs/cgroup` is
+  `cgroup2`.
+- In the agent: `/run/cilium/cgroupv2` is a `cgroup2 none` mount, the agent's own cgroup reads
+  `0::/`, the mount has 0 child cgroups, and `bpftool cgroup show /run/cilium/cgroupv2` lists all
+  `cil_sock*` programs. k3s runs in `0::/system.slice/k3s.service`, and the agent container in
+  `/kubepods.slice/…/cri-containerd-3ad04cde….scope`.
+- **The decisive test**, run on k3s-prod-1 in the same netns: raw TCP connect to
+  `192.168.2.20:443`, `10.43.225.79:443` and `10.43.59.141:8080` CONNECTED from inside the agent
+  container (covered cgroup) and FAILED from the host (`No route to host` / timeout).
+- Blast radius: host-network pods are cilium, cilium-envoy, cilium-operator (API via
+  `192.168.2.9:6443`, not a Service), kube-vip (`127.0.0.1:6443`, `svc_enable=false`) and
+  node-exporter (no outbound). Nothing depends on host Service translation being absent; the
+  change only makes Service/LB addresses reachable from the host where they currently fail.
+- `helm template cilium/cilium --version 1.20.1` with the new values, in the toolbox: the
+  DaemonSet bind-mounts hostPath `/sys/fs/cgroup` into `cilium-agent` and `clean-cilium-state`,
+  the cilium-config shows `cgroup-root: "/sys/fs/cgroup"`, and the init containers are unchanged
+  (no `mount-cgroup`).
+- Toolbox gate: see the PR description.
+
+## What I did NOT verify
+
+- **The rollout itself.** After merge, the agent DaemonSet rolls one node at a time
+  (`maxUnavailable: 1`). On each node, afterwards: host TCP connects to the three targets above
+  must succeed. On k3s-prod-1, the apiserver's background OIDC init retry must stop logging
+  `authenticator not initialized` with no k3s restart.
+- That the old programs, attached to the previous agent container's cgroup, disappear with it.
+  Expected, because cgroup BPF attachments die with the cgroup, but not observed.
+- An end-to-end kubectl OIDC login (needs a browser).
+- No regression test was added. A natural one is an L2 conformance check that a host-netns
+  connect to the Gateway VIP succeeds on the lease holder, but `conformance/**` is a sensitive
+  path. Left as a follow-up.

infra/ansible/roles/cilium/templates/cilium-values.yaml.j2

@@ -51,11 +51,14 @@ hubble:
 l2announcements:
   enabled: true
 
-# k3s ships its own cgroup mount; point Cilium at it instead of auto-mounting.
+# Point Cilium at the host's real cgroup2 root (/sys/fs/cgroup, mounted by systemd) instead of
+# auto-mounting. NOT /run/cilium/cgroupv2: nothing mounts cgroup2 there, so the agent mounted its
+# OWN namespaced cgroup and socket-LB covered only itself — host processes (the apiserver) lost
+# Service/LB translation. [parity] platform/cilium/helmrelease.yaml explains (2026-10-03).
 cgroup:
   autoMount:
     enabled: false
-  hostRoot: /run/cilium/cgroupv2
+  hostRoot: /sys/fs/cgroup
 
 # Roll the cilium agent DaemonSet ONE node at a time. [parity] must match the
 # platform/cilium HelmRelease. The cilium 1.20.1 chart defaults this to 2, which on a

platform/cilium/helmrelease.yaml

@@ -106,14 +106,33 @@ spec:
     # CiliumL2AnnouncementPolicy (reserved 10.73.0.240-.250) authored by WS-NET in Phase 2.
     l2announcements:
       enabled: true
-    # --- cgroup: k3s ships its own cgroup mount; point Cilium at it, don't auto-mount ---
-    # [parity] MUST equal Ansible cgroup block. Not in the deliverable's headline list, but it IS
-    # part of the working CNI config on k3s: reconciling without it lets Cilium try to auto-mount
-    # cgroupv2 and break on adopt. Kept in parity for a safe Phase-2 adoption.
+    # --- cgroup: point Cilium at the HOST's cgroup2 root, which systemd mounts at /sys/fs/cgroup ---
+    # [parity] MUST equal Ansible cgroup block (roles/cilium/templates/cilium-values.yaml.j2).
+    #
+    # FIXED 2026-10-03 — hostRoot WAS /run/cilium/cgroupv2, AND THAT SILENTLY BROKE SOCKET-LB FOR
+    # EVERY HOST PROCESS. With autoMount disabled, the chart only bind-mounts whatever already exists
+    # at hostRoot ("it's expected that users have mounted cgroup2 filesystem at the specified
+    # cgroup.hostRoot" — cilium 1.20.1 values.yaml). Nothing ever mounted cgroup2 at
+    # /run/cilium/cgroupv2 on these nodes (it is an empty DirectoryOrCreate hostPath), so the agent
+    # mounted one ITSELF from inside its own private cgroup namespace. That mount's root is the
+    # agent container's OWN leaf cgroup, so cil_sock4_connect & co. were attached there and covered
+    # only the agent. Every other host-netns process (the k3s apiserver in
+    # /system.slice/k3s.service, curl over SSH) was untranslated: Service ClusterIPs timed out from
+    # every node, and the node holding the Gateway's L2 lease could not reach the Gateway VIP at all
+    # ("no route to host"). That is why k3s-prod-1's apiserver logged `oidc: authenticator not
+    # initialized` from 2026-09-12 on — kubectl SSO through the kube-vip VIP failed outright.
+    # Measured on k3s-prod-1 before this change: TCP connect to 192.168.2.20:443, 10.43.225.79:443
+    # and 10.43.59.141:8080 SUCCEEDED from inside the agent container (the covered cgroup) and
+    # FAILED from the host, same node, same netns. Full write-up:
+    # decisions/2026-10-03-cilium-cgroup-hostroot.md.
+    #
+    # /sys/fs/cgroup IS a cgroup2 mount in the host's root cgroup namespace on every node (verified
+    # with findmnt on all three), so bind-mounting it gives the agent the real host root. autoMount
+    # stays off because the mount it would create already exists.
     cgroup:
       autoMount:
         enabled: false
-      hostRoot: /run/cilium/cgroupv2
+      hostRoot: /sys/fs/cgroup
     # --- Agent rollout: ONE node at a time (blast-radius bound) — 2026-09-01 incident ---
     # [parity] MUST equal Ansible: updateStrategy.rollingUpdate.maxUnavailable: 1.
     # The chart's own default is 2 (cilium 1.20.1 values.yaml: updateStrategy.rollingUpdate.

platform/cilium/values.yaml

@@ -41,11 +41,13 @@ hubble:
 # [parity] ingress VIP announcement on the flat LAN (docs/09, OQ-1)
 l2announcements:
   enabled: true
-# [parity] k3s provides the cgroupv2 mount; do not auto-mount
+# [parity] the host's cgroup2 root is /sys/fs/cgroup (systemd mounts it); do not auto-mount.
+# NOT /run/cilium/cgroupv2: nothing mounts cgroup2 there, so socket-LB attached to the agent's own
+# cgroup and host processes lost Service/LB translation (2026-10-03 — see helmrelease.yaml).
 cgroup:
   autoMount:
     enabled: false
-  hostRoot: /run/cilium/cgroupv2
+  hostRoot: /sys/fs/cgroup
 # [parity] roll the agent DaemonSet ONE node at a time. Overrides the cilium 1.20.1 chart
 # default of 2, which on 3 nodes lets a single bad render break 2/3 agents at once — as the
 # 2026-09-01 incident did (k8sServiceHost: "" → the `config` init container fell back to the