Enter host mount namespace

Use the tools installed in the host to effect reboots, and allow
the execution of commands such as `needs-restart` to determine if
reboots are required.

cmd/kured/Dockerfile

@@ -1,5 +1,5 @@
-FROM ubuntu:16.04
-RUN apt-get update && apt-get install -y ca-certificates && rm -rf /var/cache/apt
+FROM alpine:3.8
+RUN apk update && apk add ca-certificates && rm -rf /var/cache/apk/*
 # NB: you may need to update RBAC permissions when upgrading kubectl - see kured-rbac.yaml for details
 ADD https://storage.googleapis.com/kubernetes-release/release/v1.12.1/bin/linux/amd64/kubectl /usr/bin/kubectl
 RUN chmod 0755 /usr/bin/kubectl

cmd/kured/main.go

@@ -97,16 +97,23 @@ func newCommand(name string, arg ...string) *exec.Cmd {
 }
 
 func sentinelExists() bool {
-	_, err := os.Stat(rebootSentinel)
-	switch {
-	case err == nil:
-		return true
-	case os.IsNotExist(err):
-		return false
-	default:
-		log.Fatalf("Unable to determine existence of sentinel: %v", err)
-		return false // unreachable; prevents compilation error
+	// Relies on hostPID:true and privileged:true to enter host mount space
+	sentinelCmd := newCommand("/usr/bin/nsenter", "-m/proc/1/ns/mnt", "--", "/usr/bin/test", "-f", rebootSentinel)
+	if err := sentinelCmd.Run(); err != nil {
+		switch err := err.(type) {
+		case *exec.ExitError:
+			// We assume a non-zero exit code means 'reboot not required', but of course
+			// the user could have misconfigured the sentinel command or something else
+			// went wrong during its execution. In that case, not entering a reboot loop
+			// is the right thing to do, and we are logging stdout/stderr of the command
+			// so it should be obvious what is wrong.
+			return false
+		default:
+			// Something was grossly misconfigured, such as the command path being wrong.
+			log.Fatalf("Error invoking sentinel command: %v", err)
+		}
 	}
+	return true
 }
 
 func rebootRequired() bool {
@@ -205,8 +212,8 @@ func commandReboot(nodeID string) {
 		}
 	}
 
-	// Relies on /var/run/dbus/system_bus_socket bind mount to talk to systemd
-	rebootCmd := newCommand("/bin/systemctl", "reboot")
+	// Relies on hostPID:true and privileged:true to enter host mount space
+	rebootCmd := newCommand("/usr/bin/nsenter", "-m/proc/1/ns/mnt", "/bin/systemctl", "reboot")
 	if err := rebootCmd.Run(); err != nil {
 		log.Fatalf("Error invoking reboot command: %v", err)
 	}

kured-ds.yaml

@@ -25,13 +25,23 @@ spec:
       tolerations:
         - key: node-role.kubernetes.io/master
           effect: NoSchedule
+      hostPID: true # Facilitate entering the host mount namespace via init
+      restartPolicy: Always
       containers:
         - name: kured
           image: quay.io/weaveworks/kured
           imagePullPolicy: IfNotPresent
+          securityContext:
+            privileged: true # Give permission to nsenter /proc/1/ns/mnt
+          env:
+            # Pass in the name of the node on which this pod is scheduled
+            # for use with drain/uncordon operations and lock acquisition
+            - name: KURED_NODE_ID
+              valueFrom:
+                fieldRef:
+                  fieldPath: spec.nodeName
           command:
             - /usr/bin/kured
-#          args:
 #            - --alert-filter-regexp=^RebootRequired$
 #            - --ds-name=kured
 #            - --ds-namespace=kube-system
@@ -41,23 +51,3 @@ spec:
 #            - --reboot-sentinel=/var/run/reboot-required
 #            - --slack-hook-url=https://hooks.slack.com/...
 #            - --slack-username=prod
-#
-# NO USER SERVICEABLE PARTS BEYOND THIS POINT
-          env:
-            # Pass in the name of the node on which this pod is scheduled
-            # for use with drain/uncordon operations and lock acquisition
-            - name: KURED_NODE_ID
-              valueFrom:
-                fieldRef:
-                  fieldPath: spec.nodeName
-          volumeMounts:
-            # Needed for two purposes:
-            # * Testing for the existence of /var/run/reboot-required
-            # * Accessing /var/run/dbus/system_bus_socket to effect reboot
-            - name: hostrun
-              mountPath: /var/run
-      restartPolicy: Always
-      volumes:
-        - name: hostrun
-          hostPath:
-            path: /var/run