kubeadm: Extended KubeletVersionCheck

KubeletVersionCheck now able to detect if kubelet version
is higher than control plane. As this might lead to malfunctional
cluster setups, kubeadm will give warning.

Fixes: kubernetes/kubeadm#496

cmd/kubeadm/app/preflight/checks.go

@@ -498,7 +498,9 @@ func (kubever KubernetesVersionCheck) Check() (warnings, errors []error) {
 }
 
 // KubeletVersionCheck validates installed kubelet version
-type KubeletVersionCheck struct{}
+type KubeletVersionCheck struct {
+	KubernetesVersion string
+}
 
 // Check validates kubelet version. It should be not less than minimal supported version
 func (kubever KubeletVersionCheck) Check() (warnings, errors []error) {
@@ -509,7 +511,17 @@ func (kubever KubeletVersionCheck) Check() (warnings, errors []error) {
 	if kubeletVersion.LessThan(kubeadmconstants.MinimumKubeletVersion) {
 		return nil, []error{fmt.Errorf("Kubelet version %q is lower than kubadm can support. Please upgrade kubelet", kubeletVersion)}
 	}
-	return nil, []error{}
+
+	if kubever.KubernetesVersion != "" {
+		k8sVersion, err := versionutil.ParseSemantic(kubever.KubernetesVersion)
+		if err != nil {
+			return nil, []error{fmt.Errorf("couldn't parse kubernetes version %q: %v", kubever.KubernetesVersion, err)}
+		}
+		if kubeletVersion.Major() > k8sVersion.Major() || kubeletVersion.Minor() > k8sVersion.Minor() {
+			return nil, []error{fmt.Errorf("the kubelet version is higher than the control plane version. This is not a supported version skew and may lead to a malfunctional cluster. Kubelet version: %q Control plane version: %q", kubeletVersion, k8sVersion)}
+		}
+	}
+	return nil, nil
 }
 
 // SwapCheck warns if swap is enabled
@@ -688,7 +700,7 @@ func RunInitMasterChecks(cfg *kubeadmapi.MasterConfiguration) error {
 		SystemVerificationCheck{},
 		IsPrivilegedUserCheck{},
 		HostnameCheck{nodeName: cfg.NodeName},
-		KubeletVersionCheck{},
+		KubeletVersionCheck{KubernetesVersion: cfg.KubernetesVersion},
 		ServiceCheck{Service: "kubelet", CheckIfActive: false},
 		ServiceCheck{Service: "docker", CheckIfActive: true},
 		FirewalldCheck{ports: []int{int(cfg.API.BindPort), 10250}},

cmd/kubeadm/app/preflight/checks_test.go

@@ -20,6 +20,7 @@ import (
 	"bytes"
 	"fmt"
 	"io/ioutil"
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -556,3 +557,59 @@ func restoreEnv(e map[string]string) {
 		os.Setenv(k, v)
 	}
 }
+
+func TestKubeletVersionCheck(t *testing.T) {
+	type T struct {
+		kubeletVersion string
+		k8sVersion     string
+		expectErrors   bool
+		expectWarnings bool
+	}
+
+	cases := []T{
+		{"v1.10.2", "", false, false},              // check minimally supported version when there is no information about control plane
+		{"v1.7.3", "v1.7.8", true, false},          // too old kubelet (older than kubeadmconstants.MinimumKubeletVersion), should fail.
+		{"v1.9.0", "v1.9.5", false, false},         // kubelet within same major.minor as control plane
+		{"v1.9.5", "v1.9.1", false, false},         // kubelet is newer, but still within same major.minor as control plane
+		{"v1.9.0", "v1.10.1", false, false},        // kubelet is lower than control plane, but newer than minimally supported
+		{"v1.10.0-alpha.1", "v1.9.1", true, false}, // kubelet is newer (development build) than control plane, should fail.
+		{"v1.10.0", "v1.9.5", true, false},         // kubelet is newer (release) than control plane, should fail.
+	}
+
+	dir, err := ioutil.TempDir("", "test-kubelet-version-check")
+	if err != nil {
+		t.Errorf("Failed to create directory for testing GetKubeletVersion: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	// We don't want to call real kubelet or something else in $PATH
+	oldPATH := os.Getenv("PATH")
+	defer os.Setenv("PATH", oldPATH)
+
+	os.Setenv("PATH", dir)
+
+	kubeletFn := filepath.Join(dir, "kubelet")
+	for _, tc := range cases {
+
+		content := []byte(fmt.Sprintf("#!/bin/sh\necho 'Kubernetes %s'", tc.kubeletVersion))
+		if err := ioutil.WriteFile(kubeletFn, content, 0755); err != nil {
+			t.Errorf("Error creating test stub file %s: %v", kubeletFn, err)
+		}
+
+		check := KubeletVersionCheck{KubernetesVersion: tc.k8sVersion}
+		warnings, errors := check.Check()
+
+		switch {
+		case warnings != nil && !tc.expectWarnings:
+			t.Errorf("KubeletVersionCheck: unexpected warnings for kubelet version %q and kubernetes version %q. Warnings: %v", tc.kubeletVersion, tc.k8sVersion, warnings)
+		case warnings == nil && tc.expectWarnings:
+			t.Errorf("KubeletVersionCheck: expected warnings for kubelet version %q and kubernetes version %q but got nothing", tc.kubeletVersion, tc.k8sVersion)
+		case errors != nil && !tc.expectErrors:
+			t.Errorf("KubeletVersionCheck: unexpected errors for kubelet version %q and kubernetes version %q. errors: %v", tc.kubeletVersion, tc.k8sVersion, errors)
+		case errors == nil && tc.expectErrors:
+			t.Errorf("KubeletVersionCheck: expected errors for kubelet version %q and kubernetes version %q but got nothing", tc.kubeletVersion, tc.k8sVersion)
+		}
+
+	}
+
+}