Merge pull request #66702 from xlgao-zju/search-kubeconfig

Automatic merge from submit-queue (batch tested with PRs 62441, 66702, 67254, 67421, 65309). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

[kubeadm] Apply means to search for existing KubeConfig files in standard locations across kubeadm

**What this PR does / why we need it**:
Apply means to search for existing KubeConfig files in standard locations across kubeadm.

Tasklist:

- [x] Use the Join of Dir and File name instead of DefaultKubeConfig
- [x] Refactor token.go::findExistingKubeConfig() into a utility in cmd/kubeadm/app/cmd/util
- [x] Use the utility in config, upgrade and some of the phases commands that use --kubeconfig

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
ref [kubernetes/kubeadm/#805](https://github.com/kubernetes/kubeadm/issues/805)

**Special notes for your reviewer**:
/assign @neolit123 

**Release note**:

```release-note
NONE
```

cmd/kubeadm/app/cmd/BUILD

@@ -66,7 +66,6 @@ go_library(
         "//staging/src/k8s.io/client-go/kubernetes:go_default_library",
         "//staging/src/k8s.io/client-go/tools/bootstrap/token/api:go_default_library",
         "//staging/src/k8s.io/client-go/tools/bootstrap/token/util:go_default_library",
-        "//staging/src/k8s.io/client-go/tools/clientcmd:go_default_library",
         "//staging/src/k8s.io/client-go/tools/clientcmd/api:go_default_library",
         "//staging/src/k8s.io/client-go/util/cert:go_default_library",
         "//vendor/github.com/ghodss/yaml:go_default_library",

cmd/kubeadm/app/cmd/config.go

@@ -34,6 +34,7 @@ import (
 	kubeadmscheme "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/scheme"
 	kubeadmapiv1alpha2 "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1alpha2"
 	kubeadmapiv1alpha3 "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1alpha3"
+	"k8s.io/kubernetes/cmd/kubeadm/app/cmd/options"
 	phaseutil "k8s.io/kubernetes/cmd/kubeadm/app/cmd/phases"
 	cmdutil "k8s.io/kubernetes/cmd/kubeadm/app/cmd/util"
 	"k8s.io/kubernetes/cmd/kubeadm/app/componentconfigs"
@@ -60,8 +61,8 @@ var (
 
 // NewCmdConfig returns cobra.Command for "kubeadm config" command
 func NewCmdConfig(out io.Writer) *cobra.Command {
+	kubeConfigFile := constants.GetAdminKubeConfigPath()
 
-	var kubeConfigFile string
 	cmd := &cobra.Command{
 		Use:   "config",
 		Short: "Manage configuration for a kubeadm cluster persisted in a ConfigMap in the cluster.",
@@ -79,8 +80,9 @@ func NewCmdConfig(out io.Writer) *cobra.Command {
 		RunE: cmdutil.SubCmdRunE("config"),
 	}
 
-	cmd.PersistentFlags().StringVar(&kubeConfigFile, "kubeconfig", "/etc/kubernetes/admin.conf", "The KubeConfig file to use when talking to the cluster.")
+	options.AddKubeConfigFlag(cmd.PersistentFlags(), &kubeConfigFile)
 
+	kubeConfigFile = cmdutil.FindExistingKubeConfig(kubeConfigFile)
 	cmd.AddCommand(NewCmdConfigPrintDefault(out))
 	cmd.AddCommand(NewCmdConfigMigrate(out))
 	cmd.AddCommand(NewCmdConfigUpload(out, &kubeConfigFile))

cmd/kubeadm/app/cmd/options/generic.go

@@ -20,7 +20,7 @@ import "github.com/spf13/pflag"
 
 // AddKubeConfigFlag adds the --kubeconfig flag to the given flagset
 func AddKubeConfigFlag(fs *pflag.FlagSet, kubeConfigFile *string) {
-	fs.StringVar(kubeConfigFile, "kubeconfig", *kubeConfigFile, "The KubeConfig file to use when talking to the cluster")
+	fs.StringVar(kubeConfigFile, "kubeconfig", *kubeConfigFile, "The KubeConfig file to use when talking to the cluster. If the flag is not set, a set of standard locations are searched for an existing KubeConfig file.")
 }
 
 // AddConfigFlag adds the --config flag to the given flagset

cmd/kubeadm/app/cmd/phases/addons.go

@@ -27,7 +27,9 @@ import (
 	kubeadmscheme "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/scheme"
 	kubeadmapiv1alpha3 "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1alpha3"
 	"k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/validation"
+	"k8s.io/kubernetes/cmd/kubeadm/app/cmd/options"
 	cmdutil "k8s.io/kubernetes/cmd/kubeadm/app/cmd/util"
+	kubeadmconstants "k8s.io/kubernetes/cmd/kubeadm/app/constants"
 	"k8s.io/kubernetes/cmd/kubeadm/app/features"
 	dnsaddon "k8s.io/kubernetes/cmd/kubeadm/app/phases/addons/dns"
 	proxyaddon "k8s.io/kubernetes/cmd/kubeadm/app/phases/addons/proxy"
@@ -98,8 +100,9 @@ func getAddonsSubCommands() []*cobra.Command {
 	// Default values for the cobra help text
 	kubeadmscheme.Scheme.Default(cfg)
 
-	var cfgPath, kubeConfigFile, featureGatesString string
+	var cfgPath, featureGatesString string
 	var subCmds []*cobra.Command
+	kubeConfigFile := kubeadmconstants.GetAdminKubeConfigPath()
 
 	subCmdProperties := []struct {
 		use      string
@@ -136,11 +139,11 @@ func getAddonsSubCommands() []*cobra.Command {
 			Short:   properties.short,
 			Long:    properties.long,
 			Example: properties.examples,
-			Run:     runAddonsCmdFunc(properties.cmdFunc, cfg, &kubeConfigFile, &cfgPath, &featureGatesString),
+			Run:     runAddonsCmdFunc(properties.cmdFunc, cfg, kubeConfigFile, &cfgPath, &featureGatesString),
 		}
 
 		// Add flags to the command
-		cmd.Flags().StringVar(&kubeConfigFile, "kubeconfig", "/etc/kubernetes/admin.conf", "The KubeConfig file to use when talking to the cluster")
+		options.AddKubeConfigFlag(cmd.Flags(), &kubeConfigFile)
 		cmd.Flags().StringVar(&cfgPath, "config", cfgPath, "Path to a kubeadm config file. WARNING: Usage of a configuration file is experimental")
 		cmd.Flags().StringVar(&cfg.KubernetesVersion, "kubernetes-version", cfg.KubernetesVersion, `Choose a specific Kubernetes version for the control plane`)
 		cmd.Flags().StringVar(&cfg.ImageRepository, "image-repository", cfg.ImageRepository, `Choose a container registry to pull control plane images from`)
@@ -164,7 +167,7 @@ func getAddonsSubCommands() []*cobra.Command {
 }
 
 // runAddonsCmdFunc creates a cobra.Command Run function, by composing the call to the given cmdFunc with necessary additional steps (e.g preparation of input parameters)
-func runAddonsCmdFunc(cmdFunc func(cfg *kubeadmapi.InitConfiguration, client clientset.Interface) error, cfg *kubeadmapiv1alpha3.InitConfiguration, kubeConfigFile *string, cfgPath *string, featureGatesString *string) func(cmd *cobra.Command, args []string) {
+func runAddonsCmdFunc(cmdFunc func(cfg *kubeadmapi.InitConfiguration, client clientset.Interface) error, cfg *kubeadmapiv1alpha3.InitConfiguration, kubeConfigFile string, cfgPath *string, featureGatesString *string) func(cmd *cobra.Command, args []string) {
 
 	// the following statement build a clousure that wraps a call to a cmdFunc, binding
 	// the function itself with the specific parameters of each sub command.
@@ -183,7 +186,8 @@ func runAddonsCmdFunc(cmdFunc func(cfg *kubeadmapi.InitConfiguration, client cli
 
 		internalcfg := &kubeadmapi.InitConfiguration{}
 		kubeadmscheme.Scheme.Convert(cfg, internalcfg, nil)
-		client, err := kubeconfigutil.ClientSetFromFile(*kubeConfigFile)
+		kubeConfigFile = cmdutil.FindExistingKubeConfig(kubeConfigFile)
+		client, err := kubeconfigutil.ClientSetFromFile(kubeConfigFile)
 		kubeadmutil.CheckErr(err)
 		internalcfg, err = configutil.ConfigFileAndDefaultsToInternalConfig(*cfgPath, cfg)
 		kubeadmutil.CheckErr(err)

cmd/kubeadm/app/cmd/phases/bootstraptoken.go

@@ -31,6 +31,7 @@ import (
 	"k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/validation"
 	"k8s.io/kubernetes/cmd/kubeadm/app/cmd/options"
 	cmdutil "k8s.io/kubernetes/cmd/kubeadm/app/cmd/util"
+	kubeadmconstants "k8s.io/kubernetes/cmd/kubeadm/app/constants"
 	"k8s.io/kubernetes/cmd/kubeadm/app/phases/bootstraptoken/clusterinfo"
 	"k8s.io/kubernetes/cmd/kubeadm/app/phases/bootstraptoken/node"
 	kubeadmutil "k8s.io/kubernetes/cmd/kubeadm/app/util"
@@ -86,7 +87,8 @@ var (
 
 // NewCmdBootstrapToken returns the Cobra command for running the mark-master phase
 func NewCmdBootstrapToken() *cobra.Command {
-	var kubeConfigFile string
+	kubeConfigFile := kubeadmconstants.GetAdminKubeConfigPath()
+
 	cmd := &cobra.Command{
 		Use:     "bootstrap-token",
 		Short:   "Manage kubeadm-specific bootstrap token functions",
@@ -94,9 +96,10 @@ func NewCmdBootstrapToken() *cobra.Command {
 		Aliases: []string{"bootstraptoken"},
 	}
 
-	cmd.PersistentFlags().StringVar(&kubeConfigFile, "kubeconfig", "/etc/kubernetes/admin.conf", "The KubeConfig file to use when talking to the cluster")
+	options.AddKubeConfigFlag(cmd.PersistentFlags(), &kubeConfigFile)
 
 	// Add subcommands
+	kubeConfigFile = cmdutil.FindExistingKubeConfig(kubeConfigFile)
 	cmd.AddCommand(NewSubCmdBootstrapTokenAll(&kubeConfigFile))
 	cmd.AddCommand(NewSubCmdBootstrapToken(&kubeConfigFile))
 	cmd.AddCommand(NewSubCmdClusterInfo(&kubeConfigFile))

cmd/kubeadm/app/cmd/phases/kubelet.go

@@ -208,6 +208,7 @@ func NewCmdKubeletConfigUpload() *cobra.Command {
 			internalcfg, err := configutil.ConfigFileAndDefaultsToInternalConfig(cfgPath, cfg)
 			kubeadmutil.CheckErr(err)
 
+			kubeConfigFile = cmdutil.FindExistingKubeConfig(kubeConfigFile)
 			client, err := kubeconfigutil.ClientSetFromFile(kubeConfigFile)
 			kubeadmutil.CheckErr(err)
 
@@ -310,6 +311,7 @@ func NewCmdKubeletConfigEnableDynamic() *cobra.Command {
 			kubeletVersion, err := version.ParseSemantic(kubeletVersionStr)
 			kubeadmutil.CheckErr(err)
 
+			kubeConfigFile = cmdutil.FindExistingKubeConfig(kubeConfigFile)
 			client, err := kubeconfigutil.ClientSetFromFile(kubeConfigFile)
 			kubeadmutil.CheckErr(err)
 

cmd/kubeadm/app/cmd/phases/markmaster.go

@@ -22,7 +22,9 @@ import (
 	kubeadmscheme "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/scheme"
 	kubeadmapiv1alpha3 "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1alpha3"
 	"k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/validation"
+	"k8s.io/kubernetes/cmd/kubeadm/app/cmd/options"
 	cmdutil "k8s.io/kubernetes/cmd/kubeadm/app/cmd/util"
+	kubeadmconstants "k8s.io/kubernetes/cmd/kubeadm/app/constants"
 	markmasterphase "k8s.io/kubernetes/cmd/kubeadm/app/phases/markmaster"
 	kubeadmutil "k8s.io/kubernetes/cmd/kubeadm/app/util"
 	configutil "k8s.io/kubernetes/cmd/kubeadm/app/util/config"
@@ -51,7 +53,9 @@ func NewCmdMarkMaster() *cobra.Command {
 	// Default values for the cobra help text
 	kubeadmscheme.Scheme.Default(cfg)
 
-	var cfgPath, kubeConfigFile string
+	var cfgPath string
+	kubeConfigFile := kubeadmconstants.GetAdminKubeConfigPath()
+
 	cmd := &cobra.Command{
 		Use:     "mark-master",
 		Short:   "Mark a node as master",
@@ -63,6 +67,7 @@ func NewCmdMarkMaster() *cobra.Command {
 				kubeadmutil.CheckErr(err)
 			}
 
+			kubeConfigFile = cmdutil.FindExistingKubeConfig(kubeConfigFile)
 			client, err := kubeconfigutil.ClientSetFromFile(kubeConfigFile)
 			kubeadmutil.CheckErr(err)
 
@@ -80,7 +85,7 @@ func NewCmdMarkMaster() *cobra.Command {
 		},
 	}
 
-	cmd.Flags().StringVar(&kubeConfigFile, "kubeconfig", "/etc/kubernetes/admin.conf", "The KubeConfig file to use when talking to the cluster")
+	options.AddKubeConfigFlag(cmd.Flags(), &kubeConfigFile)
 	cmd.Flags().StringVar(&cfgPath, "config", cfgPath, "Path to kubeadm config file. WARNING: Usage of a configuration file is experimental")
 	cmd.Flags().StringVar(&cfg.NodeRegistration.Name, "node-name", cfg.NodeRegistration.Name, `The node name to which label and taints should apply`)
 

cmd/kubeadm/app/cmd/phases/selfhosting.go

@@ -26,6 +26,7 @@ import (
 	kubeadmscheme "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/scheme"
 	kubeadmapiv1alpha3 "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1alpha3"
 	"k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/validation"
+	"k8s.io/kubernetes/cmd/kubeadm/app/cmd/options"
 	cmdutil "k8s.io/kubernetes/cmd/kubeadm/app/cmd/util"
 	"k8s.io/kubernetes/cmd/kubeadm/app/constants"
 	"k8s.io/kubernetes/cmd/kubeadm/app/features"
@@ -74,7 +75,8 @@ func getSelfhostingSubCommand() *cobra.Command {
 	// Default values for the cobra help text
 	kubeadmscheme.Scheme.Default(cfg)
 
-	var cfgPath, kubeConfigFile, featureGatesString string
+	var cfgPath, featureGatesString string
+	kubeConfigFile := constants.GetAdminKubeConfigPath()
 
 	// Creates the UX Command
 	cmd := &cobra.Command{
@@ -94,6 +96,7 @@ func getSelfhostingSubCommand() *cobra.Command {
 			}
 
 			// Gets the kubernetes client
+			kubeConfigFile = cmdutil.FindExistingKubeConfig(kubeConfigFile)
 			client, err := kubeconfigutil.ClientSetFromFile(kubeConfigFile)
 			kubeadmutil.CheckErr(err)
 
@@ -122,7 +125,7 @@ func getSelfhostingSubCommand() *cobra.Command {
 
 	// flags that are not bound to the configuration object
 	// Note: All flags that are not bound to the cfg object should be whitelisted in cmd/kubeadm/app/apis/kubeadm/validation/validation.go
-	cmd.Flags().StringVar(&kubeConfigFile, "kubeconfig", "/etc/kubernetes/admin.conf", "The KubeConfig file to use when talking to the cluster")
+	options.AddKubeConfigFlag(cmd.Flags(), &kubeConfigFile)
 
 	return cmd
 }

cmd/kubeadm/app/cmd/phases/uploadconfig.go

@@ -23,6 +23,7 @@ import (
 
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	kubeadmapiv1alpha3 "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1alpha3"
+	"k8s.io/kubernetes/cmd/kubeadm/app/cmd/options"
 	cmdutil "k8s.io/kubernetes/cmd/kubeadm/app/cmd/util"
 	kubeadmconstants "k8s.io/kubernetes/cmd/kubeadm/app/constants"
 	"k8s.io/kubernetes/cmd/kubeadm/app/phases/uploadconfig"
@@ -49,7 +50,8 @@ var (
 // NewCmdUploadConfig returns the Cobra command for running the uploadconfig phase
 func NewCmdUploadConfig() *cobra.Command {
 	cfg := &kubeadmapiv1alpha3.InitConfiguration{}
-	var cfgPath, kubeConfigFile string
+	kubeConfigFile := kubeadmconstants.GetAdminKubeConfigPath()
+	var cfgPath string
 
 	cmd := &cobra.Command{
 		Use:     "upload-config",
@@ -61,6 +63,8 @@ func NewCmdUploadConfig() *cobra.Command {
 			if len(cfgPath) == 0 {
 				kubeadmutil.CheckErr(fmt.Errorf("the --config flag is mandatory"))
 			}
+
+			kubeConfigFile = cmdutil.FindExistingKubeConfig(kubeConfigFile)
 			client, err := kubeconfigutil.ClientSetFromFile(kubeConfigFile)
 			kubeadmutil.CheckErr(err)
 
@@ -77,7 +81,7 @@ func NewCmdUploadConfig() *cobra.Command {
 		},
 	}
 
-	cmd.Flags().StringVar(&kubeConfigFile, "kubeconfig", "/etc/kubernetes/admin.conf", "The KubeConfig file to use when talking to the cluster")
+	options.AddKubeConfigFlag(cmd.Flags(), &kubeConfigFile)
 	cmd.Flags().StringVar(&cfgPath, "config", "", "Path to a kubeadm config file. WARNING: Usage of a configuration file is experimental")
 
 	return cmd

cmd/kubeadm/app/cmd/token.go

@@ -34,7 +34,6 @@ import (
 	clientset "k8s.io/client-go/kubernetes"
 	bootstrapapi "k8s.io/client-go/tools/bootstrap/token/api"
 	bootstraputil "k8s.io/client-go/tools/bootstrap/token/util"
-	"k8s.io/client-go/tools/clientcmd"
 	kubeadmapi "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm"
 	kubeadmscheme "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/scheme"
 	kubeadmapiv1alpha3 "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1alpha3"
@@ -42,6 +41,7 @@ import (
 	"k8s.io/kubernetes/cmd/kubeadm/app/cmd/options"
 	phaseutil "k8s.io/kubernetes/cmd/kubeadm/app/cmd/phases"
 	cmdutil "k8s.io/kubernetes/cmd/kubeadm/app/cmd/util"
+	kubeadmconstants "k8s.io/kubernetes/cmd/kubeadm/app/constants"
 	tokenphase "k8s.io/kubernetes/cmd/kubeadm/app/phases/bootstraptoken/node"
 	kubeadmutil "k8s.io/kubernetes/cmd/kubeadm/app/util"
 	"k8s.io/kubernetes/cmd/kubeadm/app/util/apiclient"
@@ -49,11 +49,9 @@ import (
 	kubeconfigutil "k8s.io/kubernetes/cmd/kubeadm/app/util/kubeconfig"
 )
 
-const defaultKubeConfig = "/etc/kubernetes/admin.conf"
-
 // NewCmdToken returns cobra.Command for token management
 func NewCmdToken(out io.Writer, errW io.Writer) *cobra.Command {
-	var kubeConfigFile string
+	kubeConfigFile := kubeadmconstants.GetAdminKubeConfigPath()
 	var dryRun bool
 	tokenCmd := &cobra.Command{
 		Use:   "token",
@@ -85,8 +83,7 @@ func NewCmdToken(out io.Writer, errW io.Writer) *cobra.Command {
 		RunE: cmdutil.SubCmdRunE("token"),
 	}
 
-	tokenCmd.PersistentFlags().StringVar(&kubeConfigFile,
+	options.AddKubeConfigFlag(tokenCmd.PersistentFlags(), &kubeConfigFile)
-		"kubeconfig", defaultKubeConfig, "The KubeConfig file to use when talking to the cluster. If the flag is not set a set of standard locations are searched for an existing KubeConfig file")
 	tokenCmd.PersistentFlags().BoolVar(&dryRun,
 		"dry-run", dryRun, "Whether to enable dry-run mode or not")
 
@@ -123,7 +120,7 @@ func NewCmdToken(out io.Writer, errW io.Writer) *cobra.Command {
 			kubeadmutil.CheckErr(err)
 
 			glog.V(1).Infoln("[token] getting Clientsets from KubeConfig file")
-			kubeConfigFile = findExistingKubeConfig(kubeConfigFile)
+			kubeConfigFile = cmdutil.FindExistingKubeConfig(kubeConfigFile)
 			client, err := getClientset(kubeConfigFile, dryRun)
 			kubeadmutil.CheckErr(err)
 
@@ -150,7 +147,7 @@ func NewCmdToken(out io.Writer, errW io.Writer) *cobra.Command {
 			This command will list all bootstrap tokens for you.
 		`),
 		Run: func(tokenCmd *cobra.Command, args []string) {
-			kubeConfigFile = findExistingKubeConfig(kubeConfigFile)
+			kubeConfigFile = cmdutil.FindExistingKubeConfig(kubeConfigFile)
 			client, err := getClientset(kubeConfigFile, dryRun)
 			kubeadmutil.CheckErr(err)
 
@@ -174,7 +171,7 @@ func NewCmdToken(out io.Writer, errW io.Writer) *cobra.Command {
 			if len(args) < 1 {
 				kubeadmutil.CheckErr(fmt.Errorf("missing subcommand; 'token delete' is missing token of form %q", bootstrapapi.BootstrapTokenIDPattern))
 			}
-			kubeConfigFile = findExistingKubeConfig(kubeConfigFile)
+			kubeConfigFile = cmdutil.FindExistingKubeConfig(kubeConfigFile)
 			client, err := getClientset(kubeConfigFile, dryRun)
 			kubeadmutil.CheckErr(err)
 
@@ -357,16 +354,3 @@ func getClientset(file string, dryRun bool) (clientset.Interface, error) {
 	}
 	return kubeconfigutil.ClientSetFromFile(file)
 }
-
-func findExistingKubeConfig(file string) string {
-	// The user did provide a --kubeconfig flag. Respect that and threat it as an
-	// explicit path without building a DefaultClientConfigLoadingRules object.
-	if file != defaultKubeConfig {
-		return file
-	}
-	// The user did not provide a --kubeconfig flag. Find a config in the standard
-	// locations using DefaultClientConfigLoadingRules, but also consider `defaultKubeConfig`.
-	rules := clientcmd.NewDefaultClientConfigLoadingRules()
-	rules.Precedence = append(rules.Precedence, defaultKubeConfig)
-	return rules.GetDefaultFilename()
-}

cmd/kubeadm/app/cmd/upgrade/upgrade.go

@@ -26,6 +26,7 @@ import (
 	"k8s.io/apimachinery/pkg/util/sets"
 	"k8s.io/kubernetes/cmd/kubeadm/app/cmd/options"
 	cmdutil "k8s.io/kubernetes/cmd/kubeadm/app/cmd/util"
+	kubeadmconstants "k8s.io/kubernetes/cmd/kubeadm/app/constants"
 	"k8s.io/kubernetes/cmd/kubeadm/app/features"
 )
 
@@ -45,7 +46,7 @@ type applyPlanFlags struct {
 // NewCmdUpgrade returns the cobra command for `kubeadm upgrade`
 func NewCmdUpgrade(out io.Writer) *cobra.Command {
 	flags := &applyPlanFlags{
-		kubeConfigPath:            "/etc/kubernetes/admin.conf",
+		kubeConfigPath:            kubeadmconstants.GetAdminKubeConfigPath(),
 		cfgPath:                   "",
 		featureGatesString:        "",
 		allowExperimentalUpgrades: false,
@@ -61,6 +62,7 @@ func NewCmdUpgrade(out io.Writer) *cobra.Command {
 		RunE:  cmdutil.SubCmdRunE("upgrade"),
 	}
 
+	flags.kubeConfigPath = cmdutil.FindExistingKubeConfig(flags.kubeConfigPath)
 	cmd.AddCommand(NewCmdApply(flags))
 	cmd.AddCommand(NewCmdPlan(flags))
 	cmd.AddCommand(NewCmdDiff(out))

cmd/kubeadm/app/cmd/util/BUILD

@@ -10,6 +10,7 @@ go_library(
     importpath = "k8s.io/kubernetes/cmd/kubeadm/app/cmd/util",
     visibility = ["//visibility:public"],
     deps = [
+        "//cmd/kubeadm/app/constants:go_default_library",
         "//cmd/kubeadm/app/util/kubeconfig:go_default_library",
         "//cmd/kubeadm/app/util/pubkeypin:go_default_library",
         "//pkg/util/normalizer:go_default_library",

cmd/kubeadm/app/cmd/util/cmdutil.go

@@ -20,6 +20,9 @@ import (
 	"fmt"
 
 	"github.com/spf13/cobra"
+
+	"k8s.io/client-go/tools/clientcmd"
+	kubeadmconstants "k8s.io/kubernetes/cmd/kubeadm/app/constants"
 )
 
 // SubCmdRunE returns a function that handles a case where a subcommand must be specified
@@ -57,3 +60,17 @@ func ValidateExactArgNumber(args []string, supportedArgs []string) error {
 	}
 	return nil
 }
+
+// FindExistingKubeConfig returns the localtion of kubeconfig
+func FindExistingKubeConfig(file string) string {
+	// The user did provide a --kubeconfig flag. Respect that and threat it as an
+	// explicit path without building a DefaultClientConfigLoadingRules object.
+	if file != kubeadmconstants.GetAdminKubeConfigPath() {
+		return file
+	}
+	// The user did not provide a --kubeconfig flag. Find a config in the standard
+	// locations using DefaultClientConfigLoadingRules, but also consider the default config path.
+	rules := clientcmd.NewDefaultClientConfigLoadingRules()
+	rules.Precedence = append(rules.Precedence, kubeadmconstants.GetAdminKubeConfigPath())
+	return rules.GetDefaultFilename()
+}

cmd/kubeadm/app/constants/constants_test.go

@@ -18,6 +18,7 @@ package constants
 
 import (
 	"fmt"
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -38,7 +39,7 @@ func TestGetStaticPodDirectory(t *testing.T) {
 }
 
 func TestGetAdminKubeConfigPath(t *testing.T) {
-	expected := "/etc/kubernetes/admin.conf"
+	expected := filepath.Join(KubernetesDir, AdminKubeConfigFileName)
 	actual := GetAdminKubeConfigPath()
 
 	if actual != expected {