Merge pull request #62951 from dims/support-nsenter-better-in-non-systemd-envs

Automatic merge from submit-queue. 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>.

Support nsenter in non-systemd environments

**What this PR does / why we need it**:

In our CI, we run kubekins image for most of the jobs. This is a
debian image with upstart and does not enable systemd. So we should

* Bailout if any binary is missing other than systemd-run.
* SupportsSystemd should check the binary path to correctly
  identify if the systemd-run is present or not
* Pass the errors back to the callers so kubelet is forced to
  fail early when there is a problem. We currently assume
  that all binaries are in the root directory by default which
  is wrong.


**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```

cmd/kubelet/app/server.go

@@ -335,7 +335,10 @@ func UnsecuredDependencies(s *options.KubeletServer) (*kubelet.Dependencies, err
 	var writer kubeio.Writer = &kubeio.StdWriter{}
 	if s.Containerized {
 		glog.V(2).Info("Running kubelet in containerized mode")
-		mounter = mount.NewNsenterMounter()
+		mounter, err = mount.NewNsenterMounter()
+		if err != nil {
+			return nil, err
+		}
 		writer = &kubeio.NsenterWriter{}
 	}
 

pkg/util/io/writer.go

@@ -55,7 +55,10 @@ type NsenterWriter struct{}
 // WriteFile calls 'nsenter cat - > <the file>' and 'nsenter chmod' to create a
 // file on the host.
 func (writer *NsenterWriter) WriteFile(filename string, data []byte, perm os.FileMode) error {
-	ne := nsenter.NewNsenter()
+	ne, err := nsenter.NewNsenter()
+	if err != nil {
+		return err
+	}
 	echoArgs := []string{"-c", fmt.Sprintf("cat > %s", filename)}
 	glog.V(5).Infof("nsenter: write data to file %s by nsenter", filename)
 	command := ne.Exec("sh", echoArgs)

pkg/util/mount/nsenter_mount.go

@@ -52,8 +52,12 @@ type NsenterMounter struct {
 	ne *nsenter.Nsenter
 }
 
-func NewNsenterMounter() *NsenterMounter {
+func NewNsenterMounter() (*NsenterMounter, error) {
-	return &NsenterMounter{ne: nsenter.NewNsenter()}
+	ne, err := nsenter.NewNsenter()
+	if err != nil {
+		return nil, err
+	}
+	return &NsenterMounter{ne: ne}, nil
 }
 
 // NsenterMounter implements mount.Interface

pkg/util/mount/nsenter_mount_unsupported.go

@@ -25,8 +25,8 @@ import (
 
 type NsenterMounter struct{}
 
-func NewNsenterMounter() *NsenterMounter {
+func NewNsenterMounter() (*NsenterMounter, error) {
-	return &NsenterMounter{}
+	return &NsenterMounter{}, nil
 }
 
 var _ = Interface(&NsenterMounter{})

pkg/util/nsenter/nsenter.go

@@ -57,7 +57,8 @@ const (
 //     contents. TODO: remove this requirement.
 // 6.  The host image must have "mount", "findmnt", "umount", "stat", "touch",
 //     "mkdir", "ls", "sh" and "chmod" binaries in /bin, /usr/sbin, or /usr/bin
-// 7.  The host image should have systemd-run in /bin, /usr/sbin, or /usr/bin
+// 7.  The host image should have systemd-run in /bin, /usr/sbin, or /usr/bin if
+//     systemd is installed/enabled in the operating system.
 // For more information about mount propagation modes, see:
 //   https://www.kernel.org/doc/Documentation/filesystems/sharedsubtree.txt
 type Nsenter struct {
@@ -66,7 +67,7 @@ type Nsenter struct {
 }
 
 // NewNsenter constructs a new instance of Nsenter
-func NewNsenter() *Nsenter {
+func NewNsenter() (*Nsenter, error) {
 	ne := &Nsenter{
 		paths: map[string]string{
 			"mount":       "",
@@ -83,9 +84,8 @@ func NewNsenter() *Nsenter {
 	}
 	// search for the required commands in other locations besides /usr/bin
 	for binary := range ne.paths {
-		// default to root
+		// check for binary under the following directories
-		ne.paths[binary] = filepath.Join("/", binary)
+		for _, path := range []string{"/", "/bin", "/usr/sbin", "/usr/bin"} {
-		for _, path := range []string{"/bin", "/usr/sbin", "/usr/bin"} {
 			binPath := filepath.Join(path, binary)
 			if _, err := os.Stat(filepath.Join(hostRootFsPath, binPath)); err != nil {
 				continue
@@ -93,10 +93,12 @@ func NewNsenter() *Nsenter {
 			ne.paths[binary] = binPath
 			break
 		}
-		// TODO: error, so that the kubelet can stop if the paths don't exist
+		// systemd-run is optional, bailout if we don't find any of the other binaries
-		// (don't forget that systemd-run is optional)
+		if ne.paths[binary] == "" && binary != "systemd-run" {
+			return nil, fmt.Errorf("unable to find %v", binary)
+		}
 	}
-	return ne
+	return ne, nil
 }
 
 // Exec executes nsenter commands in hostProcMountNsPath mount namespace
@@ -119,6 +121,6 @@ func (ne *Nsenter) AbsHostPath(command string) string {
 
 // SupportsSystemd checks whether command systemd-run exists
 func (ne *Nsenter) SupportsSystemd() (string, bool) {
-	systemdRunPath, hasSystemd := ne.paths["systemd-run"]
+	systemdRunPath, ok := ne.paths["systemd-run"]
-	return systemdRunPath, hasSystemd
+	return systemdRunPath, ok && systemdRunPath != ""
 }

pkg/util/nsenter/nsenter_unsupported.go

@@ -30,8 +30,8 @@ type Nsenter struct {
 }
 
 // NewNsenter constructs a new instance of Nsenter
-func NewNsenter() *Nsenter {
+func NewNsenter() (*Nsenter, error) {
-	return &Nsenter{}
+	return &Nsenter{}, nil
 }
 
 // Exec executes nsenter commands in hostProcMountNsPath mount namespace