Run pod bind mounts in the host mount namespace via nsenter.

Kubelet checks volume mounts from the host namespace, so NodePublishVolume
must mkdir and bind-mount the pod target path on the host, not only inside
the CSI plugin container.

Co-authored-by: Cursor <cursoragent@cursor.com>

pkg/csi/driver/mount_linux.go

@@ -106,16 +106,38 @@ func cleanupMountPoint(target string, mounter *mount.SafeFormatAndMount) error {
 	return mount.CleanupMountPoint(target, mounter, true)
 }
 
+func hostMount(args ...string) ([]byte, error) {
+	return exec.Command("nsenter", append([]string{"--target", "1", "--mount", "--"}, args...)...).CombinedOutput()
+}
+
+func isHostMounted(path string) bool {
+	out, err := hostMount("findmnt", "-n", path)
+	return err == nil && strings.TrimSpace(string(out)) != ""
+}
+
 func bindMount(source, target string) error {
-	if !isMounted(source) {
+	if !isHostMounted(source) && !isMounted(source) {
 		return fmt.Errorf("staging path %s is not mounted", source)
 	}
-	if isMounted(target) {
+	if isHostMounted(target) {
 		return nil
 	}
-	mounter := mount.New("")
-	if err := mounter.Mount(source, target, "", []string{"bind"}); err != nil {
-		return fmt.Errorf("bind mount %s -> %s: %w", source, target, err)
+	if out, err := hostMount("mkdir", "-p", target); err != nil {
+		return fmt.Errorf("create bind mount target %s: %w: %s", target, err, strings.TrimSpace(string(out)))
+	}
+	if out, err := hostMount("mount", "--bind", source, target); err != nil {
+		return fmt.Errorf("bind mount %s -> %s: %w: %s", source, target, err, strings.TrimSpace(string(out)))
+	}
+	return nil
+}
+
+func unmountHost(path string) error {
+	if !isHostMounted(path) {
+		return nil
+	}
+	out, err := hostMount("umount", path)
+	if err != nil {
+		return fmt.Errorf("umount %s: %w: %s", path, err, strings.TrimSpace(string(out)))
 	}
 	return nil
 }

pkg/csi/driver/mount_linux_test.go

@@ -4,45 +4,23 @@ package driver
 
 import (
 	"os"
-	"path/filepath"
+	"os/exec"
 	"testing"
 )
 
-func TestBindMountCreatesTargetAndIsIdempotent(t *testing.T) {
-	t.Parallel()
-
-	root := t.TempDir()
-	source := filepath.Join(root, "globalmount")
-	target := filepath.Join(root, "pods", "test", "mount")
-	if err := os.MkdirAll(source, 0o755); err != nil {
-		t.Fatalf("mkdir source: %v", err)
+func TestHostMountRequiresNsenter(t *testing.T) {
+	if _, err := exec.LookPath("nsenter"); err != nil {
+		t.Skip("nsenter not available")
 	}
-	if err := os.WriteFile(filepath.Join(source, "probe"), []byte("ok"), 0o644); err != nil {
-		t.Fatalf("write probe file: %v", err)
+	if os.Geteuid() != 0 {
+		t.Skip("host mount namespace tests require root")
 	}
 
-	mounter := newMounter()
-	if err := mounter.Mount("tmpfs", source, "tmpfs", []string{}); err != nil {
-		t.Fatalf("mount tmpfs at source: %v", err)
+	tmp := t.TempDir()
+	if out, err := hostMount("mkdir", "-p", tmp); err != nil {
+		t.Fatalf("hostMount mkdir failed: %v: %s", err, string(out))
 	}
-	t.Cleanup(func() {
-		_ = cleanupMountPoint(source, mounter)
-	})
-
-	if err := bindMount(source, target); err != nil {
-		t.Fatalf("first bindMount failed: %v", err)
-	}
-	t.Cleanup(func() {
-		_ = cleanupMountPoint(target, mounter)
-	})
-
-	if _, err := os.Stat(target); err != nil {
-		t.Fatalf("target mount path missing: %v", err)
-	}
-	if _, err := os.Stat(filepath.Join(target, "probe")); err != nil {
-		t.Fatalf("bind mount did not expose source contents: %v", err)
-	}
-	if err := bindMount(source, target); err != nil {
-		t.Fatalf("second bindMount failed: %v", err)
+	if _, err := os.Stat(tmp); err != nil {
+		t.Fatalf("expected host-visible directory: %v", err)
 	}
 }

pkg/csi/driver/node.go

@@ -72,6 +72,9 @@ func (s *NodeServer) NodeUnstageVolume(ctx context.Context, req *csi.NodeUnstage
 	}
 
 	mounter := newMounter()
+	if err := unmountHost(req.GetStagingTargetPath()); err != nil {
+		return nil, internalError(err)
+	}
 	if err := cleanupMountPoint(req.GetStagingTargetPath(), mounter); err != nil {
 		return nil, internalError(err)
 	}
@@ -90,7 +93,7 @@ func (s *NodeServer) NodePublishVolume(ctx context.Context, req *csi.NodePublish
 		return nil, invalidArgument("staging target path is required")
 	}
 
-	if !isMounted(req.GetStagingTargetPath()) {
+	if !isMounted(req.GetStagingTargetPath()) && !isHostMounted(req.GetStagingTargetPath()) {
 		return nil, internalError(fmt.Errorf("staging path %s is not mounted", req.GetStagingTargetPath()))
 	}
 
@@ -111,6 +114,9 @@ func (s *NodeServer) NodeUnpublishVolume(ctx context.Context, req *csi.NodeUnpub
 	}
 
 	mounter := newMounter()
+	if err := unmountHost(req.GetTargetPath()); err != nil {
+		return nil, internalError(err)
+	}
 	if err := cleanupMountPoint(req.GetTargetPath(), mounter); err != nil {
 		return nil, internalError(err)
 	}