containerd/pkg/server/container_create.go

/*
Copyright 2017 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package server

import (
	"path/filepath"
	"strconv"
	"strings"
	"time"

	"github.com/containerd/containerd"
	"github.com/containerd/containerd/containers"
	"github.com/containerd/containerd/contrib/apparmor"
	"github.com/containerd/containerd/contrib/seccomp"
	"github.com/containerd/containerd/log"
	"github.com/containerd/containerd/oci"
	"github.com/containerd/cri/pkg/annotations"
	"github.com/containerd/cri/pkg/config"
	customopts "github.com/containerd/cri/pkg/containerd/opts"
	ctrdutil "github.com/containerd/cri/pkg/containerd/util"
	cio "github.com/containerd/cri/pkg/server/io"
	containerstore "github.com/containerd/cri/pkg/store/container"
	"github.com/containerd/cri/pkg/util"
	"github.com/containerd/typeurl"
	"github.com/davecgh/go-spew/spew"
	imagespec "github.com/opencontainers/image-spec/specs-go/v1"
	runtimespec "github.com/opencontainers/runtime-spec/specs-go"
	"github.com/pkg/errors"
	"golang.org/x/net/context"
	runtime "k8s.io/cri-api/pkg/apis/runtime/v1alpha2"
)

const (
	// profileNamePrefix is the prefix for loading profiles on a localhost. Eg. AppArmor localhost/profileName.
	profileNamePrefix = "localhost/" // TODO (mikebrow): get localhost/ & runtime/default from CRI kubernetes/kubernetes#51747
	// runtimeDefault indicates that we should use or create a runtime default profile.
	runtimeDefault = "runtime/default"
	// dockerDefault indicates that we should use or create a docker default profile.
	dockerDefault = "docker/default"
	// appArmorDefaultProfileName is name to use when creating a default apparmor profile.
	appArmorDefaultProfileName = "cri-containerd.apparmor.d"
	// unconfinedProfile is a string indicating one should run a pod/containerd without a security profile
	unconfinedProfile = "unconfined"
	// seccompDefaultProfile is the default seccomp profile.
	seccompDefaultProfile = dockerDefault
)

func init() {
	typeurl.Register(&containerstore.Metadata{},
		"github.com/containerd/cri/pkg/store/container", "Metadata")
}

// CreateContainer creates a new container in the given PodSandbox.
func (c *criService) CreateContainer(ctx context.Context, r *runtime.CreateContainerRequest) (_ *runtime.CreateContainerResponse, retErr error) {
	config := r.GetConfig()
	log.G(ctx).Debugf("Container config %+v", config)
	sandboxConfig := r.GetSandboxConfig()
	sandbox, err := c.sandboxStore.Get(r.GetPodSandboxId())
	if err != nil {
		return nil, errors.Wrapf(err, "failed to find sandbox id %q", r.GetPodSandboxId())
	}
	sandboxID := sandbox.ID
	s, err := sandbox.Container.Task(ctx, nil)
	if err != nil {
		return nil, errors.Wrap(err, "failed to get sandbox container task")
	}
	sandboxPid := s.Pid()

	// Generate unique id and name for the container and reserve the name.
	// Reserve the container name to avoid concurrent `CreateContainer` request creating
	// the same container.
	id := util.GenerateID()
	metadata := config.GetMetadata()
	if metadata == nil {
		return nil, errors.New("container config must include metadata")
	}
	name := makeContainerName(metadata, sandboxConfig.GetMetadata())
	log.G(ctx).Debugf("Generated id %q for container %q", id, name)
	if err = c.containerNameIndex.Reserve(name, id); err != nil {
		return nil, errors.Wrapf(err, "failed to reserve container name %q", name)
	}
	defer func() {
		// Release the name if the function returns with an error.
		if retErr != nil {
			c.containerNameIndex.ReleaseByName(name)
		}
	}()

	// Create initial internal container metadata.
	meta := containerstore.Metadata{
		ID:        id,
		Name:      name,
		SandboxID: sandboxID,
		Config:    config,
	}

	// Prepare container image snapshot. For container, the image should have
	// been pulled before creating the container, so do not ensure the image.
	image, err := c.localResolve(config.GetImage().GetImage())
	if err != nil {
		return nil, errors.Wrapf(err, "failed to resolve image %q", config.GetImage().GetImage())
	}
	containerdImage, err := c.toContainerdImage(ctx, image)
	if err != nil {
		return nil, errors.Wrapf(err, "failed to get image from containerd %q", image.ID)
	}

	// Run container using the same runtime with sandbox.
	sandboxInfo, err := sandbox.Container.Info(ctx)
	if err != nil {
		return nil, errors.Wrapf(err, "failed to get sandbox %q info", sandboxID)
	}

	// Create container root directory.
	containerRootDir := c.getContainerRootDir(id)
	if err = c.os.MkdirAll(containerRootDir, 0755); err != nil {
		return nil, errors.Wrapf(err, "failed to create container root directory %q",
			containerRootDir)
	}
	defer func() {
		if retErr != nil {
			// Cleanup the container root directory.
			if err = c.os.RemoveAll(containerRootDir); err != nil {
				log.G(ctx).WithError(err).Errorf("Failed to remove container root directory %q",
					containerRootDir)
			}
		}
	}()
	volatileContainerRootDir := c.getVolatileContainerRootDir(id)
	if err = c.os.MkdirAll(volatileContainerRootDir, 0755); err != nil {
		return nil, errors.Wrapf(err, "failed to create volatile container root directory %q",
			volatileContainerRootDir)
	}
	defer func() {
		if retErr != nil {
			// Cleanup the volatile container root directory.
			if err = c.os.RemoveAll(volatileContainerRootDir); err != nil {
				log.G(ctx).WithError(err).Errorf("Failed to remove volatile container root directory %q",
					volatileContainerRootDir)
			}
		}
	}()

	// Create container volumes mounts.
	volumeMounts := c.generateVolumeMounts(containerRootDir, config.GetMounts(), &image.ImageSpec.Config)

	// Generate container runtime spec.
	mounts := c.generateContainerMounts(sandboxID, config)

	ociRuntime, err := c.getSandboxRuntime(sandboxConfig, sandbox.Metadata.RuntimeHandler)
	if err != nil {
		return nil, errors.Wrap(err, "failed to get sandbox runtime")
	}
	log.G(ctx).Debugf("Use OCI runtime %+v for sandbox %q and container %q", ociRuntime, sandboxID, id)

	spec, err := c.generateContainerSpec(id, sandboxID, sandboxPid, config, sandboxConfig,
		&image.ImageSpec.Config, append(mounts, volumeMounts...), ociRuntime)
	if err != nil {
		return nil, errors.Wrapf(err, "failed to generate container %q spec", id)
	}

	log.G(ctx).Debugf("Container %q spec: %#+v", id, spew.NewFormatter(spec))

	// Set snapshotter before any other options.
	opts := []containerd.NewContainerOpts{
		containerd.WithSnapshotter(c.config.ContainerdConfig.Snapshotter),
		// Prepare container rootfs. This is always writeable even if
		// the container wants a readonly rootfs since we want to give
		// the runtime (runc) a chance to modify (e.g. to create mount
		// points corresponding to spec.Mounts) before making the
		// rootfs readonly (requested by spec.Root.Readonly).
		customopts.WithNewSnapshot(id, containerdImage),
	}

	if len(volumeMounts) > 0 {
		mountMap := make(map[string]string)
		for _, v := range volumeMounts {
			mountMap[filepath.Clean(v.HostPath)] = v.ContainerPath
		}
		opts = append(opts, customopts.WithVolumes(mountMap))
	}
	meta.ImageRef = image.ID
	meta.StopSignal = image.ImageSpec.Config.StopSignal

	// Validate log paths and compose full container log path.
	if sandboxConfig.GetLogDirectory() != "" && config.GetLogPath() != "" {
		meta.LogPath = filepath.Join(sandboxConfig.GetLogDirectory(), config.GetLogPath())
		log.G(ctx).Debugf("Composed container full log path %q using sandbox log dir %q and container log path %q",
			meta.LogPath, sandboxConfig.GetLogDirectory(), config.GetLogPath())
	} else {
		log.G(ctx).Infof("Logging will be disabled due to empty log paths for sandbox (%q) or container (%q)",
			sandboxConfig.GetLogDirectory(), config.GetLogPath())
	}

	containerIO, err := cio.NewContainerIO(id,
		cio.WithNewFIFOs(volatileContainerRootDir, config.GetTty(), config.GetStdin()))
	if err != nil {
		return nil, errors.Wrap(err, "failed to create container io")
	}
	defer func() {
		if retErr != nil {
			if err := containerIO.Close(); err != nil {
				log.G(ctx).WithError(err).Errorf("Failed to close container io %q", id)
			}
		}
	}()

	var specOpts []oci.SpecOpts
	securityContext := config.GetLinux().GetSecurityContext()
	// Set container username. This could only be done by containerd, because it needs
	// access to the container rootfs. Pass user name to containerd, and let it overwrite
	// the spec for us.
	userstr, err := generateUserString(
		securityContext.GetRunAsUsername(),
		securityContext.GetRunAsUser(),
		securityContext.GetRunAsGroup())

	if err != nil {
		return nil, errors.Wrap(err, "failed to generate user string")
	}
	if userstr == "" {
		// Lastly, since no user override was passed via CRI try to set via OCI
		// Image
		userstr = image.ImageSpec.Config.User
	}
	if userstr != "" {
		specOpts = append(specOpts, oci.WithUser(userstr))
	}

	if securityContext.GetRunAsUsername() != "" {
		userstr = securityContext.GetRunAsUsername()
	} else {
		// Even if RunAsUser is not set, we still call `GetValue` to get uid 0.
		// Because it is still useful to get additional gids for uid 0.
		userstr = strconv.FormatInt(securityContext.GetRunAsUser().GetValue(), 10)
	}
	specOpts = append(specOpts, customopts.WithAdditionalGIDs(userstr))

	apparmorSpecOpts, err := generateApparmorSpecOpts(
		securityContext.GetApparmorProfile(),
		securityContext.GetPrivileged(),
		c.apparmorEnabled)
	if err != nil {
		return nil, errors.Wrap(err, "failed to generate apparmor spec opts")
	}
	if apparmorSpecOpts != nil {
		specOpts = append(specOpts, apparmorSpecOpts)
	}

	seccompSpecOpts, err := generateSeccompSpecOpts(
		securityContext.GetSeccompProfilePath(),
		securityContext.GetPrivileged(),
		c.seccompEnabled)
	if err != nil {
		return nil, errors.Wrap(err, "failed to generate seccomp spec opts")
	}
	if seccompSpecOpts != nil {
		specOpts = append(specOpts, seccompSpecOpts)
	}
	containerLabels := buildLabels(config.Labels, containerKindContainer)

	runtimeOptions, err := getRuntimeOptions(sandboxInfo)
	if err != nil {
		return nil, errors.Wrap(err, "failed to get runtime options")
	}
	opts = append(opts,
		containerd.WithSpec(spec, specOpts...),
		containerd.WithRuntime(sandboxInfo.Runtime.Name, runtimeOptions),
		containerd.WithContainerLabels(containerLabels),
		containerd.WithContainerExtension(containerMetadataExtension, &meta))
	var cntr containerd.Container
	if cntr, err = c.client.NewContainer(ctx, id, opts...); err != nil {
		return nil, errors.Wrap(err, "failed to create containerd container")
	}
	defer func() {
		if retErr != nil {
			deferCtx, deferCancel := ctrdutil.DeferContext()
			defer deferCancel()
			if err := cntr.Delete(deferCtx, containerd.WithSnapshotCleanup); err != nil {
				log.G(ctx).WithError(err).Errorf("Failed to delete containerd container %q", id)
			}
		}
	}()

	status := containerstore.Status{CreatedAt: time.Now().UnixNano()}
	container, err := containerstore.NewContainer(meta,
		containerstore.WithStatus(status, containerRootDir),
		containerstore.WithContainer(cntr),
		containerstore.WithContainerIO(containerIO),
	)
	if err != nil {
		return nil, errors.Wrapf(err, "failed to create internal container object for %q", id)
	}
	defer func() {
		if retErr != nil {
			// Cleanup container checkpoint on error.
			if err := container.Delete(); err != nil {
				log.G(ctx).WithError(err).Errorf("Failed to cleanup container checkpoint for %q", id)
			}
		}
	}()

	// Add container into container store.
	if err := c.containerStore.Add(container); err != nil {
		return nil, errors.Wrapf(err, "failed to add container %q into store", id)
	}

	return &runtime.CreateContainerResponse{ContainerId: id}, nil
}

func (c *criService) generateContainerSpec(id string, sandboxID string, sandboxPid uint32, config *runtime.ContainerConfig,
	sandboxConfig *runtime.PodSandboxConfig, imageConfig *imagespec.ImageConfig, extraMounts []*runtime.Mount,
	ociRuntime config.Runtime) (*runtimespec.Spec, error) {

	specOpts := []oci.SpecOpts{
		customopts.WithoutRunMount,
		customopts.WithoutDefaultSecuritySettings,
		customopts.WithRelativeRoot(relativeRootfsPath),
		customopts.WithProcessArgs(config, imageConfig),
		// this will be set based on the security context below
		oci.WithNewPrivileges,
	}
	if config.GetWorkingDir() != "" {
		specOpts = append(specOpts, oci.WithProcessCwd(config.GetWorkingDir()))
	} else if imageConfig.WorkingDir != "" {
		specOpts = append(specOpts, oci.WithProcessCwd(imageConfig.WorkingDir))
	}

	if config.GetTty() {
		specOpts = append(specOpts, oci.WithTTY)
	}

	// Add HOSTNAME env.
	var (
		err      error
		hostname = sandboxConfig.GetHostname()
	)
	if hostname == "" {
		if hostname, err = c.os.Hostname(); err != nil {
			return nil, err
		}
	}
	specOpts = append(specOpts, oci.WithEnv([]string{hostnameEnv + "=" + hostname}))

	// Apply envs from image config first, so that envs from container config
	// can override them.
	env := imageConfig.Env
	for _, e := range config.GetEnvs() {
		env = append(env, e.GetKey()+"="+e.GetValue())
	}
	specOpts = append(specOpts, oci.WithEnv(env))

	securityContext := config.GetLinux().GetSecurityContext()
	selinuxOpt := securityContext.GetSelinuxOptions()
	processLabel, mountLabel, err := initSelinuxOpts(selinuxOpt)
	if err != nil {
		return nil, errors.Wrapf(err, "failed to init selinux options %+v", securityContext.GetSelinuxOptions())
	}
	specOpts = append(specOpts, customopts.WithMounts(c.os, config, extraMounts, mountLabel))

	if !c.config.DisableProcMount {
		// Apply masked paths if specified.
		// If the container is privileged, this will be cleared later on.
		specOpts = append(specOpts, oci.WithMaskedPaths(securityContext.GetMaskedPaths()))

		// Apply readonly paths if specified.
		// If the container is privileged, this will be cleared later on.
		specOpts = append(specOpts, oci.WithReadonlyPaths(securityContext.GetReadonlyPaths()))
	}

	if securityContext.GetPrivileged() {
		if !sandboxConfig.GetLinux().GetSecurityContext().GetPrivileged() {
			return nil, errors.New("no privileged container allowed in sandbox")
		}
		specOpts = append(specOpts, oci.WithPrivileged)
		if !ociRuntime.PrivilegedWithoutHostDevices {
			specOpts = append(specOpts, customopts.WithPrivilegedDevices)
		}
	} else { // not privileged
		specOpts = append(specOpts, customopts.WithDevices(c.os, config), customopts.WithCapabilities(securityContext))
	}

	// Clear all ambient capabilities. The implication of non-root + caps
	// is not clearly defined in Kubernetes.
	// See https://github.com/kubernetes/kubernetes/issues/56374
	// Keep docker's behavior for now.
	specOpts = append(specOpts,
		customopts.WithoutAmbientCaps,
		customopts.WithSelinuxLabels(processLabel, mountLabel),
	)

	// TODO: Figure out whether we should set no new privilege for sandbox container by default
	if securityContext.GetNoNewPrivs() {
		specOpts = append(specOpts, oci.WithNoNewPrivileges)
	}
	// TODO(random-liu): [P1] Set selinux options (privileged or not).
	if securityContext.GetReadonlyRootfs() {
		specOpts = append(specOpts, oci.WithRootFSReadonly())
	}

	if c.config.DisableCgroup {
		specOpts = append(specOpts, customopts.WithDisabledCgroups)
	} else {
		specOpts = append(specOpts, customopts.WithResources(config.GetLinux().GetResources()))
		if sandboxConfig.GetLinux().GetCgroupParent() != "" {
			cgroupsPath := getCgroupsPath(sandboxConfig.GetLinux().GetCgroupParent(), id)
			specOpts = append(specOpts, oci.WithCgroup(cgroupsPath))
		}
	}

	supplementalGroups := securityContext.GetSupplementalGroups()

	for pKey, pValue := range getPassthroughAnnotations(sandboxConfig.Annotations,
		ociRuntime.PodAnnotations) {
		specOpts = append(specOpts, customopts.WithAnnotation(pKey, pValue))
	}

	specOpts = append(specOpts,
		customopts.WithOOMScoreAdj(config, c.config.RestrictOOMScoreAdj),
		customopts.WithPodNamespaces(securityContext, sandboxPid),
		customopts.WithSupplementalGroups(supplementalGroups),
		customopts.WithAnnotation(annotations.ContainerType, annotations.ContainerTypeContainer),
		customopts.WithAnnotation(annotations.SandboxID, sandboxID),
	)

	return runtimeSpec(id, specOpts...)
}

// generateVolumeMounts sets up image volumes for container. Rely on the removal of container
// root directory to do cleanup. Note that image volume will be skipped, if there is criMounts
// specified with the same destination.
func (c *criService) generateVolumeMounts(containerRootDir string, criMounts []*runtime.Mount, config *imagespec.ImageConfig) []*runtime.Mount {
	if len(config.Volumes) == 0 {
		return nil
	}
	var mounts []*runtime.Mount
	for dst := range config.Volumes {
		if isInCRIMounts(dst, criMounts) {
			// Skip the image volume, if there is CRI defined volume mapping.
			// TODO(random-liu): This should be handled by Kubelet in the future.
			// Kubelet should decide what to use for image volume, and also de-duplicate
			// the image volume and user mounts.
			continue
		}
		volumeID := util.GenerateID()
		src := filepath.Join(containerRootDir, "volumes", volumeID)
		// addOCIBindMounts will create these volumes.
		mounts = append(mounts, &runtime.Mount{
			ContainerPath: dst,
			HostPath:      src,
			// Use default mount propagation.
			// TODO(random-liu): What about selinux relabel?
		})
	}
	return mounts
}

// generateContainerMounts sets up necessary container mounts including /dev/shm, /etc/hosts
// and /etc/resolv.conf.
func (c *criService) generateContainerMounts(sandboxID string, config *runtime.ContainerConfig) []*runtime.Mount {
	var mounts []*runtime.Mount
	securityContext := config.GetLinux().GetSecurityContext()
	if !isInCRIMounts(etcHostname, config.GetMounts()) {
		// /etc/hostname is added since 1.1.6, 1.2.4 and 1.3.
		// For in-place upgrade, the old sandbox doesn't have the hostname file,
		// do not mount this in that case.
		// TODO(random-liu): Remove the check and always mount this when
		// containerd 1.1 and 1.2 are deprecated.
		hostpath := c.getSandboxHostname(sandboxID)
		if _, err := c.os.Stat(hostpath); err == nil {
			mounts = append(mounts, &runtime.Mount{
				ContainerPath: etcHostname,
				HostPath:      hostpath,
				Readonly:      securityContext.GetReadonlyRootfs(),
			})
		}
	}

	if !isInCRIMounts(etcHosts, config.GetMounts()) {
		mounts = append(mounts, &runtime.Mount{
			ContainerPath: etcHosts,
			HostPath:      c.getSandboxHosts(sandboxID),
			Readonly:      securityContext.GetReadonlyRootfs(),
		})
	}

	// Mount sandbox resolv.config.
	// TODO: Need to figure out whether we should always mount it as read-only
	if !isInCRIMounts(resolvConfPath, config.GetMounts()) {
		mounts = append(mounts, &runtime.Mount{
			ContainerPath: resolvConfPath,
			HostPath:      c.getResolvPath(sandboxID),
			Readonly:      securityContext.GetReadonlyRootfs(),
		})
	}

	if !isInCRIMounts(devShm, config.GetMounts()) {
		sandboxDevShm := c.getSandboxDevShm(sandboxID)
		if securityContext.GetNamespaceOptions().GetIpc() == runtime.NamespaceMode_NODE {
			sandboxDevShm = devShm
		}
		mounts = append(mounts, &runtime.Mount{
			ContainerPath: devShm,
			HostPath:      sandboxDevShm,
			Readonly:      false,
		})
	}
	return mounts
}

// runtimeSpec returns a default runtime spec used in cri-containerd.
func runtimeSpec(id string, opts ...oci.SpecOpts) (*runtimespec.Spec, error) {
	// GenerateSpec needs namespace.
	ctx := ctrdutil.NamespacedContext()
	spec, err := oci.GenerateSpec(ctx, nil, &containers.Container{ID: id}, opts...)
	if err != nil {
		return nil, err
	}
	return spec, nil
}

// generateSeccompSpecOpts generates containerd SpecOpts for seccomp.
func generateSeccompSpecOpts(seccompProf string, privileged, seccompEnabled bool) (oci.SpecOpts, error) {
	if privileged {
		// Do not set seccomp profile when container is privileged
		return nil, nil
	}
	// Set seccomp profile
	if seccompProf == runtimeDefault || seccompProf == dockerDefault {
		// use correct default profile (Eg. if not configured otherwise, the default is docker/default)
		seccompProf = seccompDefaultProfile
	}
	if !seccompEnabled {
		if seccompProf != "" && seccompProf != unconfinedProfile {
			return nil, errors.New("seccomp is not supported")
		}
		return nil, nil
	}
	switch seccompProf {
	case "", unconfinedProfile:
		// Do not set seccomp profile.
		return nil, nil
	case dockerDefault:
		// Note: WithDefaultProfile specOpts must be added after capabilities
		return seccomp.WithDefaultProfile(), nil
	default:
		// Require and Trim default profile name prefix
		if !strings.HasPrefix(seccompProf, profileNamePrefix) {
			return nil, errors.Errorf("invalid seccomp profile %q", seccompProf)
		}
		return seccomp.WithProfile(strings.TrimPrefix(seccompProf, profileNamePrefix)), nil
	}
}

// generateApparmorSpecOpts generates containerd SpecOpts for apparmor.
func generateApparmorSpecOpts(apparmorProf string, privileged, apparmorEnabled bool) (oci.SpecOpts, error) {
	if !apparmorEnabled {
		// Should fail loudly if user try to specify apparmor profile
		// but we don't support it.
		if apparmorProf != "" && apparmorProf != unconfinedProfile {
			return nil, errors.New("apparmor is not supported")
		}
		return nil, nil
	}
	switch apparmorProf {
	// Based on kubernetes#51746, default apparmor profile should be applied
	// for when apparmor is not specified.
	case runtimeDefault, "":
		if privileged {
			// Do not set apparmor profile when container is privileged
			return nil, nil
		}
		// TODO (mikebrow): delete created apparmor default profile
		return apparmor.WithDefaultProfile(appArmorDefaultProfileName), nil
	case unconfinedProfile:
		return nil, nil
	default:
		// Require and Trim default profile name prefix
		if !strings.HasPrefix(apparmorProf, profileNamePrefix) {
			return nil, errors.Errorf("invalid apparmor profile %q", apparmorProf)
		}
		return apparmor.WithProfile(strings.TrimPrefix(apparmorProf, profileNamePrefix)), nil
	}
}

// generateUserString generates valid user string based on OCI Image Spec
// v1.0.0.
//
// CRI defines that the following combinations are valid:
//
// (none) -> ""
// username -> username
// username, uid -> username
// username, uid, gid -> username:gid
// username, gid -> username:gid
// uid -> uid
// uid, gid -> uid:gid
// gid -> error
//
// TODO(random-liu): Add group name support in CRI.
func generateUserString(username string, uid, gid *runtime.Int64Value) (string, error) {
	var userstr, groupstr string
	if uid != nil {
		userstr = strconv.FormatInt(uid.GetValue(), 10)
	}
	if username != "" {
		userstr = username
	}
	if gid != nil {
		groupstr = strconv.FormatInt(gid.GetValue(), 10)
	}
	if userstr == "" {
		if groupstr != "" {
			return "", errors.Errorf("user group %q is specified without user", groupstr)
		}
		return "", nil
	}
	if groupstr != "" {
		userstr = userstr + ":" + groupstr
	}
	return userstr, nil
}