userns: add new option --userns=keep-id

it creates a namespace where the current UID:GID on the host is mapped
to the same UID:GID in the container.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

cmd/podman/libpodruntime/runtime.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/containers/libpod/cmd/podman/cliconfig"
 	"github.com/containers/libpod/libpod"
+	"github.com/containers/libpod/pkg/namespaces"
 	"github.com/containers/libpod/pkg/rootless"
 	"github.com/containers/libpod/pkg/util"
 	"github.com/containers/storage"
@@ -37,11 +38,12 @@ func getRuntime(ctx context.Context, c *cliconfig.PodmanCommand, renumber bool,
 	subgidname := c.Flags().Lookup("subgidname")
 	if (uidmapFlag != nil && gidmapFlag != nil && subuidname != nil && subgidname != nil) &&
 		(uidmapFlag.Changed || gidmapFlag.Changed || subuidname.Changed || subgidname.Changed) {
+		userns, _ := c.Flags().GetString("userns")
 		uidmapVal, _ := c.Flags().GetStringSlice("uidmap")
 		gidmapVal, _ := c.Flags().GetStringSlice("gidmap")
 		subuidVal, _ := c.Flags().GetString("subuidname")
 		subgidVal, _ := c.Flags().GetString("subgidname")
-		mappings, err := util.ParseIDMapping(uidmapVal, gidmapVal, subuidVal, subgidVal)
+		mappings, err := util.ParseIDMapping(namespaces.UsernsMode(userns), uidmapVal, gidmapVal, subuidVal, subgidVal)
 		if err != nil {
 			return nil, err
 		}

cmd/podman/shared/create.go

@@ -19,6 +19,7 @@ import (
 	ann "github.com/containers/libpod/pkg/annotations"
 	"github.com/containers/libpod/pkg/inspect"
 	ns "github.com/containers/libpod/pkg/namespaces"
+	"github.com/containers/libpod/pkg/rootless"
 	cc "github.com/containers/libpod/pkg/spec"
 	"github.com/containers/libpod/pkg/util"
 	"github.com/docker/docker/pkg/signal"
@@ -283,7 +284,7 @@ func ParseCreateOpts(ctx context.Context, c *GenericCLIResults, runtime *libpod.
 		namespaces                                               map[string]string
 	)
 
-	idmappings, err := util.ParseIDMapping(c.StringSlice("uidmap"), c.StringSlice("gidmap"), c.String("subuidname"), c.String("subgidname"))
+	idmappings, err := util.ParseIDMapping(ns.UsernsMode(c.String("userns")), c.StringSlice("uidmap"), c.StringSlice("gidmap"), c.String("subuidname"), c.String("subgidname"))
 	if err != nil {
 		return nil, err
 	}
@@ -451,7 +452,9 @@ func ParseCreateOpts(ctx context.Context, c *GenericCLIResults, runtime *libpod.
 	// USER
 	user := c.String("user")
 	if user == "" {
-		if data == nil {
+		if usernsMode.IsKeepID() {
+			user = fmt.Sprintf("%d:%d", rootless.GetRootlessUID(), rootless.GetRootlessGID())
+		} else if data == nil {
 			user = "0"
 		} else {
 			user = data.Config.User

docs/podman-create.1.md

@@ -727,11 +727,13 @@ The followings examples are all valid:
 Without this argument the command will be run as root in the container.
 
 **--userns**=host
+**--userns**=keep-id
 **--userns**=ns:my_namespace
 
 Set the user namespace mode for the container. The use of userns is disabled by default.
 
 - `host`: run in the user namespace of the caller. This is the default if no user namespace options are set. The processes running in the container will have the same privileges on the host as any other process launched by the calling user.
+- `keep-id`: creates a user namespace where the current rootless user's UID:GID are mapped to the same values in the container. This option is ignored for containers created by the root user.
 - `ns`: run the container in the given existing user namespace.
 
 This option is incompatible with --gidmap, --uidmap, --subuid and --subgid

docs/podman-run.1.md

@@ -763,11 +763,13 @@ The followings examples are all valid:
 Without this argument the command will be run as root in the container.
 
 **--userns**=host
+**--userns**=keep-id
 **--userns**=ns:my_namespace
 
 Set the user namespace mode for the container. The use of userns is disabled by default.
 
 - `host`: run in the user namespace of the caller. This is the default if no user namespace options are set. The processes running in the container will have the same privileges on the host as any other process launched by the calling user.
+- `keep-id`: creates a user namespace where the current rootless user's UID:GID are mapped to the same values in the container. This option is ignored for containers created by the root user.
 - `ns`: run the container in the given existing user namespace.
 
 This option is incompatible with --gidmap, --uidmap, --subuid and --subgid

pkg/namespaces/namespaces.go

@@ -12,6 +12,11 @@ func (n UsernsMode) IsHost() bool {
 	return n == "host"
 }
 
+// IsKeepID indicates whether container uses a mapping where the (uid, gid) on the host is lept inside of the namespace.
+func (n UsernsMode) IsKeepID() bool {
+	return n == "keep-id"
+}
+
 // IsPrivate indicates whether the container uses the a private userns.
 func (n UsernsMode) IsPrivate() bool {
 	return !(n.IsHost())
@@ -21,7 +26,7 @@ func (n UsernsMode) IsPrivate() bool {
 func (n UsernsMode) Valid() bool {
 	parts := strings.Split(string(n), ":")
 	switch mode := parts[0]; mode {
-	case "", "host":
+	case "", "host", "keep-id":
 	default:
 		return false
 	}

pkg/util/utils.go

@@ -3,6 +3,7 @@ package util
 import (
 	"fmt"
 	"os"
+	ouser "os/user"
 	"path/filepath"
 	"strings"
 	"sync"
@@ -11,6 +12,8 @@ import (
 	"github.com/BurntSushi/toml"
 	"github.com/containers/image/types"
 	"github.com/containers/libpod/cmd/podman/cliconfig"
+	"github.com/containers/libpod/pkg/namespaces"
+	"github.com/containers/libpod/pkg/rootless"
 	"github.com/containers/storage"
 	"github.com/containers/storage/pkg/idtools"
 	"github.com/opencontainers/image-spec/specs-go/v1"
@@ -131,11 +134,59 @@ func GetImageConfig(changes []string) (v1.ImageConfig, error) {
 }
 
 // ParseIDMapping takes idmappings and subuid and subgid maps and returns a storage mapping
-func ParseIDMapping(UIDMapSlice, GIDMapSlice []string, subUIDMap, subGIDMap string) (*storage.IDMappingOptions, error) {
+func ParseIDMapping(mode namespaces.UsernsMode, UIDMapSlice, GIDMapSlice []string, subUIDMap, subGIDMap string) (*storage.IDMappingOptions, error) {
 	options := storage.IDMappingOptions{
 		HostUIDMapping: true,
 		HostGIDMapping: true,
 	}
+
+	if mode.IsKeepID() {
+		if len(UIDMapSlice) > 0 || len(GIDMapSlice) > 0 {
+			return nil, errors.New("cannot specify custom mappings with --userns=keep-id")
+		}
+		if len(subUIDMap) > 0 || len(subGIDMap) > 0 {
+			return nil, errors.New("cannot specify subuidmap or subgidmap with --userns=keep-id")
+		}
+		if rootless.IsRootless() {
+			uid := rootless.GetRootlessUID()
+			gid := rootless.GetRootlessGID()
+
+			username := os.Getenv("USER")
+			if username == "" {
+				user, err := ouser.LookupId(fmt.Sprintf("%d", uid))
+				if err == nil {
+					username = user.Username
+				}
+			}
+			mappings, err := idtools.NewIDMappings(username, username)
+			if err != nil {
+				return nil, errors.Wrapf(err, "cannot find mappings for user %s", username)
+			}
+			maxUID, maxGID := 0, 0
+			for _, u := range mappings.UIDs() {
+				maxUID += u.Size
+			}
+			for _, g := range mappings.GIDs() {
+				maxGID += g.Size
+			}
+
+			options.UIDMap, options.GIDMap = nil, nil
+
+			options.UIDMap = append(options.UIDMap, idtools.IDMap{ContainerID: 0, HostID: 1, Size: uid})
+			options.UIDMap = append(options.UIDMap, idtools.IDMap{ContainerID: uid, HostID: 0, Size: 1})
+			options.UIDMap = append(options.UIDMap, idtools.IDMap{ContainerID: uid + 1, HostID: uid + 1, Size: maxUID - uid})
+
+			options.GIDMap = append(options.GIDMap, idtools.IDMap{ContainerID: 0, HostID: 1, Size: gid})
+			options.GIDMap = append(options.GIDMap, idtools.IDMap{ContainerID: gid, HostID: 0, Size: 1})
+			options.GIDMap = append(options.GIDMap, idtools.IDMap{ContainerID: gid + 1, HostID: gid + 1, Size: maxGID - gid})
+
+			options.HostUIDMapping = false
+			options.HostGIDMapping = false
+		}
+		// Simply ignore the setting and do not setup an inner namespace for root as it is a no-op
+		return &options, nil
+	}
+
 	if subGIDMap == "" && subUIDMap != "" {
 		subGIDMap = subUIDMap
 	}

test/e2e/run_userns_test.go

@@ -3,6 +3,7 @@
 package integration
 
 import (
+	"fmt"
 	"os"
 
 	. "github.com/containers/libpod/test/utils"
@@ -76,4 +77,12 @@ var _ = Describe("Podman UserNS support", func() {
 		Expect(ok).To(BeTrue())
 	})
 
+	It("podman --userns=keep-id", func() {
+		session := podmanTest.Podman([]string{"run", "--userns=keep-id", "alpine", "id", "-u"})
+		session.WaitWithDefaultTimeout()
+		Expect(session.ExitCode()).To(Equal(0))
+		uid := fmt.Sprintf("%d", os.Geteuid())
+		ok, _ := session.GrepString(uid)
+		Expect(ok).To(BeTrue())
+	})
 })