podman: add uid and gid options to keep-id

add two new options to the keep-id user namespace option:

- uid: allow to override the UID used inside the container.
- gid: allow to override the GID used inside the container.

For example, the following command will map the rootless user (that
has UID=0 inside the rootless user namespace) to the UID=11 inside the
container user namespace:

$ podman run --userns=keep-id:uid=11 --rm -ti  fedora cat /proc/self/uid_map
         0          1         11
        11          0          1
        12         12      65525

Closes: https://github.com/containers/podman/issues/15294

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

docs/source/markdown/podman-create.1.md.in

@@ -687,6 +687,11 @@ Podman allocates unique ranges of UIDs and GIDs from the `containers` subordinat
 
 **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 not allowed for containers created by the root user.
 
+  Valid `keep-id` options:
+
+  - *uid*=UID: override the UID inside the container that will be used to map the current rootless user to.
+  - *gid*=GID: override the GID inside the container that will be used to map the current rootless user to.
+
 **nomap**: creates a user namespace where the current rootless user's UID:GID are not mapped into the container. This option is not allowed for containers created by the root user.
 
 **ns:**_namespace_: run the container in the given existing user namespace.

docs/source/markdown/podman-kube-play.1.md.in

@@ -260,6 +260,11 @@ Podman allocates unique ranges of UIDs and GIDs from the `containers` subordinat
 
 **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 not allowed for containers created by the root user.
 
+  Valid `keep-id` options:
+
+  - *uid*=UID: override the UID inside the container that will be used to map the current rootless user to.
+  - *gid*=GID: override the GID inside the container that will be used to map the current rootless user to.
+
 **nomap**: creates a user namespace where the current rootless user's UID:GID are not mapped into the container. This option is not allowed for containers created by the root user.
 
 **ns:**_namespace_: run the pod in the given existing user namespace.

docs/source/markdown/podman-run.1.md.in

@@ -745,6 +745,11 @@ The rootless option `--userns=keep-id` uses all the subuids and subgids of the u
 
 **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 not allowed for containers created by the root user.
 
+  Valid `keep-id` options:
+
+  - *uid*=UID: override the UID inside the container that will be used to map the current rootless user to.
+  - *gid*=GID: override the GID inside the container that will be used to map the current rootless user to.
+
 **nomap**: creates a user namespace where the current rootless user's UID:GID are not mapped into the container. This option is not allowed for containers created by the root user.
 
 **ns:**_namespace_: run the container in the given existing user namespace.

pkg/namespaces/namespaces.go

@@ -21,6 +21,14 @@ const (
 	slirpType     = "slirp4netns"
 )
 
+// KeepIDUserNsOptions defines how to keepIDmatically create a user namespace.
+type KeepIDUserNsOptions struct {
+	// UID is the target uid in the user namespace.
+	UID *uint32
+	// GID is the target uid in the user namespace.
+	GID *uint32
+}
+
 // CgroupMode represents cgroup mode in the container.
 type CgroupMode string
 
@@ -93,7 +101,8 @@ func (n UsernsMode) IsHost() bool {
 
 // IsKeepID indicates whether container uses a mapping where the (uid, gid) on the host is kept inside of the namespace.
 func (n UsernsMode) IsKeepID() bool {
-	return n == "keep-id"
+	parts := strings.Split(string(n), ":")
+	return parts[0] == "keep-id"
 }
 
 // IsNoMap indicates whether container uses a mapping where the (uid, gid) on the host is not present in the namespace.
@@ -154,6 +163,44 @@ func (n UsernsMode) GetAutoOptions() (*types.AutoUserNsOptions, error) {
 	return &options, nil
 }
 
+// GetKeepIDOptions returns a KeepIDUserNsOptions with the settings to keepIDmatically set up
+// a user namespace.
+func (n UsernsMode) GetKeepIDOptions() (*KeepIDUserNsOptions, error) {
+	parts := strings.SplitN(string(n), ":", 2)
+	if parts[0] != "keep-id" {
+		return nil, fmt.Errorf("wrong user namespace mode")
+	}
+	options := KeepIDUserNsOptions{}
+	if len(parts) == 1 {
+		return &options, nil
+	}
+	for _, o := range strings.Split(parts[1], ",") {
+		v := strings.SplitN(o, "=", 2)
+		if len(v) != 2 {
+			return nil, fmt.Errorf("invalid option specified: %q", o)
+		}
+		switch v[0] {
+		case "uid":
+			s, err := strconv.ParseUint(v[1], 10, 32)
+			if err != nil {
+				return nil, err
+			}
+			v := uint32(s)
+			options.UID = &v
+		case "gid":
+			s, err := strconv.ParseUint(v[1], 10, 32)
+			if err != nil {
+				return nil, err
+			}
+			v := uint32(s)
+			options.GID = &v
+		default:
+			return nil, fmt.Errorf("unknown option specified: %q", v[0])
+		}
+	}
+	return &options, nil
+}
+
 // IsPrivate indicates whether the container uses the a private userns.
 func (n UsernsMode) IsPrivate() bool {
 	return !(n.IsHost() || n.IsContainer())

pkg/specgen/generate/namespaces.go

@@ -11,6 +11,7 @@ import (
 	"github.com/containers/common/pkg/config"
 	"github.com/containers/podman/v4/libpod"
 	"github.com/containers/podman/v4/libpod/define"
+	"github.com/containers/podman/v4/pkg/namespaces"
 	"github.com/containers/podman/v4/pkg/rootless"
 	"github.com/containers/podman/v4/pkg/specgen"
 	"github.com/containers/podman/v4/pkg/util"
@@ -198,12 +199,18 @@ func namespaceOptions(s *specgen.SpecGenerator, rt *libpod.Runtime, pod *libpod.
 		if !rootless.IsRootless() {
 			return nil, errors.New("keep-id is only supported in rootless mode")
 		}
-		toReturn = append(toReturn, libpod.WithAddCurrentUserPasswdEntry())
+		opts, err := namespaces.UsernsMode(s.UserNS.String()).GetKeepIDOptions()
+		if err != nil {
+			return nil, err
+		}
+		if opts.UID == nil && opts.GID == nil {
+			toReturn = append(toReturn, libpod.WithAddCurrentUserPasswdEntry())
+		}
 
 		// If user is not overridden, set user in the container
 		// to user running Podman.
 		if s.User == "" {
-			_, uid, gid, err := util.GetKeepIDMapping()
+			_, uid, gid, err := util.GetKeepIDMapping(opts)
 			if err != nil {
 				return nil, err
 			}

pkg/specgen/namespaces.go

@@ -11,6 +11,7 @@ import (
 	"github.com/containers/common/pkg/cgroups"
 	cutil "github.com/containers/common/pkg/util"
 	"github.com/containers/podman/v4/libpod/define"
+	"github.com/containers/podman/v4/pkg/namespaces"
 	"github.com/containers/podman/v4/pkg/util"
 	"github.com/containers/storage"
 	spec "github.com/opencontainers/runtime-spec/specs-go"
@@ -308,6 +309,14 @@ func ParseUserNamespace(ns string) (Namespace, error) {
 	case ns == "keep-id":
 		toReturn.NSMode = KeepID
 		return toReturn, nil
+	case strings.HasPrefix(ns, "keep-id:"):
+		split := strings.SplitN(ns, ":", 2)
+		if len(split) != 2 {
+			return toReturn, errors.New("invalid setting for keep-id: mode")
+		}
+		toReturn.NSMode = KeepID
+		toReturn.Value = split[1]
+		return toReturn, nil
 	case ns == "nomap":
 		toReturn.NSMode = NoMap
 		return toReturn, nil
@@ -490,7 +499,11 @@ func SetupUserNS(idmappings *storage.IDMappingOptions, userns Namespace, g *gene
 			return user, err
 		}
 	case KeepID:
-		mappings, uid, gid, err := util.GetKeepIDMapping()
+		opts, err := namespaces.UsernsMode(userns.String()).GetKeepIDOptions()
+		if err != nil {
+			return user, err
+		}
+		mappings, uid, gid, err := util.GetKeepIDMapping(opts)
 		if err != nil {
 			return user, err
 		}

pkg/util/utils.go

@@ -342,7 +342,7 @@ func ParseSignal(rawSignal string) (syscall.Signal, error) {
 }
 
 // GetKeepIDMapping returns the mappings and the user to use when keep-id is used
-func GetKeepIDMapping() (*stypes.IDMappingOptions, int, int, error) {
+func GetKeepIDMapping(opts *namespaces.KeepIDUserNsOptions) (*stypes.IDMappingOptions, int, int, error) {
 	if !rootless.IsRootless() {
 		return nil, -1, -1, errors.New("keep-id is only supported in rootless mode")
 	}
@@ -359,6 +359,12 @@ func GetKeepIDMapping() (*stypes.IDMappingOptions, int, int, error) {
 
 	uid := rootless.GetRootlessUID()
 	gid := rootless.GetRootlessGID()
+	if opts.UID != nil {
+		uid = int(*opts.UID)
+	}
+	if opts.GID != nil {
+		gid = int(*opts.GID)
+	}
 
 	uids, gids, err := rootless.GetConfiguredMappings()
 	if err != nil {

test/e2e/run_userns_test.go

@@ -113,6 +113,16 @@ var _ = Describe("Podman UserNS support", func() {
 		Expect(session).Should(Exit(0))
 		uid := fmt.Sprintf("%d", os.Geteuid())
 		Expect(session.OutputToString()).To(ContainSubstring(uid))
+
+		session = podmanTest.Podman([]string{"run", "--userns=keep-id:uid=10,gid=12", "alpine", "sh", "-c", "echo $(id -u):$(id -g)"})
+		session.WaitWithDefaultTimeout()
+		if os.Geteuid() == 0 {
+			Expect(session).Should(Exit(125))
+			return
+		}
+
+		Expect(session).Should(Exit(0))
+		Expect(session.OutputToString()).To(ContainSubstring("10:12"))
 	})
 
 	It("podman --userns=keep-id check passwd", func() {