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>

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())