Add utility/support package for user namespace support

The `pkg/idtools` package supports the creation of user(s) for
retrieving /etc/sub{u,g}id ranges and creation of the UID/GID mappings
provided to clone() to add support for user namespaces in Docker.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

idtools/idtools.go

@@ -0,0 +1,207 @@
+package idtools
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"sort"
+	"strconv"
+	"strings"
+
+	"github.com/docker/docker/pkg/system"
+)
+
+// IDMap contains a single entry for user namespace range remapping. An array
+// of IDMap entries represents the structure that will be provided to the Linux
+// kernel for creating a user namespace.
+type IDMap struct {
+	ContainerID int `json:"container_id"`
+	HostID      int `json:"host_id"`
+	Size        int `json:"size"`
+}
+
+type subIDRange struct {
+	Start  int
+	Length int
+}
+
+type ranges []subIDRange
+
+func (e ranges) Len() int           { return len(e) }
+func (e ranges) Swap(i, j int)      { e[i], e[j] = e[j], e[i] }
+func (e ranges) Less(i, j int) bool { return e[i].Start < e[j].Start }
+
+const (
+	subuidFileName string = "/etc/subuid"
+	subgidFileName string = "/etc/subgid"
+)
+
+// MkdirAllAs creates a directory (include any along the path) and then modifies
+// ownership to the requested uid/gid.  If the directory already exists, this
+// function will still change ownership to the requested uid/gid pair.
+func MkdirAllAs(path string, mode os.FileMode, ownerUID, ownerGID int) error {
+	return mkdirAs(path, mode, ownerUID, ownerGID, true)
+}
+
+// MkdirAs creates a directory and then modifies ownership to the requested uid/gid.
+// If the directory already exists, this function still changes ownership
+func MkdirAs(path string, mode os.FileMode, ownerUID, ownerGID int) error {
+	return mkdirAs(path, mode, ownerUID, ownerGID, false)
+}
+
+func mkdirAs(path string, mode os.FileMode, ownerUID, ownerGID int, mkAll bool) error {
+	if mkAll {
+		if err := system.MkdirAll(path, mode); err != nil && !os.IsExist(err) {
+			return err
+		}
+	} else {
+		if err := os.Mkdir(path, mode); err != nil && !os.IsExist(err) {
+			return err
+		}
+	}
+	// even if it existed, we will chown to change ownership as requested
+	if err := os.Chown(path, ownerUID, ownerGID); err != nil {
+		return err
+	}
+	return nil
+}
+
+// GetRootUIDGID retrieves the remapped root uid/gid pair from the set of maps.
+// If the maps are empty, then the root uid/gid will default to "real" 0/0
+func GetRootUIDGID(uidMap, gidMap []IDMap) (int, int, error) {
+	var uid, gid int
+
+	if uidMap != nil {
+		xUID, err := ToHost(0, uidMap)
+		if err != nil {
+			return -1, -1, err
+		}
+		uid = xUID
+	}
+	if gidMap != nil {
+		xGID, err := ToHost(0, gidMap)
+		if err != nil {
+			return -1, -1, err
+		}
+		gid = xGID
+	}
+	return uid, gid, nil
+}
+
+// ToContainer takes an id mapping, and uses it to translate a
+// host ID to the remapped ID. If no map is provided, then the translation
+// assumes a 1-to-1 mapping and returns the passed in id
+func ToContainer(hostID int, idMap []IDMap) (int, error) {
+	if idMap == nil {
+		return hostID, nil
+	}
+	for _, m := range idMap {
+		if (hostID >= m.HostID) && (hostID <= (m.HostID + m.Size - 1)) {
+			contID := m.ContainerID + (hostID - m.HostID)
+			return contID, nil
+		}
+	}
+	return -1, fmt.Errorf("Host ID %d cannot be mapped to a container ID", hostID)
+}
+
+// ToHost takes an id mapping and a remapped ID, and translates the
+// ID to the mapped host ID. If no map is provided, then the translation
+// assumes a 1-to-1 mapping and returns the passed in id #
+func ToHost(contID int, idMap []IDMap) (int, error) {
+	if idMap == nil {
+		return contID, nil
+	}
+	for _, m := range idMap {
+		if (contID >= m.ContainerID) && (contID <= (m.ContainerID + m.Size - 1)) {
+			hostID := m.HostID + (contID - m.ContainerID)
+			return hostID, nil
+		}
+	}
+	return -1, fmt.Errorf("Container ID %d cannot be mapped to a host ID", contID)
+}
+
+// CreateIDMappings takes a requested user and group name and
+// using the data from /etc/sub{uid,gid} ranges, creates the
+// proper uid and gid remapping ranges for that user/group pair
+func CreateIDMappings(username, groupname string) ([]IDMap, []IDMap, error) {
+	subuidRanges, err := parseSubuid(username)
+	if err != nil {
+		return nil, nil, err
+	}
+	subgidRanges, err := parseSubgid(groupname)
+	if err != nil {
+		return nil, nil, err
+	}
+	if len(subuidRanges) == 0 {
+		return nil, nil, fmt.Errorf("No subuid ranges found for user %q", username)
+	}
+	if len(subgidRanges) == 0 {
+		return nil, nil, fmt.Errorf("No subgid ranges found for group %q", groupname)
+	}
+
+	return createIDMap(subuidRanges), createIDMap(subgidRanges), nil
+}
+
+func createIDMap(subidRanges ranges) []IDMap {
+	idMap := []IDMap{}
+
+	// sort the ranges by lowest ID first
+	sort.Sort(subidRanges)
+	containerID := 0
+	for _, idrange := range subidRanges {
+		idMap = append(idMap, IDMap{
+			ContainerID: containerID,
+			HostID:      idrange.Start,
+			Size:        idrange.Length,
+		})
+		containerID = containerID + idrange.Length
+	}
+	return idMap
+}
+
+func parseSubuid(username string) (ranges, error) {
+	return parseSubidFile(subuidFileName, username)
+}
+
+func parseSubgid(username string) (ranges, error) {
+	return parseSubidFile(subgidFileName, username)
+}
+
+func parseSubidFile(path, username string) (ranges, error) {
+	var rangeList ranges
+
+	subidFile, err := os.Open(path)
+	if err != nil {
+		return rangeList, err
+	}
+	defer subidFile.Close()
+
+	s := bufio.NewScanner(subidFile)
+	for s.Scan() {
+		if err := s.Err(); err != nil {
+			return rangeList, err
+		}
+
+		text := strings.TrimSpace(s.Text())
+		if text == "" {
+			continue
+		}
+		parts := strings.Split(text, ":")
+		if len(parts) != 3 {
+			return rangeList, fmt.Errorf("Cannot parse subuid/gid information: Format not correct for %s file", path)
+		}
+		if parts[0] == username {
+			// return the first entry for a user; ignores potential for multiple ranges per user
+			startid, err := strconv.Atoi(parts[1])
+			if err != nil {
+				return rangeList, fmt.Errorf("String to int conversion failed during subuid/gid parsing of %s: %v", path, err)
+			}
+			length, err := strconv.Atoi(parts[2])
+			if err != nil {
+				return rangeList, fmt.Errorf("String to int conversion failed during subuid/gid parsing of %s: %v", path, err)
+			}
+			rangeList = append(rangeList, subIDRange{startid, length})
+		}
+	}
+	return rangeList, nil
+}