Add user namespace enable flag --userns-remap in experimental build

This adds the capability to turn on user namespace support when using an
experimental build Docker daemon binary using the `--userns-remap` flag.

Also documentation is added to the experimental docs.

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

daemon/config_experimental.go

@@ -0,0 +1,119 @@
+// +build experimental
+
+package daemon
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/docker/docker/pkg/idtools"
+	flag "github.com/docker/docker/pkg/mflag"
+	"github.com/opencontainers/runc/libcontainer/user"
+)
+
+func (config *Config) attachExperimentalFlags(cmd *flag.FlagSet, usageFn func(string) string) {
+	cmd.StringVar(&config.RemappedRoot, []string{"-userns-remap"}, "", usageFn("User/Group setting for user namespaces"))
+}
+
+const (
+	defaultIDSpecifier string = "default"
+	defaultRemappedID  string = "dockremap"
+)
+
+// Parse the remapped root (user namespace) option, which can be one of:
+//   username            - valid username from /etc/passwd
+//   username:groupname  - valid username; valid groupname from /etc/group
+//   uid                 - 32-bit unsigned int valid Linux UID value
+//   uid:gid             - uid value; 32-bit unsigned int Linux GID value
+//
+//  If no groupname is specified, and a username is specified, an attempt
+//  will be made to lookup a gid for that username as a groupname
+//
+//  If names are used, they are verified to exist in passwd/group
+func parseRemappedRoot(usergrp string) (string, string, error) {
+
+	var (
+		userID, groupID     int
+		username, groupname string
+	)
+
+	idparts := strings.Split(usergrp, ":")
+	if len(idparts) > 2 {
+		return "", "", fmt.Errorf("Invalid user/group specification in --userns-remap: %q", usergrp)
+	}
+
+	if uid, err := strconv.ParseInt(idparts[0], 10, 32); err == nil {
+		// must be a uid; take it as valid
+		userID = int(uid)
+		luser, err := user.LookupUid(userID)
+		if err != nil {
+			return "", "", fmt.Errorf("Uid %d has no entry in /etc/passwd: %v", userID, err)
+		}
+		username = luser.Name
+		if len(idparts) == 1 {
+			// if the uid was numeric and no gid was specified, take the uid as the gid
+			groupID = userID
+			lgrp, err := user.LookupGid(groupID)
+			if err != nil {
+				return "", "", fmt.Errorf("Gid %d has no entry in /etc/group: %v", groupID, err)
+			}
+			groupname = lgrp.Name
+		}
+	} else {
+		lookupName := idparts[0]
+		// special case: if the user specified "default", they want Docker to create or
+		// use (after creation) the "dockremap" user/group for root remapping
+		if lookupName == defaultIDSpecifier {
+			lookupName = defaultRemappedID
+		}
+		luser, err := user.LookupUser(lookupName)
+		if err != nil && idparts[0] != defaultIDSpecifier {
+			// error if the name requested isn't the special "dockremap" ID
+			return "", "", fmt.Errorf("Error during uid lookup for %q: %v", lookupName, err)
+		} else if err != nil {
+			// special case-- if the username == "default", then we have been asked
+			// to create a new entry pair in /etc/{passwd,group} for which the /etc/sub{uid,gid}
+			// ranges will be used for the user and group mappings in user namespaced containers
+			_, _, err := idtools.AddNamespaceRangesUser(defaultRemappedID)
+			if err == nil {
+				return defaultRemappedID, defaultRemappedID, nil
+			}
+			return "", "", fmt.Errorf("Error during %q user creation: %v", defaultRemappedID, err)
+		}
+		userID = luser.Uid
+		username = luser.Name
+		if len(idparts) == 1 {
+			// we only have a string username, and no group specified; look up gid from username as group
+			group, err := user.LookupGroup(lookupName)
+			if err != nil {
+				return "", "", fmt.Errorf("Error during gid lookup for %q: %v", lookupName, err)
+			}
+			groupID = group.Gid
+			groupname = group.Name
+		}
+	}
+
+	if len(idparts) == 2 {
+		// groupname or gid is separately specified and must be resolved
+		// to a unsigned 32-bit gid
+		if gid, err := strconv.ParseInt(idparts[1], 10, 32); err == nil {
+			// must be a gid, take it as valid
+			groupID = int(gid)
+			lgrp, err := user.LookupGid(groupID)
+			if err != nil {
+				return "", "", fmt.Errorf("Gid %d has no entry in /etc/passwd: %v", groupID, err)
+			}
+			groupname = lgrp.Name
+		} else {
+			// not a number; attempt a lookup
+			group, err := user.LookupGroup(idparts[1])
+			if err != nil {
+				return "", "", fmt.Errorf("Error during gid lookup for %q: %v", idparts[1], err)
+			}
+			groupID = group.Gid
+			groupname = idparts[1]
+		}
+	}
+	return username, groupname, nil
+}