libcontainer: add parser for /etc/sub{u,g}id and /proc/PID/{u,g}id_map

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

libcontainer/system/linux.go

@@ -3,13 +3,11 @@
 package system
 
 import (
-	"bufio"
-	"fmt"
-	"os"
 	"os/exec"
 	"syscall" // only for exec
 	"unsafe"
 
+	"github.com/opencontainers/runc/libcontainer/user"
 	"golang.org/x/sys/unix"
 )
 
@@ -102,29 +100,22 @@ func Setctty() error {
 }
 
 // RunningInUserNS detects whether we are currently running in a user namespace.
-// Copied from github.com/lxc/lxd/shared/util.go
+// Originally copied from github.com/lxc/lxd/shared/util.go
 func RunningInUserNS() bool {
-	file, err := os.Open("/proc/self/uid_map")
+	uidmap, err := user.CurrentProcessUIDMap()
 	if err != nil {
 		// This kernel-provided file only exists if user namespaces are supported
 		return false
 	}
-	defer file.Close()
+	return UIDMapInUserNS(uidmap)
+}
 
-	buf := bufio.NewReader(file)
-	l, _, err := buf.ReadLine()
-	if err != nil {
-		return false
-	}
-
-	line := string(l)
-	var a, b, c int64
-	fmt.Sscanf(line, "%d %d %d", &a, &b, &c)
+func UIDMapInUserNS(uidmap []user.IDMap) bool {
 	/*
 	 * We assume we are in the initial user namespace if we have a full
 	 * range - 4294967295 uids starting at uid 0.
 	 */
-	if a == 0 && b == 0 && c == 4294967295 {
+	if len(uidmap) == 1 && uidmap[0].ID == 0 && uidmap[0].ParentID == 0 && uidmap[0].Count == 4294967295 {
 		return false
 	}
 	return true

libcontainer/system/linux_test.go

@@ -0,0 +1,45 @@
+// +build linux
+
+package system
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/opencontainers/runc/libcontainer/user"
+)
+
+func TestUIDMapInUserNS(t *testing.T) {
+	cases := []struct {
+		s        string
+		expected bool
+	}{
+		{
+			s:        "         0          0 4294967295\n",
+			expected: false,
+		},
+		{
+			s:        "         0          0          1\n",
+			expected: true,
+		},
+		{
+			s:        "         0       1001          1\n         1     231072      65536\n",
+			expected: true,
+		},
+		{
+			// file exist but empty (the initial state when userns is created. see man 7 user_namespaces)
+			s:        "",
+			expected: true,
+		},
+	}
+	for _, c := range cases {
+		uidmap, err := user.ParseIDMap(strings.NewReader(c.s))
+		if err != nil {
+			t.Fatal(err)
+		}
+		actual := UIDMapInUserNS(uidmap)
+		if c.expected != actual {
+			t.Fatalf("expected %v, got %v for %q", c.expected, actual, c.s)
+		}
+	}
+}

libcontainer/system/unsupported.go

@@ -2,8 +2,16 @@
 
 package system
 
+import "github.com/opencontainers/runc/libcontainer/user"
+
 // RunningInUserNS is a stub for non-Linux systems
 // Always returns false
 func RunningInUserNS() bool {
 	return false
 }
+
+// UIDMapInUserNS is a stub for non-Linux systems
+// Always returns false
+func UIDMapInUserNS(uidmap []user.IDMap) bool {
+	return false
+}

libcontainer/user/lookup_unix.go

@@ -114,3 +114,29 @@ func CurrentUser() (User, error) {
 func CurrentGroup() (Group, error) {
 	return LookupGid(unix.Getgid())
 }
+
+func CurrentUserSubUIDs() ([]SubID, error) {
+	u, err := CurrentUser()
+	if err != nil {
+		return nil, err
+	}
+	return ParseSubIDFileFilter("/etc/subuid",
+		func(entry SubID) bool { return entry.Name == u.Name })
+}
+
+func CurrentGroupSubGIDs() ([]SubID, error) {
+	g, err := CurrentGroup()
+	if err != nil {
+		return nil, err
+	}
+	return ParseSubIDFileFilter("/etc/subgid",
+		func(entry SubID) bool { return entry.Name == g.Name })
+}
+
+func CurrentProcessUIDMap() ([]IDMap, error) {
+	return ParseIDMapFile("/proc/self/uid_map")
+}
+
+func CurrentProcessGIDMap() ([]IDMap, error) {
+	return ParseIDMapFile("/proc/self/gid_map")
+}

libcontainer/user/user.go

@@ -75,12 +75,29 @@ func groupFromOS(g *user.Group) (Group, error) {
 	return newGroup, nil
 }
 
+// SubID represents an entry in /etc/sub{u,g}id
+type SubID struct {
+	Name  string
+	SubID int
+	Count int
+}
+
+// IDMap represents an entry in /proc/PID/{u,g}id_map
+type IDMap struct {
+	ID       int
+	ParentID int
+	Count    int
+}
+
 func parseLine(line string, v ...interface{}) {
-	if line == "" {
+	parseParts(strings.Split(line, ":"), v...)
+}
+
+func parseParts(parts []string, v ...interface{}) {
+	if len(parts) == 0 {
 		return
 	}
 
-	parts := strings.Split(line, ":")
 	for i, p := range parts {
 		// Ignore cases where we don't have enough fields to populate the arguments.
 		// Some configuration files like to misbehave.
@@ -479,3 +496,111 @@ func GetAdditionalGroupsPath(additionalGroups []string, groupPath string) ([]int
 	}
 	return GetAdditionalGroups(additionalGroups, group)
 }
+
+func ParseSubIDFile(path string) ([]SubID, error) {
+	subid, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer subid.Close()
+	return ParseSubID(subid)
+}
+
+func ParseSubID(subid io.Reader) ([]SubID, error) {
+	return ParseSubIDFilter(subid, nil)
+}
+
+func ParseSubIDFileFilter(path string, filter func(SubID) bool) ([]SubID, error) {
+	subid, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer subid.Close()
+	return ParseSubIDFilter(subid, filter)
+}
+
+func ParseSubIDFilter(r io.Reader, filter func(SubID) bool) ([]SubID, error) {
+	if r == nil {
+		return nil, fmt.Errorf("nil source for subid-formatted data")
+	}
+
+	var (
+		s   = bufio.NewScanner(r)
+		out = []SubID{}
+	)
+
+	for s.Scan() {
+		if err := s.Err(); err != nil {
+			return nil, err
+		}
+
+		line := strings.TrimSpace(s.Text())
+		if line == "" {
+			continue
+		}
+
+		// see: man 5 subuid
+		p := SubID{}
+		parseLine(line, &p.Name, &p.SubID, &p.Count)
+
+		if filter == nil || filter(p) {
+			out = append(out, p)
+		}
+	}
+
+	return out, nil
+}
+
+func ParseIDMapFile(path string) ([]IDMap, error) {
+	r, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer r.Close()
+	return ParseIDMap(r)
+}
+
+func ParseIDMap(r io.Reader) ([]IDMap, error) {
+	return ParseIDMapFilter(r, nil)
+}
+
+func ParseIDMapFileFilter(path string, filter func(IDMap) bool) ([]IDMap, error) {
+	r, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer r.Close()
+	return ParseIDMapFilter(r, filter)
+}
+
+func ParseIDMapFilter(r io.Reader, filter func(IDMap) bool) ([]IDMap, error) {
+	if r == nil {
+		return nil, fmt.Errorf("nil source for idmap-formatted data")
+	}
+
+	var (
+		s   = bufio.NewScanner(r)
+		out = []IDMap{}
+	)
+
+	for s.Scan() {
+		if err := s.Err(); err != nil {
+			return nil, err
+		}
+
+		line := strings.TrimSpace(s.Text())
+		if line == "" {
+			continue
+		}
+
+		// see: man 7 user_namespaces
+		p := IDMap{}
+		parseParts(strings.Fields(line), &p.ID, &p.ParentID, &p.Count)
+
+		if filter == nil || filter(p) {
+			out = append(out, p)
+		}
+	}
+
+	return out, nil
+}