3 files changed, 325 insertions, 0 deletions
diff --git a/pkg/chrootuser/user.go b/pkg/chrootuser/user.go
new file mode 100644
index 000000000..a024877a5
--- /dev/null
+++ b/pkg/chrootuser/user.go
@@ -0,0 +1,71 @@
+package chrootuser
+
+import (
+	"os/user"
+	"strconv"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+// GetUser will return the uid, gid of the user specified in the userspec
+// it will use the /etc/password and /etc/shadow files inside of the rootdir
+// to return this information.
+// userspace format [user | user:group | uid | uid:gid | user:gid | uid:group ]
+func GetUser(rootdir, userspec string) (uint32, uint32, error) {
+	var gid64 uint64
+	var gerr error = user.UnknownGroupError("error looking up group")
+
+	spec := strings.SplitN(userspec, ":", 2)
+	userspec = spec[0]
+	groupspec := ""
+	if userspec == "" {
+		return 0, 0, nil
+	}
+	if len(spec) > 1 {
+		groupspec = spec[1]
+	}
+
+	uid64, uerr := strconv.ParseUint(userspec, 10, 32)
+	if uerr == nil && groupspec == "" {
+		// We parsed the user name as a number, and there's no group
+		// component, so try to look up the primary GID of the user who
+		// has this UID.
+		var name string
+		name, gid64, gerr = lookupGroupForUIDInContainer(rootdir, uid64)
+		if gerr == nil {
+			userspec = name
+		} else {
+			// Leave userspec alone, but swallow the error and just
+			// use GID 0.
+			gid64 = 0
+			gerr = nil
+		}
+	}
+	if uerr != nil {
+		// The user ID couldn't be parsed as a number, so try to look
+		// up the user's UID and primary GID.
+		uid64, gid64, uerr = lookupUserInContainer(rootdir, userspec)
+		gerr = uerr
+	}
+
+	if groupspec != "" {
+		// We have a group name or number, so parse it.
+		gid64, gerr = strconv.ParseUint(groupspec, 10, 32)
+		if gerr != nil {
+			// The group couldn't be parsed as a number, so look up
+			// the group's GID.
+			gid64, gerr = lookupGroupInContainer(rootdir, groupspec)
+		}
+	}
+
+	if uerr == nil && gerr == nil {
+		return uint32(uid64), uint32(gid64), nil
+	}
+
+	err := errors.Wrapf(uerr, "error determining run uid")
+	if uerr == nil {
+		err = errors.Wrapf(gerr, "error determining run gid")
+	}
+	return 0, 0, err
+}
diff --git a/pkg/chrootuser/user_basic.go b/pkg/chrootuser/user_basic.go
new file mode 100644
index 000000000..4f89af557
--- /dev/null
+++ b/pkg/chrootuser/user_basic.go
@@ -0,0 +1,19 @@
+// +build !linux
+
+package chrootuser
+
+import (
+	"github.com/pkg/errors"
+)
+
+func lookupUserInContainer(rootdir, username string) (uint64, uint64, error) {
+	return 0, 0, errors.New("user lookup not supported")
+}
+
+func lookupGroupInContainer(rootdir, groupname string) (uint64, error) {
+	return 0, errors.New("group lookup not supported")
+}
+
+func lookupGroupForUIDInContainer(rootdir string, userid uint64) (string, uint64, error) {
+	return "", 0, errors.New("primary group lookup by uid not supported")
+}
diff --git a/pkg/chrootuser/user_linux.go b/pkg/chrootuser/user_linux.go
new file mode 100644
index 000000000..2baf9ea33
--- /dev/null
+++ b/pkg/chrootuser/user_linux.go
@@ -0,0 +1,235 @@
+// +build linux
+
+package chrootuser
+
+import (
+	"bufio"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"os/user"
+	"strconv"
+	"strings"
+	"sync"
+
+	"github.com/containers/storage/pkg/reexec"
+	"github.com/sirupsen/logrus"
+	"golang.org/x/sys/unix"
+)
+
+const (
+	openChrootedCommand = "chrootuser-open"
+)
+
+func init() {
+	reexec.Register(openChrootedCommand, openChrootedFileMain)
+}
+
+func openChrootedFileMain() {
+	status := 0
+	flag.Parse()
+	if len(flag.Args()) < 1 {
+		os.Exit(1)
+	}
+	// Our first parameter is the directory to chroot into.
+	if err := unix.Chdir(flag.Arg(0)); err != nil {
+		fmt.Fprintf(os.Stderr, "chdir(): %v", err)
+		os.Exit(1)
+	}
+	if err := unix.Chroot(flag.Arg(0)); err != nil {
+		fmt.Fprintf(os.Stderr, "chroot(): %v", err)
+		os.Exit(1)
+	}
+	// Anything else is a file we want to dump out.
+	for _, filename := range flag.Args()[1:] {
+		f, err := os.Open(filename)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "open(%q): %v", filename, err)
+			status = 1
+			continue
+		}
+		_, err = io.Copy(os.Stdout, f)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "read(%q): %v", filename, err)
+		}
+		f.Close()
+	}
+	os.Exit(status)
+}
+
+func openChrootedFile(rootdir, filename string) (*exec.Cmd, io.ReadCloser, error) {
+	// The child process expects a chroot and one or more filenames that
+	// will be consulted relative to the chroot directory and concatenated
+	// to its stdout.  Start it up.
+	cmd := reexec.Command(openChrootedCommand, rootdir, filename)
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return nil, nil, err
+	}
+	err = cmd.Start()
+	if err != nil {
+		return nil, nil, err
+	}
+	// Hand back the child's stdout for reading, and the child to reap.
+	return cmd, stdout, nil
+}
+
+var (
+	lookupUser, lookupGroup sync.Mutex
+)
+
+type lookupPasswdEntry struct {
+	name string
+	uid  uint64
+	gid  uint64
+}
+type lookupGroupEntry struct {
+	name string
+	gid  uint64
+}
+
+func readWholeLine(rc *bufio.Reader) ([]byte, error) {
+	line, isPrefix, err := rc.ReadLine()
+	if err != nil {
+		return nil, err
+	}
+	for isPrefix {
+		// We didn't get a whole line.  Keep reading chunks until we find an end of line, and discard them.
+		for isPrefix {
+			logrus.Debugf("discarding partial line %q", string(line))
+			_, isPrefix, err = rc.ReadLine()
+			if err != nil {
+				return nil, err
+			}
+		}
+		// That last read was the end of a line, so now we try to read the (beginning of?) the next line.
+		line, isPrefix, err = rc.ReadLine()
+		if err != nil {
+			return nil, err
+		}
+	}
+	return line, nil
+}
+
+func parseNextPasswd(rc *bufio.Reader) *lookupPasswdEntry {
+	line, err := readWholeLine(rc)
+	if err != nil {
+		return nil
+	}
+	fields := strings.Split(string(line), ":")
+	if len(fields) < 7 {
+		return nil
+	}
+	uid, err := strconv.ParseUint(fields[2], 10, 32)
+	if err != nil {
+		return nil
+	}
+	gid, err := strconv.ParseUint(fields[3], 10, 32)
+	if err != nil {
+		return nil
+	}
+	return &lookupPasswdEntry{
+		name: fields[0],
+		uid:  uid,
+		gid:  gid,
+	}
+}
+
+func parseNextGroup(rc *bufio.Reader) *lookupGroupEntry {
+	line, err := readWholeLine(rc)
+	if err != nil {
+		return nil
+	}
+	fields := strings.Split(string(line), ":")
+	if len(fields) < 4 {
+		return nil
+	}
+	gid, err := strconv.ParseUint(fields[2], 10, 32)
+	if err != nil {
+		return nil
+	}
+	return &lookupGroupEntry{
+		name: fields[0],
+		gid:  gid,
+	}
+}
+
+func lookupUserInContainer(rootdir, username string) (uid uint64, gid uint64, err error) {
+	cmd, f, err := openChrootedFile(rootdir, "/etc/passwd")
+	if err != nil {
+		return 0, 0, err
+	}
+	defer func() {
+		_ = cmd.Wait()
+	}()
+	rc := bufio.NewReader(f)
+	defer f.Close()
+
+	lookupUser.Lock()
+	defer lookupUser.Unlock()
+
+	pwd := parseNextPasswd(rc)
+	for pwd != nil {
+		if pwd.name != username {
+			pwd = parseNextPasswd(rc)
+			continue
+		}
+		return pwd.uid, pwd.gid, nil
+	}
+
+	return 0, 0, user.UnknownUserError(fmt.Sprintf("error looking up user %q", username))
+}
+
+func lookupGroupForUIDInContainer(rootdir string, userid uint64) (username string, gid uint64, err error) {
+	cmd, f, err := openChrootedFile(rootdir, "/etc/passwd")
+	if err != nil {
+		return "", 0, err
+	}
+	defer func() {
+		_ = cmd.Wait()
+	}()
+	rc := bufio.NewReader(f)
+	defer f.Close()
+
+	lookupUser.Lock()
+	defer lookupUser.Unlock()
+
+	pwd := parseNextPasswd(rc)
+	for pwd != nil {
+		if pwd.uid != userid {
+			pwd = parseNextPasswd(rc)
+			continue
+		}
+		return pwd.name, pwd.gid, nil
+	}
+
+	return "", 0, user.UnknownUserError(fmt.Sprintf("error looking up user with UID %d", userid))
+}
+
+func lookupGroupInContainer(rootdir, groupname string) (gid uint64, err error) {
+	cmd, f, err := openChrootedFile(rootdir, "/etc/group")
+	if err != nil {
+		return 0, err
+	}
+	defer func() {
+		_ = cmd.Wait()
+	}()
+	rc := bufio.NewReader(f)
+	defer f.Close()
+
+	lookupGroup.Lock()
+	defer lookupGroup.Unlock()
+
+	grp := parseNextGroup(rc)
+	for grp != nil {
+		if grp.name != groupname {
+			grp = parseNextGroup(rc)
+			continue
+		}
+		return grp.gid, nil
+	}
+
+	return 0, user.UnknownGroupError(fmt.Sprintf("error looking up group %q", groupname))
+}