forked from u-root/u-root
/
chroot.go
201 lines (164 loc) · 3.69 KB
/
chroot.go
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// Copyright 2018 the u-root Authors. All rights reserved
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build !windows !plan9
package main
import (
"bytes"
"flag"
"fmt"
"log"
"os"
"os/exec"
"path/filepath"
"strconv"
"strings"
"syscall"
)
type userSpec struct {
uid uint32
gid uint32
}
func (u *userSpec) Set(s string) error {
var err error
userspecSplit := strings.Split(s, ":")
if len(userspecSplit) != 2 || userspecSplit[1] == "" {
return fmt.Errorf("expected user spec flag to be \":\" separated values received %s", s)
}
u.uid, err = stringToUint32(userspecSplit[0])
if err != nil {
return err
}
u.gid, err = stringToUint32(userspecSplit[1])
if err != nil {
return err
}
return nil
}
func (u *userSpec) Get() interface{} {
return *u
}
func (u *userSpec) String() string {
return fmt.Sprintf("%d:%d", u.uid, u.gid)
}
func defaultUser() userSpec {
return userSpec{
uid: uint32(os.Getuid()),
gid: uint32(os.Getgid()),
}
}
type groupsSpec struct {
groups []uint32
}
func (g *groupsSpec) Set(s string) error {
groupStrs := strings.Split(s, ",")
g.groups = make([]uint32, len(groupStrs))
for index, group := range groupStrs {
gid, err := stringToUint32(group)
if err != nil {
return err
}
g.groups[index] = gid
}
return nil
}
func (g *groupsSpec) Get() interface{} {
return *g
}
func (g *groupsSpec) String() string {
var buffer bytes.Buffer
for index, gid := range g.groups {
buffer.WriteString(fmt.Sprint(gid))
if index < len(g.groups)-1 {
buffer.WriteString(",")
}
}
return buffer.String()
}
var (
skipchdirFlag bool
user = defaultUser()
groups = groupsSpec{}
)
func init() {
flag.Var(&user, "u", "specify user and group (ID only) as USER:GROUP")
flag.Var(&groups, "g", "specify supplementary group ids as g1,g2,..,gN")
flag.BoolVar(&skipchdirFlag, "s", false, fmt.Sprint("Use this option to not change",
"the working directory to / after changing the root directory to newroot, i.e., ",
"inside the chroot. This option is only permitted when newroot is the old / directory."))
}
func stringToUint32(str string) (uint32, error) {
ret, err := strconv.ParseUint(str, 10, 32)
if err != nil {
return 0, err
}
return uint32(ret), nil
}
func parseCommand(args []string) []string {
if len(args) > 1 {
return args[1:]
}
return []string{"/bin/sh", "-i"}
}
func parseRoot(args []string) (root string, err error) {
if len(args) < 1 {
return "", fmt.Errorf("missing operand")
}
return filepath.Abs(args[0])
}
func isRoot(dir string) (bool, error) {
realPath, err := filepath.EvalSymlinks(dir)
if err != nil {
return false, err
}
absolutePath, err := filepath.Abs(realPath)
if err != nil {
return false, err
}
if absolutePath == "/" {
return true, nil
}
return false, nil
}
func main() {
var (
newRoot string
isOldroot bool
err error
)
flag.Parse()
if flag.NFlag() == 0 && flag.NArg() == 0 {
flag.PrintDefaults()
os.Exit(1)
}
newRoot, err = parseRoot(flag.Args())
if err != nil {
log.Fatal(err)
}
isOldroot, err = isRoot(newRoot)
if err != nil {
log.Fatal(err)
}
if !skipchdirFlag {
err = os.Chdir(newRoot)
if err != nil {
log.Fatal(err)
}
} else if !isOldroot {
log.Fatal("The -s option is only permitted when newroot is the old / directory")
}
argv := parseCommand(flag.Args())
cmd := exec.Command(argv[0], argv[1:]...)
cmd.Stdin, cmd.Stdout, cmd.Stderr = os.Stdin, os.Stdout, os.Stderr
cmd.SysProcAttr = &syscall.SysProcAttr{
Credential: &syscall.Credential{
Uid: user.uid,
Gid: user.gid,
Groups: groups.groups,
},
Chroot: newRoot,
}
if err = cmd.Run(); err != nil {
log.Fatal(err)
}
}