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chroot.go
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chroot.go
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// Copyright 2015 The rkt Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package tar
import (
"archive/tar"
"encoding/json"
"fmt"
"io"
"io/ioutil"
"os"
"path/filepath"
"strconv"
"syscall"
"github.com/coreos/rkt/pkg/multicall"
"github.com/coreos/rkt/pkg/sys"
)
const (
multicallName = "extracttar"
fileMapFdNum = 3
)
var mcEntrypoint multicall.Entrypoint
func init() {
mcEntrypoint = multicall.Add(multicallName, extractTarCommand)
}
func extractTarCommand() error {
if len(os.Args) != 3 {
return fmt.Errorf("incorrect number of arguments. Usage: %s DIR {true|false}", multicallName)
}
if !sys.HasChrootCapability() {
return fmt.Errorf("chroot capability not available.")
}
dir := os.Args[1]
if !filepath.IsAbs(dir) {
return fmt.Errorf("dir %s must be an absolute path", dir)
}
overwrite, err := strconv.ParseBool(os.Args[2])
if err != nil {
return fmt.Errorf("error parsing overwrite argument: %v", err)
}
if err := syscall.Chroot(dir); err != nil {
return fmt.Errorf("failed to chroot in %s: %v", dir, err)
}
if err := syscall.Chdir("/"); err != nil {
return fmt.Errorf("failed to chdir: %v", err)
}
fileMapFile := os.NewFile(uintptr(fileMapFdNum), "fileMap")
fileMap := map[string]struct{}{}
if err := json.NewDecoder(fileMapFile).Decode(&fileMap); err != nil {
return fmt.Errorf("error decoding fileMap: %v", err)
}
if err := extractTar(tar.NewReader(os.Stdin), overwrite, fileMap); err != nil {
return fmt.Errorf("error extracting tar: %v", err)
}
// flush remaining bytes
io.Copy(ioutil.Discard, os.Stdin)
return nil
}
// ExtractTar extracts a tarball (from a io.Reader) into the given directory
// if pwl is not nil, only the paths in the map are extracted.
// If overwrite is true, existing files will be overwritten.
// The extraction is executed by fork/exec()ing a new process. The new process
// needs the CAP_SYS_CHROOT capability.
func ExtractTar(rs io.Reader, dir string, overwrite bool, pwl PathWhitelistMap) error {
r, w, err := os.Pipe()
if err != nil {
return err
}
defer w.Close()
enc := json.NewEncoder(w)
cmd := mcEntrypoint.Cmd(dir, strconv.FormatBool(overwrite))
cmd.ExtraFiles = []*os.File{r}
cmd.Stdin = rs
encodeCh := make(chan error)
go func() {
encodeCh <- enc.Encode(pwl)
}()
out, err := cmd.CombinedOutput()
// read from blocking encodeCh to release the goroutine
encodeErr := <-encodeCh
if err != nil {
return fmt.Errorf("extracttar error: %v, output: %s", err, out)
}
if encodeErr != nil {
return fmt.Errorf("extracttar failed to json encode filemap: %v", encodeErr)
}
return nil
}