moby/pkg/chrootarchive/archive_unix.go

// +build !windows

package chrootarchive

import (
	"bytes"
	"encoding/json"
	"flag"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"runtime"
	"syscall"

	"github.com/docker/docker/pkg/archive"
	"github.com/docker/docker/pkg/reexec"
)

func chroot(path string) error {
	if err := syscall.Chroot(path); err != nil {
		return err
	}
	return syscall.Chdir("/")
}

// untar is the entry-point for docker-untar on re-exec. This is not used on
// Windows as it does not support chroot, hence no point sandboxing through
// chroot and rexec.
func untar() {
	runtime.LockOSThread()
	flag.Parse()

	var options *archive.TarOptions

	//read the options from the pipe "ExtraFiles"
	if err := json.NewDecoder(os.NewFile(3, "options")).Decode(&options); err != nil {
		fatal(err)
	}

	if err := chroot(flag.Arg(0)); err != nil {
		fatal(err)
	}

	if err := archive.Unpack(os.Stdin, "/", options); err != nil {
		fatal(err)
	}
	// fully consume stdin in case it is zero padded
	flush(os.Stdin)
	os.Exit(0)
}

func invokeUnpack(decompressedArchive io.Reader, dest string, options *archive.TarOptions) error {

	// We can't pass a potentially large exclude list directly via cmd line
	// because we easily overrun the kernel's max argument/environment size
	// when the full image list is passed (e.g. when this is used by
	// `docker load`). We will marshall the options via a pipe to the
	// child
	r, w, err := os.Pipe()
	if err != nil {
		return fmt.Errorf("Untar pipe failure: %v", err)
	}

	cmd := reexec.Command("docker-untar", dest)
	cmd.Stdin = decompressedArchive

	cmd.ExtraFiles = append(cmd.ExtraFiles, r)
	output := bytes.NewBuffer(nil)
	cmd.Stdout = output
	cmd.Stderr = output

	if err := cmd.Start(); err != nil {
		return fmt.Errorf("Untar error on re-exec cmd: %v", err)
	}
	//write the options to the pipe for the untar exec to read
	if err := json.NewEncoder(w).Encode(options); err != nil {
		return fmt.Errorf("Untar json encode to pipe failed: %v", err)
	}
	w.Close()

	if err := cmd.Wait(); err != nil {
		// when `xz -d -c -q | docker-untar ...` failed on docker-untar side,
		// we need to exhaust `xz`'s output, otherwise the `xz` side will be
		// pending on write pipe forever
		io.Copy(ioutil.Discard, decompressedArchive)

		return fmt.Errorf("Untar re-exec error: %v: output: %s", err, output)
	}
	return nil
}