github.com/iaintshine/docker@v1.8.2/pkg/chrootarchive/archive_unix.go (about)

     1  // +build !windows
     2  
     3  package chrootarchive
     4  
     5  import (
     6  	"bytes"
     7  	"encoding/json"
     8  	"flag"
     9  	"fmt"
    10  	"io"
    11  	"os"
    12  	"runtime"
    13  	"syscall"
    14  
    15  	"github.com/docker/docker/pkg/archive"
    16  	"github.com/docker/docker/pkg/reexec"
    17  )
    18  
    19  func chroot(path string) error {
    20  	if err := syscall.Chroot(path); err != nil {
    21  		return err
    22  	}
    23  	return syscall.Chdir("/")
    24  }
    25  
    26  // untar is the entry-point for docker-untar on re-exec. This is not used on
    27  // Windows as it does not support chroot, hence no point sandboxing through
    28  // chroot and rexec.
    29  func untar() {
    30  	runtime.LockOSThread()
    31  	flag.Parse()
    32  
    33  	var options *archive.TarOptions
    34  
    35  	//read the options from the pipe "ExtraFiles"
    36  	if err := json.NewDecoder(os.NewFile(3, "options")).Decode(&options); err != nil {
    37  		fatal(err)
    38  	}
    39  
    40  	if err := chroot(flag.Arg(0)); err != nil {
    41  		fatal(err)
    42  	}
    43  
    44  	if err := archive.Unpack(os.Stdin, "/", options); err != nil {
    45  		fatal(err)
    46  	}
    47  	// fully consume stdin in case it is zero padded
    48  	flush(os.Stdin)
    49  	os.Exit(0)
    50  }
    51  
    52  func invokeUnpack(decompressedArchive io.Reader, dest string, options *archive.TarOptions) error {
    53  
    54  	// We can't pass a potentially large exclude list directly via cmd line
    55  	// because we easily overrun the kernel's max argument/environment size
    56  	// when the full image list is passed (e.g. when this is used by
    57  	// `docker load`). We will marshall the options via a pipe to the
    58  	// child
    59  	r, w, err := os.Pipe()
    60  	if err != nil {
    61  		return fmt.Errorf("Untar pipe failure: %v", err)
    62  	}
    63  
    64  	cmd := reexec.Command("docker-untar", dest)
    65  	cmd.Stdin = decompressedArchive
    66  
    67  	cmd.ExtraFiles = append(cmd.ExtraFiles, r)
    68  	output := bytes.NewBuffer(nil)
    69  	cmd.Stdout = output
    70  	cmd.Stderr = output
    71  
    72  	if err := cmd.Start(); err != nil {
    73  		return fmt.Errorf("Untar error on re-exec cmd: %v", err)
    74  	}
    75  	//write the options to the pipe for the untar exec to read
    76  	if err := json.NewEncoder(w).Encode(options); err != nil {
    77  		return fmt.Errorf("Untar json encode to pipe failed: %v", err)
    78  	}
    79  	w.Close()
    80  
    81  	if err := cmd.Wait(); err != nil {
    82  		return fmt.Errorf("Untar re-exec error: %v: output: %s", err, output)
    83  	}
    84  	return nil
    85  }