dotcloud/docker

package archive // import "github.com/docker/docker/pkg/archive"

import (
    "archive/tar"
    "os"
    "path/filepath"
    "strings"

    "github.com/docker/docker/pkg/system"
    "github.com/moby/sys/userns"
    "github.com/pkg/errors"
    "golang.org/x/sys/unix"
)

func getWhiteoutConverter(format WhiteoutFormat) tarWhiteoutConverter {
    if format == OverlayWhiteoutFormat {
        return overlayWhiteoutConverter{}
    }
    return nil
}

type overlayWhiteoutConverter struct{}

func (overlayWhiteoutConverter) ConvertWrite(hdr *tar.Header, path string, fi os.FileInfo) (wo *tar.Header, err error) {
    // convert whiteouts to AUFS format
    if fi.Mode()&os.ModeCharDevice != 0 && hdr.Devmajor == 0 && hdr.Devminor == 0 {
        // we just rename the file and make it normal
        dir, filename := filepath.Split(hdr.Name)
        hdr.Name = filepath.Join(dir, WhiteoutPrefix+filename)
        hdr.Mode = 0o600
        hdr.Typeflag = tar.TypeReg
        hdr.Size = 0
    }

    if fi.Mode()&os.ModeDir != 0 {
        opaqueXattrName := "trusted.overlay.opaque"
        if userns.RunningInUserNS() {
            opaqueXattrName = "user.overlay.opaque"
        }

        // convert opaque dirs to AUFS format by writing an empty file with the prefix
        opaque, err := system.Lgetxattr(path, opaqueXattrName)
        if err != nil {
            return nil, err
        }
        if len(opaque) == 1 && opaque[0] == 'y' {
            delete(hdr.PAXRecords, paxSchilyXattr+opaqueXattrName)

            // create a header for the whiteout file
            // it should inherit some properties from the parent, but be a regular file
            wo = &tar.Header{
                Typeflag:   tar.TypeReg,
                Mode:       hdr.Mode & int64(os.ModePerm),
                Name:       filepath.Join(hdr.Name, WhiteoutOpaqueDir), // #nosec G305 -- An archive is being created, not extracted.
                Size:       0,
                Uid:        hdr.Uid,
                Uname:      hdr.Uname,
                Gid:        hdr.Gid,
                Gname:      hdr.Gname,
                AccessTime: hdr.AccessTime,
                ChangeTime: hdr.ChangeTime,
            }
        }
    }

    return
}

func (c overlayWhiteoutConverter) ConvertRead(hdr *tar.Header, path string) (bool, error) {
    base := filepath.Base(path)
    dir := filepath.Dir(path)

    // if a directory is marked as opaque by the AUFS special file, we need to translate that to overlay
    if base == WhiteoutOpaqueDir {
        opaqueXattrName := "trusted.overlay.opaque"
        if userns.RunningInUserNS() {
            opaqueXattrName = "user.overlay.opaque"
        }

        err := unix.Setxattr(dir, opaqueXattrName, []byte{'y'}, 0)
        if err != nil {
            return false, errors.Wrapf(err, "setxattr(%q, %s=y)", dir, opaqueXattrName)
        }
        // don't write the file itself
        return false, err
    }

    // if a file was deleted and we are using overlay, we need to create a character device
    if strings.HasPrefix(base, WhiteoutPrefix) {
        originalBase := base[len(WhiteoutPrefix):]
        originalPath := filepath.Join(dir, originalBase)

        if err := unix.Mknod(originalPath, unix.S_IFCHR, 0); err != nil {
            return false, errors.Wrapf(err, "failed to mknod(%q, S_IFCHR, 0)", originalPath)
        }
        if err := os.Chown(originalPath, hdr.Uid, hdr.Gid); err != nil {
            return false, err
        }

        // don't write the file itself
        return false, nil
    }

    return true, nil
}