linuxkit/src/cmd/linuxkit/initrd/initrd.go

package initrd

import (
	"archive/tar"
	"errors"
	"io"
	"path/filepath"
	"strings"

	// drop-in 100% compatible replacement and 17% faster than compress/gzip.
	gzip "github.com/klauspost/pgzip"
	"github.com/linuxkit/linuxkit/src/cmd/linuxkit/pad4"
	"github.com/surma/gocpio"
)

// Writer is an io.WriteCloser that writes to an initrd
// This is a compressed cpio archive, zero padded to 4 bytes
type Writer struct {
	pw *pad4.Writer
	gw *gzip.Writer
	cw *cpio.Writer
}

func typeconv(thdr *tar.Header) int64 {
	switch thdr.Typeflag {
	case tar.TypeReg:
		return cpio.TYPE_REG
	case tar.TypeRegA:
		return cpio.TYPE_REG
	// Currently hard links not supported very well :)
	// Convert to relative symlink as absolute will not work in container
	// cpio does support hardlinks but file contents still duplicated, so rely
	// on compression to fix that which is fairly ugly. Symlink has not caused issues.
	case tar.TypeLink:
		dir := filepath.Dir(thdr.Name)
		rel, err := filepath.Rel(dir, thdr.Linkname)
		if err != nil {
			// should never happen, but leave as full abs path
			rel = "/" + thdr.Linkname
		}
		thdr.Linkname = rel
		return cpio.TYPE_SYMLINK
	case tar.TypeSymlink:
		return cpio.TYPE_SYMLINK
	case tar.TypeChar:
		return cpio.TYPE_CHAR
	case tar.TypeBlock:
		return cpio.TYPE_BLK
	case tar.TypeDir:
		return cpio.TYPE_DIR
	case tar.TypeFifo:
		return cpio.TYPE_FIFO
	default:
		return -1
	}
}

func copyTarEntry(w *Writer, thdr *tar.Header, r io.Reader) (written int64, err error) {
	tp := typeconv(thdr)
	if tp == -1 {
		return written, errors.New("cannot convert tar file")
	}
	size := thdr.Size
	if tp == cpio.TYPE_SYMLINK {
		size = int64(len(thdr.Linkname))
	}
	chdr := cpio.Header{
		Mode:     thdr.Mode,
		Uid:      thdr.Uid,
		Gid:      thdr.Gid,
		Mtime:    thdr.ModTime.Unix(),
		Size:     size,
		Devmajor: thdr.Devmajor,
		Devminor: thdr.Devminor,
		Type:     tp,
		Name:     thdr.Name,
	}
	err = w.WriteHeader(&chdr)
	if err != nil {
		return
	}
	var n int64
	switch tp {
	case cpio.TYPE_SYMLINK:
		var count int
		count, err = w.Write([]byte(thdr.Linkname))
		n = int64(count)
	case cpio.TYPE_REG:
		n, err = io.Copy(w, r)
	}
	written += n

	return
}

// CopySplitTar copies a tar stream into an initrd, but splits out kernel, cmdline, and ucode
func CopySplitTar(w *Writer, r *tar.Reader) (kernel []byte, cmdline string, ucode []byte, err error) {
	for {
		var thdr *tar.Header
		thdr, err = r.Next()
		if err == io.EOF {
			return kernel, cmdline, ucode, nil
		}
		if err != nil {
			return
		}
		switch {
		case thdr.Name == "boot/kernel":
			kernel, err = io.ReadAll(r)
			if err != nil {
				return
			}
		case thdr.Name == "boot/cmdline":
			var buf []byte
			buf, err = io.ReadAll(r)
			if err != nil {
				return
			}
			cmdline = string(buf)
		case thdr.Name == "boot/ucode.cpio":
			ucode, err = io.ReadAll(r)
			if err != nil {
				return
			}
		case strings.HasPrefix(thdr.Name, "boot/"):
			// skip the rest of ./boot
		default:
			_, err = copyTarEntry(w, thdr, r)
			if err != nil {
				return
			}
		}
	}
}

// NewWriter creates a writer that will output an initrd stream
func NewWriter(w io.Writer) *Writer {
	initrd := new(Writer)
	initrd.pw = pad4.NewWriter(w)
	initrd.gw = gzip.NewWriter(initrd.pw)
	initrd.cw = cpio.NewWriter(initrd.gw)

	return initrd
}

// WriteHeader writes a cpio header into an initrd
func (w *Writer) WriteHeader(hdr *cpio.Header) error {
	return w.cw.WriteHeader(hdr)
}

// Write writes a cpio file into an initrd
func (w *Writer) Write(b []byte) (n int, e error) {
	return w.cw.Write(b)
}

// Close closes the writer
func (w *Writer) Close() error {
	err1 := w.cw.Close()
	err2 := w.gw.Close()
	err3 := w.pw.Close()
	if err1 != nil {
		return err1
	}
	if err2 != nil {
		return err2
	}
	if err3 != nil {
		return err3
	}
	return nil
}