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writer.go
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/
writer.go
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// Package squashfs implements writing SquashFS file system images using zlib
// compression for data blocks (inodes and directory entries are written
// uncompressed for simplicity).
//
// Note that SquashFS requires directory entries to be sorted, i.e. files and
// directories need to be added in the correct order.
//
// This package intentionally only implements a subset of SquashFS. Notably,
// block devices, character devices, FIFOs, sockets and xattrs are not
// supported.
package squashfs
import (
"bytes"
"compress/zlib"
"encoding/binary"
"io"
"os"
"path/filepath"
"time"
"golang.org/x/sys/unix"
)
// inode contains a block number + offset within that block.
type inode int64
const (
zlibCompression = 1 + iota
lzmaCompression
lzoCompression
xzCompression
lz4Compression
// Note: I tried switching from zlib compression to zstd compression for
// gokrazy root file systems, but the effect was minimal (a 30 MB zlib root
// squashfs shrunk to a 29 MB zstd root squashfs). Given that the kernel
// must support zstd, too, which not all do (notably the Raspberry Pi
// foundation kernel does not enable zstd).
zstdCompression
)
const (
invalidFragment = 0xFFFFFFFF
invalidXattr = 0xFFFFFFFF
)
type superblock struct {
Magic uint32
Inodes uint32
MkfsTime int32
BlockSize uint32
Fragments uint32
Compression uint16
BlockLog uint16
Flags uint16
NoIds uint16
Major uint16
Minor uint16
RootInode inode
BytesUsed int64
IdTableStart int64
XattrIdTableStart int64
InodeTableStart int64
DirectoryTableStart int64
FragmentTableStart int64
LookupTableStart int64
}
const (
dirType = 1 + iota
fileType
symlinkType
blkdevType
chrdevType
fifoType
socketType
// The larger types are used for e.g. sparse files, xattrs, etc.
ldirType
lregType
lsymlinkType
lblkdevType
lchrdevType
lfifoType
lsocketType
)
type inodeHeader struct {
InodeType uint16
Mode uint16
Uid uint16
Gid uint16
Mtime int32
InodeNumber uint32
}
// fileType
type regInodeHeader struct {
inodeHeader
// full byte offset from the start of the file system, e.g. 96 for first
// file contents. Not using fragments limits us to 2^32-1-96 (≈ 4GiB) bytes
// of file contents.
StartBlock uint32
Fragment uint32
Offset uint32
FileSize uint32
// Followed by a uint32 array of compressed block sizes.
}
// symlinkType
type symlinkInodeHeader struct {
inodeHeader
Nlink uint32
SymlinkSize uint32
// Followed by a byte array of SymlinkSize bytes.
}
// chrdevType and blkdevType
type devInodeHeader struct {
inodeHeader
Nlink uint32
Rdev uint32
}
// fifoType and socketType
type ipcInodeHeader struct {
inodeHeader
Nlink uint32
}
// dirType
type dirInodeHeader struct {
inodeHeader
StartBlock uint32
Nlink uint32
FileSize uint16
Offset uint16
ParentInode uint32
}
// ldirType
type ldirInodeHeader struct {
inodeHeader
Nlink uint32
FileSize uint32
StartBlock uint32
ParentInode uint32
Icount uint16
Offset uint16
Xattr uint32
}
type dirHeader struct {
Count uint32
StartBlock uint32
InodeOffset uint32
}
type dirEntry struct {
Offset uint16
InodeNumber int16
EntryType uint16
Size uint16
// Followed by a byte array of Size bytes.
}
func writeIdTable(w io.WriteSeeker, ids []uint32) (start int64, err error) {
metaOff, err := w.Seek(0, io.SeekCurrent)
if err != nil {
return 0, err
}
var buf bytes.Buffer
if err := binary.Write(&buf, binary.LittleEndian, ids); err != nil {
return 0, err
}
if err := binary.Write(w, binary.LittleEndian, uint16(buf.Len())|0x8000); err != nil {
return 0, err
}
if _, err := io.Copy(w, &buf); err != nil {
return 0, err
}
off, err := w.Seek(0, io.SeekCurrent)
if err != nil {
return 0, err
}
return off, binary.Write(w, binary.LittleEndian, metaOff)
}
type fullDirEntry struct {
startBlock uint32
offset uint16
inodeNumber uint32
entryType uint16
name string
}
const (
magic = 0x73717368
dataBlockSize = 131072
metadataBlockSize = 8192
majorVersion = 4
minorVersion = 0
)
type Writer struct {
// Root represents the file system root. Like all directories, Flush must be
// called precisely once.
Root *Directory
w io.WriteSeeker
sb superblock
inodeBuf bytes.Buffer
dirBuf bytes.Buffer
writeInodeNumTo map[string][]int64
}
// TODO: document what this is doing and what it is used for
func slog(block uint32) uint16 {
for i := uint16(12); i <= 20; i++ {
if block == (1 << i) {
return i
}
}
return 0
}
// filesystemFlags returns flags for a SquashFS file system created by this
// package (disabling most features for now).
func filesystemFlags() uint16 {
const (
noI = 1 << iota // uncompressed metadata
noD // uncompressed data
_
noF // uncompressed fragments
noFrag // never use fragments
alwaysFrag // always use fragments
duplicateChecking // de-duplication
exportable // exportable via NFS
noX // uncompressed xattrs
noXattr // no xattrs
compopt // compressor-specific options present?
)
return noI | noF | noFrag | noX | noXattr
}
// NewWriter returns a Writer which will write a SquashFS file system image to w
// once Flush is called.
//
// Create new files and directories with the corresponding methods on the Root
// directory of the Writer.
//
// File data is written to w even before Flush is called.
func NewWriter(w io.WriteSeeker, mkfsTime time.Time) (*Writer, error) {
// Skip over superblock to the data area, we come back to the superblock
// when flushing.
if _, err := w.Seek(96, io.SeekStart); err != nil {
return nil, err
}
wr := &Writer{
w: w,
sb: superblock{
Magic: magic,
MkfsTime: int32(mkfsTime.Unix()),
BlockSize: dataBlockSize,
Fragments: 0,
Compression: zlibCompression,
BlockLog: slog(dataBlockSize),
Flags: filesystemFlags(),
NoIds: 1, // just one uid/gid mapping (for root)
Major: majorVersion,
Minor: minorVersion,
XattrIdTableStart: -1, // not present
LookupTableStart: -1, // not present
},
writeInodeNumTo: make(map[string][]int64),
}
wr.Root = &Directory{
w: wr,
name: "", // root
modTime: mkfsTime,
}
return wr, nil
}
// Directory represents a SquashFS directory.
type Directory struct {
w *Writer
name string
modTime time.Time
dirEntries []fullDirEntry
parent *Directory
}
func (d *Directory) path() string {
if d.parent == nil {
return d.name
}
return filepath.Join(d.parent.path(), d.name)
}
type file struct {
w *Writer
d *Directory
off int64
size uint32
name string
modTime time.Time
mode os.FileMode
// buf accumulates at least dataBlockSize bytes, at which point a new block
// is being written.
buf bytes.Buffer
// blocksizes stores, for each block of dataBlockSize bytes (uncompressed),
// the number of bytes the block compressed down to.
blocksizes []uint32
// compBuf is used for holding a block during compression to avoid memory
// allocations.
compBuf *bytes.Buffer
// zlibWriter is re-used for each compressed block
zlibWriter *zlib.Writer
}
// Directory creates a new directory with the specified name and modTime.
func (d *Directory) Directory(name string, modTime time.Time) *Directory {
return &Directory{
w: d.w,
name: name,
modTime: modTime,
parent: d,
}
}
// File creates a file with the specified name, modTime and mode. The returned
// io.WriterCloser must be closed after writing the file.
func (d *Directory) File(name string, modTime time.Time, mode os.FileMode) (io.WriteCloser, error) {
off, err := d.w.w.Seek(0, io.SeekCurrent)
if err != nil {
return nil, err
}
// zlib.BestSpeed results in only a 2x slow-down over no compression
// (compared to >4x slow-down with DefaultCompression), but generates
// results which are in the same ball park (10% larger).
zw, err := zlib.NewWriterLevel(nil, zlib.BestSpeed)
if err != nil {
return nil, err
}
return &file{
w: d.w,
d: d,
off: off,
name: name,
modTime: modTime,
mode: mode,
compBuf: bytes.NewBuffer(make([]byte, dataBlockSize)),
zlibWriter: zw,
}, nil
}
// Symlink creates a symbolic link from newname to oldname with the specified
// modTime and mode.
func (d *Directory) Symlink(oldname, newname string, modTime time.Time, mode os.FileMode) error {
startBlock := d.w.inodeBuf.Len() / metadataBlockSize
offset := d.w.inodeBuf.Len() - startBlock*metadataBlockSize
if err := binary.Write(&d.w.inodeBuf, binary.LittleEndian, symlinkInodeHeader{
inodeHeader: inodeHeader{
InodeType: symlinkType,
Mode: uint16(mode),
Uid: 0,
Gid: 0,
Mtime: int32(modTime.Unix()),
InodeNumber: d.w.sb.Inodes + 1,
},
Nlink: 1, // TODO(later): when is this not 1?
SymlinkSize: uint32(len(oldname)),
}); err != nil {
return err
}
if _, err := d.w.inodeBuf.Write([]byte(oldname)); err != nil {
return err
}
d.dirEntries = append(d.dirEntries, fullDirEntry{
startBlock: uint32(startBlock),
offset: uint16(offset),
inodeNumber: d.w.sb.Inodes + 1,
entryType: symlinkType,
name: newname,
})
d.w.sb.Inodes++
return nil
}
// Flush writes directory entries and creates inodes for the directory.
func (d *Directory) Flush() error {
countByStartBlock := make(map[uint32]uint32)
for _, de := range d.dirEntries {
countByStartBlock[de.startBlock]++
}
dirBufStartBlock := d.w.dirBuf.Len() / metadataBlockSize
dirBufOffset := d.w.dirBuf.Len()
currentBlock := int64(-1)
currentInodeOffset := int64(-1)
var subdirs int
for _, de := range d.dirEntries {
if de.entryType == dirType {
subdirs++
}
if int64(de.startBlock) != currentBlock {
dh := dirHeader{
Count: countByStartBlock[de.startBlock] - 1,
StartBlock: de.startBlock * (metadataBlockSize + 2),
InodeOffset: de.inodeNumber,
}
if err := binary.Write(&d.w.dirBuf, binary.LittleEndian, &dh); err != nil {
return err
}
currentBlock = int64(de.startBlock)
currentInodeOffset = int64(de.inodeNumber)
}
if err := binary.Write(&d.w.dirBuf, binary.LittleEndian, &dirEntry{
Offset: de.offset,
InodeNumber: int16(de.inodeNumber - uint32(currentInodeOffset)),
EntryType: de.entryType,
Size: uint16(len(de.name) - 1),
}); err != nil {
return err
}
if _, err := d.w.dirBuf.Write([]byte(de.name)); err != nil {
return err
}
}
startBlock := d.w.inodeBuf.Len() / metadataBlockSize
offset := d.w.inodeBuf.Len() - startBlock*metadataBlockSize
inodeBufOffset := d.w.inodeBuf.Len()
// parentInodeOffset is the offset (in bytes) of the ParentInode field
// within a dirInodeHeader or ldirInodeHeader
var parentInodeOffset int64
if len(d.dirEntries) > 256 ||
d.w.dirBuf.Len()-dirBufOffset > metadataBlockSize {
parentInodeOffset = (2 + 2 + 2 + 2 + 4 + 4) + 4 + 4 + 4
if err := binary.Write(&d.w.inodeBuf, binary.LittleEndian, ldirInodeHeader{
inodeHeader: inodeHeader{
InodeType: ldirType,
Mode: unix.S_IRUSR | unix.S_IXUSR |
unix.S_IRGRP | unix.S_IXGRP |
unix.S_IROTH | unix.S_IXOTH,
Uid: 0,
Gid: 0,
Mtime: int32(d.modTime.Unix()),
InodeNumber: d.w.sb.Inodes + 1,
},
Nlink: uint32(subdirs + 2 - 1), // + 2 for . and ..
FileSize: uint32(d.w.dirBuf.Len()-dirBufOffset) + 3,
StartBlock: uint32(dirBufStartBlock * (metadataBlockSize + 2)),
ParentInode: d.w.sb.Inodes + 2, // invalid
Icount: 0, // no directory index
Offset: uint16(dirBufOffset - dirBufStartBlock*metadataBlockSize),
Xattr: invalidXattr,
}); err != nil {
return err
}
} else {
parentInodeOffset = (2 + 2 + 2 + 2 + 4 + 4) + 4 + 4 + 2 + 2
if err := binary.Write(&d.w.inodeBuf, binary.LittleEndian, dirInodeHeader{
inodeHeader: inodeHeader{
InodeType: dirType,
Mode: unix.S_IRUSR | unix.S_IXUSR |
unix.S_IRGRP | unix.S_IXGRP |
unix.S_IROTH | unix.S_IXOTH,
Uid: 0,
Gid: 0,
Mtime: int32(d.modTime.Unix()),
InodeNumber: d.w.sb.Inodes + 1,
},
StartBlock: uint32(dirBufStartBlock * (metadataBlockSize + 2)),
Nlink: uint32(subdirs + 2 - 1), // + 2 for . and ..
FileSize: uint16(d.w.dirBuf.Len()-dirBufOffset) + 3,
Offset: uint16(dirBufOffset - dirBufStartBlock*metadataBlockSize),
ParentInode: d.w.sb.Inodes + 2, // invalid
}); err != nil {
return err
}
}
path := d.path()
for _, offset := range d.w.writeInodeNumTo[path] {
// Directly manipulating unread data in bytes.Buffer via Bytes(), as per
// https://groups.google.com/d/msg/golang-nuts/1ON9XVQ1jXE/8j9RaeSYxuEJ
b := d.w.inodeBuf.Bytes()
binary.LittleEndian.PutUint32(b[offset:offset+4], d.w.sb.Inodes+1)
}
if d.parent != nil {
parentPath := filepath.Dir(d.path())
if parentPath == "." {
parentPath = ""
}
d.w.writeInodeNumTo[parentPath] = append(d.w.writeInodeNumTo[parentPath], int64(inodeBufOffset)+parentInodeOffset)
d.parent.dirEntries = append(d.parent.dirEntries, fullDirEntry{
startBlock: uint32(startBlock),
offset: uint16(offset),
inodeNumber: d.w.sb.Inodes + 1,
entryType: dirType,
name: d.name,
})
} else { // root
d.w.sb.RootInode = inode((startBlock*(metadataBlockSize+2))<<16 | offset)
}
d.w.sb.Inodes++
return nil
}
// Write implements io.Writer
func (f *file) Write(p []byte) (n int, err error) {
n, err = f.buf.Write(p)
if n > 0 {
// Keep track of the uncompressed file size.
f.size += uint32(n)
for f.buf.Len() >= dataBlockSize {
if err := f.writeBlock(); err != nil {
return 0, err
}
}
}
return n, err
}
func (f *file) writeBlock() error {
n := f.buf.Len()
if n > dataBlockSize {
n = dataBlockSize
}
// Feed dataBlockSize bytes to the compressor
b := f.buf.Bytes()
block := b[:n]
rest := b[n:]
f.compBuf.Reset()
f.zlibWriter.Reset(f.compBuf)
if _, err := f.zlibWriter.Write(block); err != nil {
return err
}
if err := f.zlibWriter.Close(); err != nil {
return err
}
size := f.compBuf.Len()
if size > len(block) {
// Copy uncompressed data: Linux returns i/o errors when it encounters a
// compressed block which is larger than the uncompressed data:
// https://github.com/torvalds/linux/blob/3ca24ce9ff764bc27bceb9b2fd8ece74846c3fd3/fs/squashfs/block.c#L150
size = len(block) | (1 << 24) // SQUASHFS_COMPRESSED_BIT_BLOCK
if _, err := f.w.w.Write(block); err != nil {
return err
}
} else {
if _, err := io.Copy(f.w.w, f.compBuf); err != nil {
return err
}
}
f.blocksizes = append(f.blocksizes, uint32(size))
// Keep the rest in f.buf for the next write
copy(b, rest)
f.buf.Truncate(len(rest))
return nil
}
// Close implements io.Closer
func (f *file) Close() error {
for f.buf.Len() > 0 {
if err := f.writeBlock(); err != nil {
return err
}
}
startBlock := f.w.inodeBuf.Len() / metadataBlockSize
offset := f.w.inodeBuf.Len() - startBlock*metadataBlockSize
if err := binary.Write(&f.w.inodeBuf, binary.LittleEndian, regInodeHeader{
inodeHeader: inodeHeader{
InodeType: fileType,
Mode: uint16(f.mode),
Uid: 0,
Gid: 0,
Mtime: int32(f.modTime.Unix()),
InodeNumber: f.w.sb.Inodes + 1,
},
StartBlock: uint32(f.off), // TODO(later): check for overflow
Fragment: invalidFragment,
Offset: 0,
FileSize: f.size,
}); err != nil {
return err
}
if err := binary.Write(&f.w.inodeBuf, binary.LittleEndian, f.blocksizes); err != nil {
return err
}
f.d.dirEntries = append(f.d.dirEntries, fullDirEntry{
startBlock: uint32(startBlock),
offset: uint16(offset),
inodeNumber: f.w.sb.Inodes + 1,
entryType: fileType,
name: f.name,
})
f.w.sb.Inodes++
return nil
}
// writeMetadataChunks copies from r to w in blocks of metadataBlockSize bytes
// each, prefixing each block with a uint16 length header, setting the
// uncompressed bit.
func (w *Writer) writeMetadataChunks(r io.Reader) error {
buf := make([]byte, metadataBlockSize)
for {
buf = buf[:metadataBlockSize]
n, err := r.Read(buf)
if err != nil {
if err == io.EOF { // done
return nil
}
return err
}
buf = buf[:n]
if err := binary.Write(w.w, binary.LittleEndian, uint16(len(buf))|0x8000); err != nil {
return err
}
if _, err := w.w.Write(buf); err != nil {
return err
}
}
}
// Flush writes the SquashFS file system. The Writer must not be used after
// calling Flush.
func (w *Writer) Flush() error {
// (1) superblock will be written later
// (2) compressor-specific options omitted
// (3) data has already been written
// (4) write inode table
off, err := w.w.Seek(0, io.SeekCurrent)
if err != nil {
return err
}
w.sb.InodeTableStart = off
if err := w.writeMetadataChunks(&w.inodeBuf); err != nil {
return err
}
// (5) write directory table
off, err = w.w.Seek(0, io.SeekCurrent)
if err != nil {
return err
}
w.sb.DirectoryTableStart = off
if err := w.writeMetadataChunks(&w.dirBuf); err != nil {
return err
}
// (6) fragment table omitted
off, err = w.w.Seek(0, io.SeekCurrent)
if err != nil {
return err
}
w.sb.FragmentTableStart = off
// (7) export table omitted
// (8) write uid/gid lookup table
idTableStart, err := writeIdTable(w.w, []uint32{0})
if err != nil {
return err
}
w.sb.IdTableStart = idTableStart
// (9) xattr table omitted
off, err = w.w.Seek(0, io.SeekCurrent)
if err != nil {
return err
}
w.sb.BytesUsed = off
// Pad to 4096, required for the kernel to be able to access all pages
if pad := off % 4096; pad > 0 {
padding := make([]byte, 4096-pad)
if _, err := w.w.Write(padding); err != nil {
return err
}
}
// (1) Write superblock
if _, err := w.w.Seek(0, io.SeekStart); err != nil {
return err
}
return binary.Write(w.w, binary.LittleEndian, &w.sb)
}