ext4 read-only for now

README.md

@@ -151,7 +151,7 @@ cat $PWD/foo.img | docker run -i --rm $INT_IMAGE mdir -i /file.img /abc
 Future plans are to add the following:
 
 * embed boot code in `mbr` e.g. `altmbr.bin` (no need for `gpt` since an ESP with `/EFI/BOOT/BOOT<arch>.EFI` will boot)
-* `ext4` filesystem
+* `ext4` filesystem writing (read-only already works)
 * `Joliet` extensions to `iso9660`
 * `Rock Ridge` sparse file support - supports the flag, but not yet reading or writing
 * `squashfs` sparse file support - currently treats sparse files as regular files

disk/disk.go

@@ -13,6 +13,7 @@ import (
 	log "github.com/sirupsen/logrus"
 
 	"github.com/diskfs/go-diskfs/filesystem"
+	"github.com/diskfs/go-diskfs/filesystem/ext4"
 	"github.com/diskfs/go-diskfs/filesystem/fat32"
 	"github.com/diskfs/go-diskfs/filesystem/iso9660"
 	"github.com/diskfs/go-diskfs/filesystem/squashfs"
@@ -185,6 +186,8 @@ func (d *Disk) CreateFilesystem(spec FilesystemSpec) (filesystem.FileSystem, err
 		return fat32.Create(d.File, size, start, d.LogicalBlocksize, spec.VolumeLabel)
 	case filesystem.TypeISO9660:
 		return iso9660.Create(d.File, size, start, d.LogicalBlocksize, spec.WorkDir)
+	case filesystem.TypeExt4:
+		return ext4.Create(d.File, size, start, d.LogicalBlocksize, nil)
 	case filesystem.TypeSquashfs:
 		return nil, errors.New("squashfs is a read-only filesystem")
 	default:

filesystem/ext4/bitmaps.go

@@ -0,0 +1,104 @@
+package ext4
+
+import "fmt"
+
+// bitmap is a structure holding a bitmap
+type bitmap struct {
+	bits []byte
+}
+
+// bitmapFromBytes create a bitmap struct from bytes
+func bitmapFromBytes(b []byte) *bitmap {
+	// just copy them over
+	bits := make([]byte, len(b))
+	copy(bits, b)
+	bm := bitmap{
+		bits: bits,
+	}
+
+	return &bm
+}
+
+// toBytes returns raw bytes ready to be written to disk
+func (bm *bitmap) toBytes() []byte {
+	b := make([]byte, len(bm.bits))
+	copy(b, bm.bits)
+
+	return b
+}
+
+func (bm *bitmap) checkFree(location int) (bool, error) {
+	byteNumber, bitNumber := findBitForIndex(location)
+	if byteNumber > len(bm.bits) {
+		return false, fmt.Errorf("location %d is not in %d size bitmap", location, len(bm.bits)*8)
+	}
+	mask := byte(0x1) << bitNumber
+	return bm.bits[byteNumber]&mask == mask, nil
+}
+
+func (bm *bitmap) free(location int) error {
+	byteNumber, bitNumber := findBitForIndex(location)
+	if byteNumber > len(bm.bits) {
+		return fmt.Errorf("location %d is not in %d size bitmap", location, len(bm.bits)*8)
+	}
+	mask := byte(0x1) << bitNumber
+	mask = ^mask
+	bm.bits[byteNumber] &= mask
+	return nil
+}
+
+func (bm *bitmap) use(location int) error {
+	byteNumber, bitNumber := findBitForIndex(location)
+	if byteNumber > len(bm.bits) {
+		return fmt.Errorf("location %d is not in %d size bitmap", location, len(bm.bits)*8)
+	}
+	mask := byte(0x1) << bitNumber
+	bm.bits[byteNumber] |= mask
+	return nil
+}
+
+func (bm *bitmap) findFirstFree() int {
+	var location = -1
+	for i, b := range bm.bits {
+		// if all used, continue to next
+		if b&0xff == 0xff {
+			continue
+		}
+		// not all used, so find first bit set to 0
+		for j := uint8(0); j < 8; j++ {
+			mask := byte(0x1) << j
+			if b&mask != mask {
+				location = 8*i + (8 - int(j))
+				break
+			}
+		}
+		break
+	}
+	return location
+}
+
+//nolint:revive // params are unused as of yet, but will be used in the future
+func (bm *bitmap) findFirstUsed() int {
+	var location int = -1
+	for i, b := range bm.bits {
+		// if all free, continue to next
+		if b == 0x00 {
+			continue
+		}
+		// not all free, so find first bit set to 1
+		for j := uint8(0); j < 8; j++ {
+			mask := byte(0x1) << j
+			mask = ^mask
+			if b|mask != mask {
+				location = 8*i + (8 - int(j))
+				break
+			}
+		}
+		break
+	}
+	return location
+}
+
+func findBitForIndex(index int) (byteNumber int, bitNumber uint8) {
+	return index / 8, uint8(index % 8)
+}

filesystem/ext4/blockgroup.go

@@ -0,0 +1,53 @@
+package ext4
+
+import (
+	"fmt"
+)
+
+// blockGroup is a structure holding the data about a single block group
+//
+//nolint:unused // will be used in the future, not yet
+type blockGroup struct {
+	inodeBitmap    *bitmap
+	blockBitmap    *bitmap
+	blockSize      int
+	number         int
+	inodeTableSize int
+	firstDataBlock int
+}
+
+// blockGroupFromBytes create a blockGroup struct from bytes
+// it does not load the inode table or data blocks into memory, rather holding pointers to where they are
+//
+//nolint:unused // will be used in the future, not yet
+func blockGroupFromBytes(b []byte, blockSize, groupNumber int) (*blockGroup, error) {
+	expectedSize := 2 * blockSize
+	actualSize := len(b)
+	if actualSize != expectedSize {
+		return nil, fmt.Errorf("expected to be passed %d bytes for 2 blocks of size %d, instead received %d", expectedSize, blockSize, actualSize)
+	}
+	inodeBitmap := bitmapFromBytes(b[0:blockSize])
+	blockBitmap := bitmapFromBytes(b[blockSize : 2*blockSize])
+
+	bg := blockGroup{
+		inodeBitmap: inodeBitmap,
+		blockBitmap: blockBitmap,
+		number:      groupNumber,
+		blockSize:   blockSize,
+	}
+	return &bg, nil
+}
+
+// toBytes returns bitmaps ready to be written to disk
+//
+//nolint:unused // will be used in the future, not yet
+func (bg *blockGroup) toBytes() ([]byte, error) {
+	b := make([]byte, 2*bg.blockSize)
+	inodeBitmapBytes := bg.inodeBitmap.toBytes()
+	blockBitmapBytes := bg.blockBitmap.toBytes()
+
+	b = append(b, inodeBitmapBytes...)
+	b = append(b, blockBitmapBytes...)
+
+	return b, nil
+}

filesystem/ext4/checksum.go

@@ -0,0 +1,48 @@
+package ext4
+
+import (
+	"encoding/binary"
+
+	"github.com/diskfs/go-diskfs/filesystem/ext4/crc"
+)
+
+// checksumAppender is a function that takes a byte slice and returns a byte slice with a checksum appended
+type checksumAppender func([]byte) []byte
+type checksummer func([]byte) uint32
+
+// directoryChecksummer returns a function that implements checksumAppender for a directory entries block
+// original calculations can be seen for e2fsprogs https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git/tree/lib/ext2fs/csum.c#n301
+// and in the linux tree https://github.com/torvalds/linux/blob/master/fs/ext4/namei.c#L376-L384
+func directoryChecksummer(seed, inodeNumber, inodeGeneration uint32) checksummer {
+	numBytes := make([]byte, 4)
+	binary.LittleEndian.PutUint32(numBytes, inodeNumber)
+	crcResult := crc.CRC32c(seed, numBytes)
+	genBytes := make([]byte, 4)
+	binary.LittleEndian.PutUint32(genBytes, inodeGeneration)
+	crcResult = crc.CRC32c(crcResult, genBytes)
+	return func(b []byte) uint32 {
+		checksum := crc.CRC32c(crcResult, b)
+		return checksum
+	}
+}
+
+// directoryChecksumAppender returns a function that implements checksumAppender for a directory entries block
+// original calculations can be seen for e2fsprogs https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git/tree/lib/ext2fs/csum.c#n301
+// and in the linux tree https://github.com/torvalds/linux/blob/master/fs/ext4/namei.c#L376-L384
+func directoryChecksumAppender(seed, inodeNumber, inodeGeneration uint32) checksumAppender {
+	fn := directoryChecksummer(seed, inodeNumber, inodeGeneration)
+	return func(b []byte) []byte {
+		checksum := fn(b)
+		checksumBytes := make([]byte, 12)
+		checksumBytes[4] = 12
+		checksumBytes[7] = 0xde
+		binary.LittleEndian.PutUint32(checksumBytes[8:12], checksum)
+		b = append(b, checksumBytes...)
+		return b
+	}
+}
+
+// nullDirectoryChecksummer does not change anything
+func nullDirectoryChecksummer(b []byte) []byte {
+	return b
+}