ext4

Signed-off-by: Avi Deitcher <[email protected]>

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/directory.go

@@ -0,0 +1,32 @@
+package ext4
+
+// Directory represents a single directory in an ext4 filesystem
+type Directory struct {
+	directoryEntry
+	root    bool
+	entries []*directoryEntry
+}
+
+// dirEntriesFromBytes loads the directory entries from the raw bytes
+func (d *Directory) entriesFromBytes(b []byte) error {
+	entries, err := parseDirEntries(b)
+	if err != nil {
+		return err
+	}
+	d.entries = entries
+	return nil
+}
+
+// toBytes convert our entries to raw bytes
+func (d *Directory) toBytes(bytesPerBlock int) []byte {
+	b := make([]byte, 0)
+	for _, de := range d.entries {
+		b2 := de.toBytes()
+		b = append(b, b2...)
+	}
+	remainder := len(b) % bytesPerBlock
+	extra := bytesPerBlock - remainder
+	zeroes := make([]byte, extra)
+	b = append(b, zeroes...)
+	return b
+}

filesystem/ext4/directoryentry.go

@@ -0,0 +1,73 @@
+package ext4
+
+import (
+	"encoding/binary"
+	"fmt"
+)
+
+const (
+	minDirEntryLength int = 12 // actually 9 for 1-byte file length, but must be multiple of 4 bytes
+	maxDirEntryLength int = 263
+)
+
+// directoryEntry is a single directory entry
+type directoryEntry struct {
+	inode    uint32
+	filename string
+	fileType fileType
+}
+
+func directoryEntryFromBytes(b []byte) (*directoryEntry, error) {
+	if len(b) < minDirEntryLength {
+		return nil, fmt.Errorf("directory entry of length %d is less than minimum %d", len(b), minDirEntryLength)
+	}
+	if len(b) > maxDirEntryLength {
+		return nil, fmt.Errorf("directory entry of length %d is greater than maximum %d", len(b), maxDirEntryLength)
+	}
+
+	//nolint:gocritic // keep this here for future reference
+	// length := binary.LittleEndian.Uint16(b[0x4:0x6])
+	nameLength := b[0x6]
+	name := b[0x8 : 0x8+nameLength]
+	de := directoryEntry{
+		inode:    binary.LittleEndian.Uint32(b[0x0:0x4]),
+		fileType: fileType(b[0x7]),
+		filename: string(name),
+	}
+	return &de, nil
+}
+
+func (de *directoryEntry) toBytes() []byte {
+	// it must be the header length + filename length rounded up to nearest multiple of 4
+	nameLength := uint8(len(de.filename))
+	entryLength := uint16(nameLength) + 8
+	if leftover := entryLength % 4; leftover > 0 {
+		entryLength += leftover
+	}
+	b := make([]byte, 0, entryLength)
+
+	binary.LittleEndian.PutUint32(b[0x0:0x4], de.inode)
+	binary.LittleEndian.PutUint16(b[0x4:0x6], entryLength)
+	b[0x6] = nameLength
+	b[0x7] = byte(de.fileType)
+	copy(b[0x8:], de.filename)
+
+	return b
+}
+
+// parse the data blocks to get the directory entries
+func parseDirEntries(b []byte) ([]*directoryEntry, error) {
+	entries := make([]*directoryEntry, 4)
+	count := 0
+	for i := 0; i < len(b); count++ {
+		// read the length of the first entry
+		length := binary.LittleEndian.Uint16(b[i+0x4 : i+0x6])
+		de, err := directoryEntryFromBytes(b[i : i+int(length)])
+		if err != nil {
+			return nil, fmt.Errorf("failed to parse directory entry %d: %v", count, err)
+		}
+		entries = append(entries, de)
+		i += int(length)
+	}
+	return entries, nil
+}