writable ext4

Signed-off-by: Avi Deitcher <avi@deitcher.net>

README.md

@@ -109,7 +109,6 @@ 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 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

filesystem/ext4/blockgroup.go

@@ -2,14 +2,16 @@ package ext4
 
 import (
 	"fmt"
+
+	"github.com/diskfs/go-diskfs/util"
 )
 
 // 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
+	inodeBitmap    *util.Bitmap
+	blockBitmap    *util.Bitmap
 	blockSize      int
 	number         int
 	inodeTableSize int
@@ -26,8 +28,8 @@ func blockGroupFromBytes(b []byte, blockSize, groupNumber int) (*blockGroup, err
 	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])
+	inodeBitmap := util.BitmapFromBytes(b[0:blockSize])
+	blockBitmap := util.BitmapFromBytes(b[blockSize : 2*blockSize])
 
 	bg := blockGroup{
 		inodeBitmap: inodeBitmap,
@@ -43,8 +45,8 @@ func blockGroupFromBytes(b []byte, blockSize, groupNumber int) (*blockGroup, err
 //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()
+	inodeBitmapBytes := bg.inodeBitmap.ToBytes()
+	blockBitmapBytes := bg.blockBitmap.ToBytes()
 
 	b = append(b, inodeBitmapBytes...)
 	b = append(b, blockBitmapBytes...)

filesystem/ext4/checksum.go

@@ -29,6 +29,8 @@ func directoryChecksummer(seed, inodeNumber, inodeGeneration uint32) checksummer
 // 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
+//
+//nolint:unparam // inodeGeneration is always 0
 func directoryChecksumAppender(seed, inodeNumber, inodeGeneration uint32) checksumAppender {
 	fn := directoryChecksummer(seed, inodeNumber, inodeGeneration)
 	return func(b []byte) []byte {

filesystem/ext4/consts.go

@@ -0,0 +1,5 @@
+package ext4
+
+const (
+	maxUint16 uint64 = 1<<16 - 1
+)

filesystem/ext4/directory.go

@@ -36,7 +36,7 @@ func (d *Directory) toBytes(bytesPerBlock uint32, checksumFunc checksumAppender)
 		switch {
 		case len(block)+len(b2) > int(bytesPerBlock)-minDirEntryLength:
 			// if adding this one will go past the end of the block, pad out the previous
-			block = b[:len(block)-previousLength]
+			block = block[:len(block)-previousLength]
 			previousB := previousEntry.toBytes(uint16(int(bytesPerBlock) - len(block) - minDirEntryLength))
 			block = append(block, previousB...)
 			// add the checksum