squashfs: fix error format when file larger than block size

https://dr-emann.github.io/squashfs/squashfs.html#_file_inodes

Due to SquashFS spec, file inode's 'blocks start' field means the offset
from the beginning of the file.

This commit also fixes the fragmentData location calculation.

Finally, due to Spec, if a file does not have fragment data, its frag
index should be all 0xF.

Fixes #282

Signed-off-by: Xynnn007 <[email protected]>

Author: Xynnn007

filesystem/squashfs/finalize.go

@@ -111,11 +111,13 @@ func (fs *FileSystem) Finalize(options FinalizeOptions) error {
 		b        []byte
 	)
 	location += superblockSize
+	dataBlocksOffset := int64(superblockSize)
 	if options.Compression != nil {
 		b = options.Compression.optionsBytes()
 		if len(b) > 0 {
 			_, _ = f.WriteAt(b, location)
 			location += int64(len(b))
+			dataBlocksOffset += int64(len(b))
 		}
 	}
 
@@ -137,11 +139,11 @@ func (fs *FileSystem) Finalize(options FinalizeOptions) error {
 	// write file fragments
 	//
 	fragmentBlockStart := location
-	fragmentBlocks, fragsWritten, err := writeFragmentBlocks(fileList, f, fs.workspace, blocksize, options, fragmentBlockStart)
+	fragmentBlocks, _, err := writeFragmentBlocks(fileList, f, fs.workspace, blocksize, options, fragmentBlockStart)
 	if err != nil {
 		return fmt.Errorf("error writing file fragment blocks: %v", err)
 	}
-	location += fragsWritten
+	location += int64(len(fragmentBlocks) * blocksize)
 
 	// extract extended attributes, and save them for later; these are written at the very end
 	// this must be done *before* creating inodes, as inodes reference these
@@ -538,6 +540,7 @@ func writeFileDataBlocks(e *finalizeFileInfo, to backend.WritableFile, ws string
 	e.dataLocation = location
 	e.blocks = blocks
 	e.startBlock = startBlock
+	e.startFromBegin = location
 
 	// how many blocks did we write?
 	blockCount = raw / blocksize
@@ -588,7 +591,9 @@ func writeDataBlocks(fileList []*finalizeFileInfo, f backend.WritableFile, ws st
 		}
 		allBlocks += blocks
 		allWritten += written
+		location += int64(written)
 	}
+
 	return allWritten, nil
 }
 
@@ -637,9 +642,10 @@ func writeFragmentBlocks(fileList []*finalizeFileInfo, f backend.WritableFile, w
 				compressed: compressed,
 				location:   location,
 			})
+			location += int64((written + blocksize - 1) / blocksize * blocksize)
 			// increment as all writes will be to next block block
 			fragmentBlockIndex++
-			fragmentData = fragmentData[:blocksize]
+			fragmentData = make([]byte, 0)
 		}
 
 		e.fragment = &fragmentRef{
@@ -1119,13 +1125,15 @@ func createInodes(fileList []*finalizeFileInfo, idtable map[uint32]uint16, optio
 			} else {
 				// use basicFile
 				bf := &basicFile{
-					startBlock: uint32(e.startBlock),
+					startBlock: uint32(e.startFromBegin),
 					fileSize:   uint32(e.Size()),
 					blockSizes: e.blocks,
 				}
 				if e.fragment != nil {
 					bf.fragmentBlockIndex = e.fragment.block
 					bf.fragmentOffset = e.fragment.offset
+				} else {
+					bf.fragmentBlockIndex = 0xffffffff
 				}
 				in = bf
 				inodeT = inodeBasicFile

filesystem/squashfs/finalizefileinfo.go

@@ -41,6 +41,7 @@ type finalizeFileInfo struct {
 	links             uint32
 	blocks            []*blockData
 	startBlock        uint64
+	startFromBegin    int64
 	fragment          *fragmentRef
 	uid               uint32
 	gid               uint32