Block diagram
This page hosts a formal specification of cramfs using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.
meta:
id: cramfs
xref:
justsolve: Cramfs
wikidata: Q747406
tags:
- filesystem
- linux
license: MIT
endian: le
seq:
- id: super_block
type: super_block_struct
instances:
page_size:
value: 4096
types:
super_block_struct:
seq:
- id: magic
contents: [0x45, 0x3D, 0xCD, 0x28]
- id: size
type: u4
- id: flags
type: u4
- id: future
type: u4
- id: signature
contents: 'Compressed ROMFS'
- id: fsid
type: info
- id: name
type: str
size: 16
encoding: ASCII
- id: root
type: inode
instances:
# flags
flag_fsid_v2:
value: (flags >> 0) & 1
flag_sorted_dirs:
value: (flags >> 1) & 1
flag_holes:
value: (flags >> 8) & 1
flag_wrong_signature:
value: (flags >> 9) & 1
flag_shifted_root_offset:
value: (flags >> 10) & 1
info:
seq:
- id: crc
type: u4
- id: edition
type: u4
- id: blocks
type: u4
- id: files
type: u4
inode:
seq:
- id: mode
type: u2
- id: uid
type: u2
- id: size_gid
type: u4
- id: namelen_offset
type: u4
- id: name
type: str
size: namelen
encoding: utf-8
instances:
# -- [mode] --
type:
value: (mode >> 12) & 0b1111
enum: file_type
attr:
value: (mode >> 9) & 0b0111
perm_u:
value: (mode >> 6) & 0b0111
perm_g:
value: (mode >> 3) & 0b0111
perm_o:
value: mode & 0b0111
# -- [size_gid] --
size:
value: size_gid & 0xFFFFFF
gid:
value: size_gid >> 24
# -- [namelen_offset] --
namelen:
value: (namelen_offset & 0x3F) << 2
offset:
value: ((namelen_offset >> 6) & 0x3FFFFFF) << 2
# -- [type dependent data] --
as_reg_file:
io: _root._io
pos: offset
type: chunked_data_inode
as_symlink:
io: _root._io
pos: offset
type: chunked_data_inode
as_dir:
io: _root._io
pos: offset
size: size
type: dir_inode
enums:
file_type:
1: fifo
2: chrdev
4: dir
6: blkdev
8: reg_file
10: symlink
12: socket
chunked_data_inode:
seq:
- id: block_end_index
type: u4
repeat: expr
repeat-expr: (_parent.size + _root.page_size - 1) / _root.page_size
# Correct decoding can't yet be described -- raw data for now.
- id: raw_blocks
size-eos: true
#- id: raw_blocks
# size: block_end_index[i] - _io.pos
# repeat: expr
# repeat-expr: (_parent.size + _root.page_size - 1) / _root.page_size
dir_inode:
seq:
- id: children
repeat: eos
type: inode
if: _io.size > 0