Android sparse image: Python parsing library

The Android sparse format is a format to more efficiently store files for for example firmware updates to save on bandwidth. Files in sparse format first have to be converted back to their original format.

A tool to create images for testing can be found in the Android source code tree:

https://android.googlesource.com/platform/system/core/+/e8d02c50d7/libsparse - img2simg.c

Note: this is not the same as the Android sparse data image format.

File extension

img

KS implementation details

License: CC0-1.0
Minimal Kaitai Struct required: 0.9

This page hosts a formal specification of Android sparse image using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

Usage

Runtime library

All parsing code for Python generated by Kaitai Struct depends on the Python runtime library. You have to install it before you can parse data.

The Python runtime library can be installed from PyPI:

python3 -m pip install kaitaistruct

Code

Parse a local file and get structure in memory:

data = AndroidSparse.from_file("path/to/local/file.img")

Or parse structure from a bytes:

from kaitaistruct import KaitaiStream, BytesIO

raw = b"\x00\x01\x02..."
data = AndroidSparse(KaitaiStream(BytesIO(raw)))

After that, one can get various attributes from the structure by invoking getter methods like:

data.header_prefix # => internal; access `_root.header` instead

Python source code to parse Android sparse image

android_sparse.py

# This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild

from pkg_resources import parse_version
import kaitaistruct
from kaitaistruct import KaitaiStruct, KaitaiStream, BytesIO
from enum import Enum


if parse_version(kaitaistruct.__version__) < parse_version('0.9'):
    raise Exception("Incompatible Kaitai Struct Python API: 0.9 or later is required, but you have %s" % (kaitaistruct.__version__))

class AndroidSparse(KaitaiStruct):
    """The Android sparse format is a format to more efficiently store files
    for for example firmware updates to save on bandwidth. Files in sparse
    format first have to be converted back to their original format.
    
    A tool to create images for testing can be found in the Android source code tree:
    
    <https://android.googlesource.com/platform/system/core/+/e8d02c50d7/libsparse> - `img2simg.c`
    
    Note: this is not the same as the Android sparse data image format.
    
    .. seealso::
       Source - https://android.googlesource.com/platform/system/core/+/e8d02c50d7/libsparse/sparse_format.h
    
    
    .. seealso::
       Source - https://source.android.com/devices/bootloader/images#sparse-image-format
    """

    class ChunkTypes(Enum):
        raw = 51905
        fill = 51906
        dont_care = 51907
        crc32 = 51908
    def __init__(self, _io, _parent=None, _root=None):
        self._io = _io
        self._parent = _parent
        self._root = _root if _root else self
        self._read()

    def _read(self):
        self.header_prefix = AndroidSparse.FileHeaderPrefix(self._io, self, self._root)
        self._raw_header = self._io.read_bytes((self.header_prefix.len_header - 10))
        _io__raw_header = KaitaiStream(BytesIO(self._raw_header))
        self.header = AndroidSparse.FileHeader(_io__raw_header, self, self._root)
        self.chunks = [None] * (self.header.num_chunks)
        for i in range(self.header.num_chunks):
            self.chunks[i] = AndroidSparse.Chunk(self._io, self, self._root)


    class FileHeaderPrefix(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            self._io = _io
            self._parent = _parent
            self._root = _root if _root else self
            self._read()

        def _read(self):
            self.magic = self._io.read_bytes(4)
            if not self.magic == b"\x3A\xFF\x26\xED":
                raise kaitaistruct.ValidationNotEqualError(b"\x3A\xFF\x26\xED", self.magic, self._io, u"/types/file_header_prefix/seq/0")
            self.version = AndroidSparse.Version(self._io, self, self._root)
            self.len_header = self._io.read_u2le()


    class FileHeader(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            self._io = _io
            self._parent = _parent
            self._root = _root if _root else self
            self._read()

        def _read(self):
            self.len_chunk_header = self._io.read_u2le()
            self.block_size = self._io.read_u4le()
            _ = self.block_size
            if not (_ % 4) == 0:
                raise kaitaistruct.ValidationExprError(self.block_size, self._io, u"/types/file_header/seq/1")
            self.num_blocks = self._io.read_u4le()
            self.num_chunks = self._io.read_u4le()
            self.checksum = self._io.read_u4le()

        @property
        def version(self):
            if hasattr(self, '_m_version'):
                return self._m_version if hasattr(self, '_m_version') else None

            self._m_version = self._root.header_prefix.version
            return self._m_version if hasattr(self, '_m_version') else None

        @property
        def len_header(self):
            """size of file header, should be 28."""
            if hasattr(self, '_m_len_header'):
                return self._m_len_header if hasattr(self, '_m_len_header') else None

            self._m_len_header = self._root.header_prefix.len_header
            return self._m_len_header if hasattr(self, '_m_len_header') else None


    class Chunk(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            self._io = _io
            self._parent = _parent
            self._root = _root if _root else self
            self._read()

        def _read(self):
            self._raw_header = self._io.read_bytes(self._root.header.len_chunk_header)
            _io__raw_header = KaitaiStream(BytesIO(self._raw_header))
            self.header = AndroidSparse.Chunk.ChunkHeader(_io__raw_header, self, self._root)
            _on = self.header.chunk_type
            if _on == AndroidSparse.ChunkTypes.crc32:
                self.body = self._io.read_u4le()
            else:
                self.body = self._io.read_bytes(self.header.len_body)

        class ChunkHeader(KaitaiStruct):
            def __init__(self, _io, _parent=None, _root=None):
                self._io = _io
                self._parent = _parent
                self._root = _root if _root else self
                self._read()

            def _read(self):
                self.chunk_type = KaitaiStream.resolve_enum(AndroidSparse.ChunkTypes, self._io.read_u2le())
                self.reserved1 = self._io.read_u2le()
                self.num_body_blocks = self._io.read_u4le()
                self.len_chunk = self._io.read_u4le()
                if not self.len_chunk == ((self._root.header.len_chunk_header + self.len_body_expected) if self.len_body_expected != -1 else self.len_chunk):
                    raise kaitaistruct.ValidationNotEqualError(((self._root.header.len_chunk_header + self.len_body_expected) if self.len_body_expected != -1 else self.len_chunk), self.len_chunk, self._io, u"/types/chunk/types/chunk_header/seq/3")

            @property
            def len_body(self):
                if hasattr(self, '_m_len_body'):
                    return self._m_len_body if hasattr(self, '_m_len_body') else None

                self._m_len_body = (self.len_chunk - self._root.header.len_chunk_header)
                return self._m_len_body if hasattr(self, '_m_len_body') else None

            @property
            def len_body_expected(self):
                """
                .. seealso::
                   Source - https://android.googlesource.com/platform/system/core/+/e8d02c50d7/libsparse/sparse_read.cpp#184
                
                
                .. seealso::
                   Source - https://android.googlesource.com/platform/system/core/+/e8d02c50d7/libsparse/sparse_read.cpp#215
                
                
                .. seealso::
                   Source - https://android.googlesource.com/platform/system/core/+/e8d02c50d7/libsparse/sparse_read.cpp#249
                
                
                .. seealso::
                   Source - https://android.googlesource.com/platform/system/core/+/e8d02c50d7/libsparse/sparse_read.cpp#270
                """
                if hasattr(self, '_m_len_body_expected'):
                    return self._m_len_body_expected if hasattr(self, '_m_len_body_expected') else None

                self._m_len_body_expected = ((self._root.header.block_size * self.num_body_blocks) if self.chunk_type == AndroidSparse.ChunkTypes.raw else (4 if self.chunk_type == AndroidSparse.ChunkTypes.fill else (0 if self.chunk_type == AndroidSparse.ChunkTypes.dont_care else (4 if self.chunk_type == AndroidSparse.ChunkTypes.crc32 else -1))))
                return self._m_len_body_expected if hasattr(self, '_m_len_body_expected') else None



    class Version(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            self._io = _io
            self._parent = _parent
            self._root = _root if _root else self
            self._read()

        def _read(self):
            self.major = self._io.read_u2le()
            if not self.major == 1:
                raise kaitaistruct.ValidationNotEqualError(1, self.major, self._io, u"/types/version/seq/0")
            self.minor = self._io.read_u2le()