qemu/tests/qemu-iotests/qcow2_format.py

# Library for manipulations with qcow2 image
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
#

import struct
import string


class QcowHeaderExtension:

    def __init__(self, magic, length, data):
        if length % 8 != 0:
            padding = 8 - (length % 8)
            data += b'\0' * padding

        self.magic = magic
        self.length = length
        self.data = data

    @classmethod
    def create(cls, magic, data):
        return QcowHeaderExtension(magic, len(data), data)


# Mapping from c types to python struct format
ctypes = {
    'u8': 'B',
    'u16': 'H',
    'u32': 'I',
    'u64': 'Q'
}


class QcowHeader:

    fields = (
        # Version 2 header fields
        ('u32', '{:#x}', 'magic'),
        ('u32', '{}', 'version'),
        ('u64', '{:#x}', 'backing_file_offset'),
        ('u32', '{:#x}', 'backing_file_size'),
        ('u32', '{}', 'cluster_bits'),
        ('u64', '{}', 'size'),
        ('u32', '{}', 'crypt_method'),
        ('u32', '{}', 'l1_size'),
        ('u64', '{:#x}', 'l1_table_offset'),
        ('u64', '{:#x}', 'refcount_table_offset'),
        ('u32', '{}', 'refcount_table_clusters'),
        ('u32', '{}', 'nb_snapshots'),
        ('u64', '{:#x}', 'snapshot_offset'),

        # Version 3 header fields
        ('u64', 'mask', 'incompatible_features'),
        ('u64', 'mask', 'compatible_features'),
        ('u64', 'mask', 'autoclear_features'),
        ('u32', '{}', 'refcount_order'),
        ('u32', '{}', 'header_length'),
    )

    fmt = '>' + ''.join(ctypes[f[0]] for f in fields)

    def __init__(self, fd):

        buf_size = struct.calcsize(QcowHeader.fmt)

        fd.seek(0)
        buf = fd.read(buf_size)

        header = struct.unpack(QcowHeader.fmt, buf)
        self.__dict__ = dict((field[2], header[i])
                             for i, field in enumerate(QcowHeader.fields))

        self.set_defaults()
        self.cluster_size = 1 << self.cluster_bits

        fd.seek(self.header_length)
        self.load_extensions(fd)

        if self.backing_file_offset:
            fd.seek(self.backing_file_offset)
            self.backing_file = fd.read(self.backing_file_size)
        else:
            self.backing_file = None

    def set_defaults(self):
        if self.version == 2:
            self.incompatible_features = 0
            self.compatible_features = 0
            self.autoclear_features = 0
            self.refcount_order = 4
            self.header_length = 72

    def load_extensions(self, fd):
        self.extensions = []

        if self.backing_file_offset != 0:
            end = min(self.cluster_size, self.backing_file_offset)
        else:
            end = self.cluster_size

        while fd.tell() < end:
            (magic, length) = struct.unpack('>II', fd.read(8))
            if magic == 0:
                break
            else:
                padded = (length + 7) & ~7
                data = fd.read(padded)
                self.extensions.append(QcowHeaderExtension(magic, length,
                                                           data))

    def update_extensions(self, fd):

        fd.seek(self.header_length)
        extensions = self.extensions
        extensions.append(QcowHeaderExtension(0, 0, b''))
        for ex in extensions:
            buf = struct.pack('>II', ex.magic, ex.length)
            fd.write(buf)
            fd.write(ex.data)

        if self.backing_file is not None:
            self.backing_file_offset = fd.tell()
            fd.write(self.backing_file)

        if fd.tell() > self.cluster_size:
            raise Exception('I think I just broke the image...')

    def update(self, fd):
        header_bytes = self.header_length

        self.update_extensions(fd)

        fd.seek(0)
        header = tuple(self.__dict__[f] for t, p, f in QcowHeader.fields)
        buf = struct.pack(QcowHeader.fmt, *header)
        buf = buf[0:header_bytes-1]
        fd.write(buf)

    def dump(self):
        for f in QcowHeader.fields:
            value = self.__dict__[f[2]]
            if f[1] == 'mask':
                bits = []
                for bit in range(64):
                    if value & (1 << bit):
                        bits.append(bit)
                value_str = str(bits)
            else:
                value_str = f[1].format(value)

            print(f'{f[2]:<25} {value_str}')

    def dump_extensions(self):
        for ex in self.extensions:

            data = ex.data[:ex.length]
            if all(c in string.printable.encode('ascii') for c in data):
                data = f"'{ data.decode('ascii') }'"
            else:
                data = '<binary>'

            print('Header extension:')
            print(f'{"magic":<25} {ex.magic:#x}')
            print(f'{"length":<25} {ex.length}')
            print(f'{"data":<25} {data}')
            print()