tests: Add the image fuzzer with qcow2 support

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+# Python version < 2.7 doesn't support empty options
+# Option values will not be interpreted
+[header]
+magic=True
+version=True
+backing_file_offset=True
+backing_file_size=True
+cluster_bits=True
+size=True
+crypt_method=True
+l1_size=True
+l1_table_offset=True
+refcount_table_offset=True
+refcount_table_clusters=True
+nb_snapshots=True
+snapshots_offset=True
+incompatible_features=True
+compatible_features=True
+autoclear_features=True
+refcount_order=True
+header_length=True
+
+[backing_file_format_ext]
+ext_magic=True
+ext_length=True
+bf_data=True
+
+[feature_name_table_ext]
+ext_magic=True
+ext_length=True
+feat_type=True
+feat_bit_number=True
+feat_name=True
+
+[end_ext]
+ext_magic=True
+ext_length=True
\ No newline at end of file

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+#!/usr/bin/env python
+
+# Tool for running fuzz tests
+#
+# Copyright (C) 2014 Maria Kustova <[email protected]>
+#
+# 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 sys, os, signal
+from time import time
+import subprocess
+import random
+from itertools import count
+from shutil import rmtree
+import getopt
+import ConfigParser
+import resource
+resource.setrlimit(resource.RLIMIT_CORE, (-1, -1))
+
+
+def multilog(msg, *output):
+    """ Write an object to all of specified file descriptors
+    """
+
+    for fd in output:
+        fd.write(msg)
+        fd.flush()
+
+
+def str_signal(sig):
+    """ Convert a numeric value of a system signal to the string one
+    defined by the current operational system
+    """
+
+    for k, v in signal.__dict__.items():
+        if v == sig:
+            return k
+
+
+class TestException(Exception):
+    """Exception for errors risen by TestEnv objects"""
+    pass
+
+
+class TestEnv(object):
+    """ Trivial test object
+
+    The class sets up test environment, generates a test image and executes
+    application under tests with specified arguments and a test image provided.
+    All logs are collected.
+    Summary log will contain short descriptions and statuses of tests in
+    a run.
+    Test log will include application (e.g. 'qemu-img') logs besides info sent
+    to the summary log.
+    """
+
+    def __init__(self, test_id, seed, work_dir, run_log, exec_bin=None,
+                 cleanup=True, log_all=False, fuzz_config=None):
+        """Set test environment in a specified work directory.
+
+        Path to qemu_img will be retrieved from 'QEMU_IMG' environment
+        variable, if a test binary is not specified.
+        """
+
+        if seed is not None:
+            self.seed = seed
+        else:
+            self.seed = hash(time())
+
+        self.init_path = os.getcwd()
+        self.work_dir = work_dir
+        self.current_dir = os.path.join(work_dir, 'test-' + test_id)
+        if exec_bin is not None:
+            self.exec_bin = exec_bin.strip().split(' ')
+        else:
+            self.exec_bin = \
+                os.environ.get('QEMU_IMG', 'qemu-img').strip().split(' ')
+
+        try:
+            os.makedirs(self.current_dir)
+        except OSError:
+            e = sys.exc_info()[1]
+            print >>sys.stderr, \
+                "Error: The working directory '%s' cannot be used. Reason: %s"\
+                % (self.work_dir, e[1])
+            raise TestException
+        self.log = open(os.path.join(self.current_dir, "test.log"), "w")
+        self.parent_log = open(run_log, "a")
+        self.result = False
+        self.cleanup = cleanup
+        self.log_all = log_all
+        self.fuzz_config = fuzz_config
+
+    def _test_app(self, q_args):
+        """ Start application under test with specified arguments and return
+        an exit code or a kill signal depending on result of an execution.
+        """
+        devnull = open('/dev/null', 'r+')
+        return subprocess.call(self.exec_bin + q_args,
+                               stdin=devnull, stdout=self.log, stderr=self.log)
+
+    def execute(self, q_args):
+        """ Execute a test.
+
+        The method creates a test image, runs the test app and analyzes
+        its exit status. If the application was killed by a signal,
+        the test is marked as failed.
+        """
+        os.chdir(self.current_dir)
+        # Seed initialization should be as close to an image generation call
+        # as posssible to avoid a corruption of the random sequence
+        random.seed(self.seed)
+        image_generator.create_image('test_image', self.fuzz_config)
+        test_summary = "Seed: %s\nCommand: %s\nTest directory: %s\n" \
+                       % (self.seed, " ".join(q_args), self.current_dir)
+        try:
+            retcode = self._test_app(q_args)
+        except OSError:
+            e = sys.exc_info()[1]
+            multilog(test_summary + "Error: Start of '%s' failed. " \
+                     "Reason: %s\n\n" % (os.path.basename(self.exec_bin[0]),
+                                         e[1]),
+                     sys.stderr, self.log, self.parent_log)
+            raise TestException
+
+        if retcode < 0:
+            multilog(test_summary + "FAIL: Test terminated by signal %s\n\n"
+                     % str_signal(-retcode), sys.stderr, self.log,
+                     self.parent_log)
+        elif self.log_all:
+            multilog(test_summary + "PASS: Application exited with the code" +
+                     " '%d'\n\n" % retcode, sys.stdout, self.log,
+                     self.parent_log)
+            self.result = True
+        else:
+            self.result = True
+
+    def finish(self):
+        """ Restore environment after a test execution. Remove folders of
+        passed tests
+        """
+        self.log.close()
+        self.parent_log.close()
+        os.chdir(self.init_path)
+        if self.result and self.cleanup:
+            rmtree(self.current_dir)
+
+if __name__ == '__main__':
+
+    def usage():
+        print """
+        Usage: runner.py [OPTION...] TEST_DIR IMG_GENERATOR
+
+        Set up test environment in TEST_DIR and run a test in it. A module for
+        test image generation should be specified via IMG_GENERATOR. Use
+        TEST_IMG alias to mark the position in the command where a test image
+        name should be placed.
+        Example:
+        python runner.py -b ./qemu-img -c 'info TEST_IMG' /tmp/test ../qcow2
+
+        Optional arguments:
+          -h, --help                    display this help and exit
+          -b, --binary=PATH             path to the application under test,
+                                        by default "qemu-img" in PATH or
+                                        QEMU_IMG environment variables
+          -c, --command=STRING          execute the tested application
+                                        with arguments specified,
+                                        by default STRING="check"
+          -s, --seed=STRING             seed for a test image generation,
+                                        by default will be generated randomly
+          --config=FILE                 take fuzzer a configuration from FILE
+          -k, --keep_passed             don't remove folders of passed tests
+          -v, --verbose                 log information about passed tests
+        """
+
+    def run_test(test_id, seed, work_dir, run_log, test_bin, cleanup, log_all,
+                 command, fuzz_config):
+        """Setup environment for one test and execute this test"""
+        try:
+            test = TestEnv(test_id, seed, work_dir, run_log, test_bin, cleanup,
+                           log_all, fuzz_config)
+        except TestException:
+            sys.exit(1)
+
+        # Python 2.4 doesn't support 'finally' and 'except' in the same 'try'
+        # block
+        try:
+            try:
+                test.execute(command)
+            # Silent exit on user break
+            except TestException:
+                sys.exit(1)
+        finally:
+            test.finish()
+
+    try:
+        opts, args = getopt.gnu_getopt(sys.argv[1:], 'c:hb:s:kv',
+                                       ['command=', 'help', 'binary=', 'seed=',
+                                        'config=', 'keep_passed', 'verbose'])
+    except getopt.error:
+        e = sys.exc_info()[1]
+        print "Error: %s\n\nTry 'runner.py --help' for more information" % e
+        sys.exit(1)
+
+    command = ['check']
+    cleanup = True
+    log_all = False
+    test_bin = None
+    seed = None
+    config_file = None
+    for opt, arg in opts:
+        if opt in ('-h', '--help'):
+            usage()
+            sys.exit()
+        elif opt in ('-c', '--command'):
+            command = arg.split(" ")
+        elif opt in ('-k', '--keep_passed'):
+            cleanup = False
+        elif opt in ('-v', '--verbose'):
+            log_all = True
+        elif opt in ('-b', '--binary'):
+            test_bin = os.path.realpath(arg)
+        elif opt in ('-s', '--seed'):
+            seed = arg
+        elif opt == '--config':
+            config_file = arg
+
+    if not len(args) == 2:
+        print "Missed parameter\nTry 'runner.py --help' " \
+            "for more information"
+        sys.exit(1)
+
+    work_dir = os.path.realpath(args[0])
+    # run_log is created in 'main', because multiple tests are expected to
+    # log in it
+    run_log = os.path.join(work_dir, 'run.log')
+
+    # Add the path to the image generator module to sys.path
+    sys.path.append(os.path.dirname(os.path.realpath(args[1])))
+    # Remove a script extension from image generator module if any
+    generator_name = os.path.splitext(os.path.basename(args[1]))[0]
+    try:
+        image_generator = __import__(generator_name)
+    except ImportError:
+        e = sys.exc_info()[1]
+        print "Error: The image generator '%s' cannot be imported.\n" \
+            "Reason: %s" % (generator_name, e)
+        sys.exit(1)
+
+    # Replace test image alias with its real name
+    for idx, item in enumerate(command):
+        if item == 'TEST_IMG':
+            command[idx] = 'test_image'
+    # Read fuzzer configuration
+    if config_file is None:
+        fuzz_config = None
+    else:
+        fuzz_config = []
+        config = ConfigParser.RawConfigParser()
+        config.read(config_file)
+        for s in config.sections():
+            opt_list = config.options(s)
+            if len(opt_list) == 0:
+                fuzz_config.append([s])
+            else:
+                for o in opt_list:
+                    fuzz_config.append([s, o])
+    # If a seed is specified, only one test will be executed.
+    # Otherwise runner will terminate after a keyboard interruption
+    for test_id in count(1):
+        try:
+            run_test(str(test_id), seed, work_dir, run_log, test_bin, cleanup,
+                     log_all, command, fuzz_config)
+        except (KeyboardInterrupt, SystemExit):
+            sys.exit(1)
+
+        if seed is not None:
+            break

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+# Fuzzing functions for qcow2 fields
+#
+# Copyright (C) 2014 Maria Kustova <[email protected]>
+#
+# 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 random
+
+
+UINT32 = 0xffffffff
+UINT64 = 0xffffffffffffffff
+# Most significant bit orders
+UINT32_M = 31
+UINT64_M = 63
+# Fuzz vectors
+UINT32_V = [0, 0x100, 0x1000, 0x10000, 0x100000, UINT32/4, UINT32/2 - 1,
+            UINT32/2, UINT32/2 + 1, UINT32 - 1, UINT32]
+UINT64_V = UINT32_V + [0x1000000, 0x10000000, 0x100000000, UINT64/4,
+                       UINT64/2 - 1, UINT64/2, UINT64/2 + 1, UINT64 - 1,
+                       UINT64]
+STRING_V = ['%s%p%x%d', '.1024d', '%.2049d', '%p%p%p%p', '%x%x%x%x',
+            '%d%d%d%d', '%s%s%s%s', '%99999999999s', '%08x', '%%20d', '%%20n',
+            '%%20x', '%%20s', '%s%s%s%s%s%s%s%s%s%s', '%p%p%p%p%p%p%p%p%p%p',
+            '%#0123456x%08x%x%s%p%d%n%o%u%c%h%l%q%j%z%Z%t%i%e%g%f%a%C%S%08x%%',
+            '%s x 129', '%x x 257']
+
+
+def random_from_intervals(intervals):
+    """Select a random integer number from the list of specified intervals
+
+    Each interval is a tuple of lower and upper limits of the interval. The
+    limits are included. Intervals in a list should not overlap.
+    """
+    total = reduce(lambda x, y: x + y[1] - y[0] + 1, intervals, 0)
+    r = random.randint(0, total-1) + intervals[0][0]
+    temp = zip(intervals, intervals[1:])
+    for x in temp:
+        r = r + (r > x[0][1])*(x[1][0] - x[0][1] - 1)
+    return r
+
+
+def random_bits(bit_ranges):
+    """Generate random binary mask with ones in the specified bit ranges
+
+    Each bit_ranges is a list of tuples of lower and upper limits of bit
+    positions will be fuzzed. The limits are included. Random amount of bits
+    in range limits will be set to ones. The mask is returned in decimal
+    integer format.
+    """
+    bit_numbers = []
+    # Select random amount of random positions in bit_ranges
+    for rng in bit_ranges:
+        bit_numbers += random.sample(range(rng[0], rng[1] + 1),
+                                     random.randint(0, rng[1] - rng[0] + 1))
+    val = 0
+    # Set bits on selected possitions to ones
+    for bit in bit_numbers:
+        val |= 1 << bit
+    return val
+
+
+def truncate_string(strings, length):
+    """Return strings truncated to the specified length"""
+    if type(strings) == list:
+        return [s[:length] for s in strings]
+    else:
+        return strings[:length]
+
+
+def int_validator(current, intervals):
+    """Return a random value from intervals not equal to the current.
+
+    This function is useful for selection from valid values except current one.
+    """
+    val = random_from_intervals(intervals)
+    if val == current:
+        return int_validator(current, intervals)
+    else:
+        return val
+
+
+def bit_validator(current, bit_ranges):
+    """Return a random bit mask not equal to the current.
+
+    This function is useful for selection from valid values except current one.
+    """
+
+    val = random_bits(bit_ranges)
+    if val == current:
+        return bit_validator(current, bit_ranges)
+    else:
+        return val
+
+
+def string_validator(current, strings):
+    """Return a random string value from the list not equal to the current.
+
+    This function is useful for selection from valid values except current one.
+    """
+    val = random.choice(strings)
+    if val == current:
+        return string_validator(current, strings)
+    else:
+        return val
+
+
+def selector(current, constraints, fmt=None):
+    """Select one value from all defined by constraints
+
+    Each constraint produces one random value satisfying to it. The function
+    randomly selects one value satisfying at least one constraint (depending on
+    constraints overlaps).
+    """
+    validate = {
+        'bitmask': bit_validator,
+        'string': string_validator
+    }.get(fmt, int_validator)
+
+    def iter_validate(c):
+        """Apply validate() only to constraints represented as lists
+
+        This auxiliary function replaces short circuit conditions not supported
+        in Python 2.4
+        """
+        if type(c) == list:
+            return validate(current, c)
+        else:
+            return c
+    fuzz_values = [iter_validate(c) for c in constraints]
+    # Remove current for cases it's implicitly specified in constraints
+    # Duplicate validator functionality to prevent decreasing of probability
+    # to get one of allowable values
+    # TODO: remove validators after implementation of intelligent selection
+    # of fields will be fuzzed
+    try:
+        fuzz_values.remove(current)
+    except ValueError:
+        pass
+    return random.choice(fuzz_values)
+
+
+def magic(current):
+    """Fuzz magic header field
+
+    The function just returns the current magic value and provides uniformity
+    of calls for all fuzzing functions
+    """
+    return current
+
+
+def version(current):
+    """Fuzz version header field"""
+    constraints = UINT32_V + [
+        [(2, 3)],  # correct values
+        [(0, 1), (4, UINT32)]
+    ]
+    return selector(current, constraints)
+
+
+def backing_file_offset(current):
+    """Fuzz backing file offset header field"""
+    constraints = UINT64_V
+    return selector(current, constraints)
+
+
+def backing_file_size(current):
+    """Fuzz backing file size header field"""
+    constraints = UINT32_V
+    return selector(current, constraints)
+
+
+def cluster_bits(current):
+    """Fuzz cluster bits header field"""
+    constraints = UINT32_V + [
+        [(9, 20)],  # correct values
+        [(0, 9), (20, UINT32)]
+    ]
+    return selector(current, constraints)
+
+
+def size(current):
+    """Fuzz image size header field"""
+    constraints = UINT64_V
+    return selector(current, constraints)
+
+
+def crypt_method(current):
+    """Fuzz crypt method header field"""
+    constraints = UINT32_V + [
+        1,
+        [(2, UINT32)]
+    ]
+    return selector(current, constraints)
+
+
+def l1_size(current):
+    """Fuzz L1 table size header field"""
+    constraints = UINT32_V
+    return selector(current, constraints)
+
+
+def l1_table_offset(current):
+    """Fuzz L1 table offset header field"""
+    constraints = UINT64_V
+    return selector(current, constraints)
+
+
+def refcount_table_offset(current):
+    """Fuzz refcount table offset header field"""
+    constraints = UINT64_V
+    return selector(current, constraints)
+
+
+def refcount_table_clusters(current):
+    """Fuzz refcount table clusters header field"""
+    constraints = UINT32_V
+    return selector(current, constraints)
+
+
+def nb_snapshots(current):
+    """Fuzz number of snapshots header field"""
+    constraints = UINT32_V
+    return selector(current, constraints)
+
+
+def snapshots_offset(current):
+    """Fuzz snapshots offset header field"""
+    constraints = UINT64_V
+    return selector(current, constraints)
+
+
+def incompatible_features(current):
+    """Fuzz incompatible features header field"""
+    constraints = [
+        [(0, 1)],  # allowable values
+        [(0, UINT64_M)]
+    ]
+    return selector(current, constraints, 'bitmask')
+
+
+def compatible_features(current):
+    """Fuzz compatible features header field"""
+    constraints = [
+        [(0, UINT64_M)]
+    ]
+    return selector(current, constraints, 'bitmask')
+
+
+def autoclear_features(current):
+    """Fuzz autoclear features header field"""
+    constraints = [
+        [(0, UINT64_M)]
+    ]
+    return selector(current, constraints, 'bitmask')
+
+
+def refcount_order(current):
+    """Fuzz number of refcount order header field"""
+    constraints = UINT32_V
+    return selector(current, constraints)
+
+
+def header_length(current):
+    """Fuzz number of refcount order header field"""
+    constraints = UINT32_V + [
+        72,
+        104,
+        [(0, UINT32)]
+    ]
+    return selector(current, constraints)
+
+
+def ext_magic(current):
+    """Fuzz magic field of a header extension"""
+    constraints = UINT32_V
+    return selector(current, constraints)
+
+
+def ext_length(current):
+    """Fuzz length field of a header extension"""
+    constraints = UINT32_V
+    return selector(current, constraints)
+
+
+def bf_data(current):
+    """Fuzz backing file format in the corresponding header extension"""
+    constraints = [
+        truncate_string(STRING_V, len(current)),
+        truncate_string(STRING_V, (len(current) + 7) & ~7)  # Fuzz padding
+    ]
+    return selector(current, constraints, 'string')
+
+
+def feat_type(current):
+    """Fuzz feature type field of a feature name table header extension"""
+    constraints = [
+        [(0, 255)]
+    ]
+    return selector(current, constraints)
+
+
+def feat_bit_number(current):
+    """Fuzz bit number field of a feature name table header extension"""
+    constraints = [
+        [(0, 255)]
+    ]
+    return selector(current, constraints)
+
+
+def feat_name(current):
+    """Fuzz feature name field of a feature name table header extension"""
+    constraints = [
+        truncate_string(STRING_V, len(current)),
+        truncate_string(STRING_V, 48)  # Fuzz padding (field length = 48)
+    ]
+    return selector(current, constraints, 'string')

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+# Generator of fuzzed qcow2 images
+#
+# Copyright (C) 2014 Maria Kustova <[email protected]>
+#
+# 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 random
+import struct
+import qcow2.fuzz
+
+MAX_IMAGE_SIZE = 10*2**20
+# Standard sizes
+UINT32_S = 4
+UINT64_S = 8
+
+# Percentage of fields will be fuzzed
+BIAS = random.uniform(0.2, 0.5)
+
+
+class Field(object):
+    """Atomic image element (field)
+
+    The class represents an image field as quadruple of a data format
+    of value necessary for its packing to binary form, an offset from
+    the beginning of the image, a value and a name.
+
+    The field can be iterated as a list [format, offset, value]
+    """
+    __slots__ = ('fmt', 'offset', 'value', 'name')
+
+    def __init__(self, fmt, offset, val, name):
+        self.fmt = fmt
+        self.offset = offset
+        self.value = val
+        self.name = name
+
+    def __iter__(self):
+        return (x for x in [self.fmt, self.offset, self.value])
+
+    def __repr__(self):
+        return "Field(fmt='%s', offset=%d, value=%s, name=%s)" % \
+            (self.fmt, self.offset, str(self.value), self.name)
+
+
+class FieldsList(object):
+    """List of fields
+
+    The class allows access to a field in the list by its name and joins
+    several list in one via in-place addition
+    """
+    def __init__(self, meta_data=None):
+        if meta_data is None:
+            self.data = []
+        else:
+            self.data = [Field(f[0], f[1], f[2], f[3])
+                         for f in meta_data]
+
+    def __getitem__(self, name):
+        return [x for x in self.data if x.name == name]
+
+    def __iter__(self):
+        return (x for x in self.data)
+
+    def __iadd__(self, other):
+        self.data += other.data
+        return self
+
+    def __len__(self):
+        return len(self.data)
+
+
+class Image(object):
+    """ Qcow2 image object
+
+    This class allows to create valid qcow2 images with random structure,
+    fuzz them via external qcow2.fuzz module and write to files.
+    """
+    @staticmethod
+    def _size_params():
+        """Generate a random file size aligned to a random correct cluster size
+        """
+        cluster_bits = random.randrange(9, 21)
+        cluster_size = 1 << cluster_bits
+        # Minimal image size is equal to 5 clusters as for qcow2 empty image
+        # created by qemu-img
+        file_size = random.randrange(5*cluster_size, MAX_IMAGE_SIZE + 1,
+                                     cluster_size)
+        return [cluster_bits, file_size]
+
+    @staticmethod
+    def _header(cluster_bits, img_size):
+        """Generate a random valid header"""
+        meta_header = [
+            ['>4s', 0, "QFI\xfb", 'magic'],
+            ['>I', 4, random.randint(2, 3), 'version'],
+            ['>Q', 8, 0, 'backing_file_offset'],
+            ['>I', 16, 0, 'backing_file_size'],
+            ['>I', 20, cluster_bits, 'cluster_bits'],
+            ['>Q', 24, img_size, 'size'],
+            ['>I', 32, 0, 'crypt_method'],
+            ['>I', 36, 0, 'l1_size'],
+            ['>Q', 40, 0, 'l1_table_offset'],
+            ['>Q', 48, 0, 'refcount_table_offset'],
+            ['>I', 56, 0, 'refcount_table_clusters'],
+            ['>I', 60, 0, 'nb_snapshots'],
+            ['>Q', 64, 0, 'snapshots_offset'],
+            ['>Q', 72, 0, 'incompatible_features'],
+            ['>Q', 80, 0, 'compatible_features'],
+            ['>Q', 88, 0, 'autoclear_features'],
+            # From the e-mail thread for [PATCH] docs: Define refcount_bits
+            # value: Only refcount_order = 4 is supported by QEMU at the moment
+            ['>I', 96, 4, 'refcount_order'],
+            ['>I', 100, 0, 'header_length']
+        ]
+        v_header = FieldsList(meta_header)
+
+        if v_header['version'][0].value == 2:
+            v_header['header_length'][0].value = 72
+        else:
+            v_header['incompatible_features'][0].value = random.getrandbits(2)
+            v_header['compatible_features'][0].value = random.getrandbits(1)
+            v_header['header_length'][0].value = 104
+
+        return v_header
+
+    @staticmethod
+    def _backing_file_format_ext(header):
+        """Generate a random header extension for name of backing file
+        format
+
+        If the header doesn't contain information about the backing file,
+        this extension is left empty
+        """
+        offset = struct.calcsize(header['header_length'][0].fmt) + \
+                 header['header_length'][0].offset
+
+        if not header['backing_file_offset'][0].value == 0:
+            # Till real backup image is not supported, a random valid fmt
+            # is set
+            ext_data = random.choice(['raw', 'qcow', 'qcow2', 'qed',
+                                      'cow', 'vdi', 'vmdk', 'vpc',
+                                      'vhdx', 'bochs', 'cloop',
+                                      'dmg', 'parallels'])
+            ext_data_len = len(ext_data)
+            ext_data_padded = '>' + str(ext_data_len) + 's' + \
+                              str(7 - (ext_data_len - 1) % 8) + 'x'
+            ext = FieldsList([
+                ['>I', offset, 0xE2792ACA, 'ext_magic'],
+                ['>I', offset + UINT32_S, ext_data_len, 'ext_length'],
+                [ext_data_padded, offset + UINT32_S*2, ext_data,
+                 'bf_data']
+            ])
+            offset = ext['bf_data'][0].offset + \
+                     struct.calcsize(ext['bf_data'][0].fmt)
+        else:
+            ext = FieldsList()
+        return (ext, offset)
+
+    @staticmethod
+    def _feature_name_table_ext(header, offset):
+        """Generate a random header extension for names of features used in
+        the image
+
+        If all features bit masks in the header are set to zeroes,
+        this extension is left empty
+        """
+        feature_tables = []
+        # Current offset + magic and length fields of the feature table
+        # extension
+        inner_offset = offset + 2*UINT32_S
+
+        # Each tuple includes bit value in the corresponding header field,
+        # feature type, number of the bit in the header field and feature name
+        feature_list = [
+            (header['incompatible_features'][0].value & 1, 0,
+             1, 'dirty bit'),
+            (header['incompatible_features'][0].value & 2, 0,
+             2, 'corrupt bit'),
+            (header['compatible_features'][0].value & 1, 1,
+             1, 'lazy refcounts bit')
+        ]
+        for item in feature_list:
+            if not item[0] == 0:
+                name_len = len(item[3])
+                name_padded_fmt = '>' + str(name_len) + 's' + \
+                                  str(46 - name_len) + 'x'
+                feature_tables += [['B', inner_offset, item[1], 'feat_type'],
+                                   ['B', inner_offset + 1,
+                                    item[2], 'feat_bit_number'],
+                                   [name_padded_fmt, inner_offset + 2,
+                                    item[3], 'feat_name']
+                ]
+                inner_offset = inner_offset + 2 + \
+                               struct.calcsize(name_padded_fmt)
+
+        if not len(feature_tables) == 0:
+            # No padding for the extension is necessary, because
+            # the extension length = 8 + 48*N is multiple of 8
+            ext = FieldsList([
+                ['>I', offset, 0x6803f857, 'ext_magic'],
+                ['>I', offset + UINT32_S, len(feature_tables)*48,'ext_length']
+            ] + feature_tables)
+            offset = inner_offset
+        else:
+            ext = FieldsList()
+
+        return (ext, offset)
+
+    @staticmethod
+    def _end_ext(offset):
+        """Generate a mandatory header extension marking end of header
+        extensions
+        """
+        ext = FieldsList([
+            ['>I', offset, 0, 'ext_magic'],
+            ['>I', offset + UINT32_S, 0, 'ext_length']
+        ])
+        return (ext, offset)
+
+    def __init__(self):
+        """Create a random valid qcow2 image with the correct inner structure
+        and allowable values
+        """
+        # Image size is necessary for writing, but the field with it can be
+        # fuzzed so it's saved separately.
+        cluster_bits, self.image_size = self._size_params()
+        self.header = self._header(cluster_bits, self.image_size)
+        self.backing_file_format_ext, \
+            offset = self._backing_file_format_ext(self.header)
+        self.feature_name_table_ext, \
+            offset = self._feature_name_table_ext(self.header, offset)
+        self.end_ext, offset = self._end_ext(offset)
+        # Container for entire image
+        self.data = FieldsList()
+
+    def __iter__(self):
+        return (x for x in [self.header, self.backing_file_format_ext,
+                            self.feature_name_table_ext, self.end_ext])
+
+    def _join(self):
+        """Join all image structure elements as header, tables, etc in one
+        list of fields
+        """
+        if len(self.data) == 0:
+            for v in self:
+                self.data += v
+
+    def fuzz(self, fields_to_fuzz=None):
+        """Fuzz an image by corrupting values of a random subset of its fields
+
+        Without parameters the method fuzzes an entire image.
+        If 'fields_to_fuzz' is specified then only fields in this list will be
+        fuzzed. 'fields_to_fuzz' can contain both individual fields and more
+        general image elements as header or tables.
+        In the first case the single field will be fuzzed always.
+        In the second a random subset of fields will be selected and fuzzed.
+        """
+        def coin():
+            """Return boolean value proportional to a portion of fields to be
+            fuzzed
+            """
+            return random.random() < BIAS
+
+        if fields_to_fuzz is None:
+            self._join()
+            for field in self.data:
+                if coin():
+                    field.value = getattr(qcow2.fuzz, field.name)(field.value)
+        else:
+            for item in fields_to_fuzz:
+                if len(item) == 1:
+                    for field in self.__dict__[item[0]]:
+                        if coin():
+                            field.value = getattr(qcow2.fuzz,
+                                                  field.name)(field.value)
+                else:
+                    for field in self.__dict__[item[0]][item[1]]:
+                        try:
+                            field.value = getattr(qcow2.fuzz, field.name)(
+                                field.value)
+                        except AttributeError:
+                            # Some fields can be skipped depending on
+                            # references, e.g. FNT header extension is not
+                            # generated for a feature mask header field
+                            # equal to zero
+                            pass
+
+    def write(self, filename):
+        """Writes an entire image to the file"""
+        image_file = open(filename, 'w')
+        # Create an empty image
+        # (sparse if FS supports it or preallocated otherwise)
+        image_file.seek(self.image_size - 1)
+        image_file.write("\0")
+        self._join()
+        for field in self.data:
+            image_file.seek(field.offset)
+            image_file.write(struct.pack(field.fmt, field.value))
+
+        image_file.close()
+
+
+def create_image(test_img_path, fields_to_fuzz=None):
+    """Create a fuzzed image and write it to the specified file"""
+    image = Image()
+    image.fuzz(fields_to_fuzz)
+    image.write(test_img_path)

new file mode 100644
index 0000000..e2ebe19
@@ -0,0 +1 @@
+from layout import create_image

new file mode 100644
index 0000000..ecbc059
@@ -0,0 +1,339 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 2, June 1991
+
+ Copyright (C) 1989, 1991 Free Software Foundation, Inc.,
+ 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+                            Preamble
+
+  The licenses for most software are designed to take away your
+freedom to share and change it.  By contrast, the GNU General Public
+License is intended to guarantee your freedom to share and change free
+software--to make sure the software is free for all its users.  This
+General Public License applies to most of the Free Software
+Foundation's software and to any other program whose authors commit to
+using it.  (Some other Free Software Foundation software is covered by
+the GNU Lesser General Public License instead.)  You can apply it to
+your programs, too.
+
+  When we speak of free software, we are referring to freedom, not
+price.  Our General Public Licenses are designed to make sure that you
+have the freedom to distribute copies of free software (and charge for
+this service if you wish), that you receive source code or can get it
+if you want it, that you can change the software or use pieces of it
+in new free programs; and that you know you can do these things.
+
+  To protect your rights, we need to make restrictions that forbid
+anyone to deny you these rights or to ask you to surrender the rights.
+These restrictions translate to certain responsibilities for you if you
+distribute copies of the software, or if you modify it.
+
+  For example, if you distribute copies of such a program, whether
+gratis or for a fee, you must give the recipients all the rights that
+you have.  You must make sure that they, too, receive or can get the
+source code.  And you must show them these terms so they know their
+rights.
+
+  We protect your rights with two steps: (1) copyright the software, and
+(2) offer you this license which gives you legal permission to copy,
+distribute and/or modify the software.
+
+  Also, for each author's protection and ours, we want to make certain
+that everyone understands that there is no warranty for this free
+software.  If the software is modified by someone else and passed on, we
+want its recipients to know that what they have is not the original, so
+that any problems introduced by others will not reflect on the original
+authors' reputations.
+
+  Finally, any free program is threatened constantly by software
+patents.  We wish to avoid the danger that redistributors of a free
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+
+  The precise terms and conditions for copying, distribution and
+modification follow.
+
+                    GNU GENERAL PUBLIC LICENSE
+   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+
+  0. This License applies to any program or other work which contains
+a notice placed by the copyright holder saying it may be distributed
+under the terms of this General Public License.  The "Program", below,
+refers to any such program or work, and a "work based on the Program"
+means either the Program or any derivative work under copyright law:
+that is to say, a work containing the Program or a portion of it,
+either verbatim or with modifications and/or translated into another
+language.  (Hereinafter, translation is included without limitation in
+the term "modification".)  Each licensee is addressed as "you".
+
+Activities other than copying, distribution and modification are not
+covered by this License; they are outside its scope.  The act of
+running the Program is not restricted, and the output from the Program
+is covered only if its contents constitute a work based on the
+Program (independent of having been made by running the Program).
+Whether that is true depends on what the Program does.
+
+  1. You may copy and distribute verbatim copies of the Program's
+source code as you receive it, in any medium, provided that you
+conspicuously and appropriately publish on each copy an appropriate
+copyright notice and disclaimer of warranty; keep intact all the
+notices that refer to this License and to the absence of any warranty;
+and give any other recipients of the Program a copy of this License
+along with the Program.
+
+You may charge a fee for the physical act of transferring a copy, and
+you may at your option offer warranty protection in exchange for a fee.
+
+  2. You may modify your copy or copies of the Program or any portion
+of it, thus forming a work based on the Program, and copy and
+distribute such modifications or work under the terms of Section 1
+above, provided that you also meet all of these conditions:
+
+    a) You must cause the modified files to carry prominent notices
+    stating that you changed the files and the date of any change.
+
+    b) You must cause any work that you distribute or publish, that in
+    whole or in part contains or is derived from the Program or any
+    part thereof, to be licensed as a whole at no charge to all third
+    parties under the terms of this License.
+
+    c) If the modified program normally reads commands interactively
+    when run, you must cause it, when started running for such
+    interactive use in the most ordinary way, to print or display an
+    announcement including an appropriate copyright notice and a
+    notice that there is no warranty (or else, saying that you provide
+    a warranty) and that users may redistribute the program under
+    these conditions, and telling the user how to view a copy of this
+    License.  (Exception: if the Program itself is interactive but
+    does not normally print such an announcement, your work based on
+    the Program is not required to print an announcement.)
+
+These requirements apply to the modified work as a whole.  If
+identifiable sections of that work are not derived from the Program,
+and can be reasonably considered independent and separate works in
+themselves, then this License, and its terms, do not apply to those
+sections when you distribute them as separate works.  But when you
+distribute the same sections as part of a whole which is a work based
+on the Program, the distribution of the whole must be on the terms of
+this License, whose permissions for other licensees extend to the
+entire whole, and thus to each and every part regardless of who wrote it.
+
+Thus, it is not the intent of this section to claim rights or contest
+your rights to work written entirely by you; rather, the intent is to
+exercise the right to control the distribution of derivative or
+collective works based on the Program.
+
+In addition, mere aggregation of another work not based on the Program
+with the Program (or with a work based on the Program) on a volume of
+a storage or distribution medium does not bring the other work under
+the scope of this License.
+
+  3. You may copy and distribute the Program (or a work based on it,
+under Section 2) in object code or executable form under the terms of
+Sections 1 and 2 above provided that you also do one of the following:
+
+    a) Accompany it with the complete corresponding machine-readable
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+
+    c) Accompany it with the information you received as to the offer
+    to distribute corresponding source code.  (This alternative is
+    allowed only for noncommercial distribution and only if you
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+    an offer, in accord with Subsection b above.)
+
+The source code for a work means the preferred form of the work for
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+associated interface definition files, plus the scripts used to
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+
+If distribution of executable or object code is made by offering
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+access to copy the source code from the same place counts as
+distribution of the source code, even though third parties are not
+compelled to copy the source along with the object code.
+
+  4. You may not copy, modify, sublicense, or distribute the Program
+except as expressly provided under this License.  Any attempt
+otherwise to copy, modify, sublicense or distribute the Program is
+void, and will automatically terminate your rights under this License.
+However, parties who have received copies, or rights, from you under
+this License will not have their licenses terminated so long as such
+parties remain in full compliance.
+
+  5. You are not required to accept this License, since you have not
+signed it.  However, nothing else grants you permission to modify or
+distribute the Program or its derivative works.  These actions are
+prohibited by law if you do not accept this License.  Therefore, by
+modifying or distributing the Program (or any work based on the
+Program), you indicate your acceptance of this License to do so, and
+all its terms and conditions for copying, distributing or modifying
+the Program or works based on it.
+
+  6. Each time you redistribute the Program (or any work based on the
+Program), the recipient automatically receives a license from the
+original licensor to copy, distribute or modify the Program subject to
+these terms and conditions.  You may not impose any further
+restrictions on the recipients' exercise of the rights granted herein.
+You are not responsible for enforcing compliance by third parties to
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+
+  7. If, as a consequence of a court judgment or allegation of patent
+infringement or for any other reason (not limited to patent issues),
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+distribute so as to satisfy simultaneously your obligations under this
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+refrain entirely from distribution of the Program.
+
+If any portion of this section is held invalid or unenforceable under
+any particular circumstance, the balance of the section is intended to
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+
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+
+This section is intended to make thoroughly clear what is believed to
+be a consequence of the rest of this License.
+
+  8. If the distribution and/or use of the Program is restricted in
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+original copyright holder who places the Program under this License
+may add an explicit geographical distribution limitation excluding
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+
+  9. The Free Software Foundation may publish revised and/or new versions
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+address new problems or concerns.
+
+Each version is given a distinguishing version number.  If the Program
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+Foundation.
+
+  10. If you wish to incorporate parts of the Program into other free
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+
+                            NO WARRANTY
+
+  11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
+FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
+OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
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+TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
+PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
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+
+  12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
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+POSSIBILITY OF SUCH DAMAGES.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+convey the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    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, write to the Free Software Foundation, Inc.,
+    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+Also add information on how to contact you by electronic and paper mail.
+
+If the program is interactive, make it output a short notice like this
+when it starts in an interactive mode:
+
+    Gnomovision version 69, Copyright (C) year name of author
+    Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, the commands you use may
+be called something other than `show w' and `show c'; they could even be
+mouse-clicks or menu items--whatever suits your program.
+
+You should also get your employer (if you work as a programmer) or your
+school, if any, to sign a "copyright disclaimer" for the program, if
+necessary.  Here is a sample; alter the names:
+
+  Yoyodyne, Inc., hereby disclaims all copyright interest in the program
+  `Gnomovision' (which makes passes at compilers) written by James Hacker.
+
+  <signature of Ty Coon>, 1 April 1989
+  Ty Coon, President of Vice
+
+This General Public License does not permit incorporating your program into
+proprietary programs.  If your program is a subroutine library, you may
+consider it more useful to permit linking proprietary applications with the
+library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.
\ No newline at end of file