Source code for glance.common.format_inspector

# Copyright 2020 Red Hat, Inc
# All Rights Reserved.
#    Licensed under the Apache License, Version 2.0 (the "License"); you may
#    not use this file except in compliance with the License. You may obtain
#    a copy of the License at
#    Unless required by applicable law or agreed to in writing, software
#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
#    License for the specific language governing permissions and limitations
#    under the License.

This is a python implementation of virtual disk format inspection routines
gathered from various public specification documents, as well as qemu disk
driver code. It attempts to store and parse the minimum amount of data
required, and in a streaming-friendly manner to collect metadata about
complex-format images.

import struct

from oslo_log import log as logging

LOG = logging.getLogger(__name__)

[docs]class CaptureRegion(object): """Represents a region of a file we want to capture. A region of a file we want to capture requires a byte offset into the file and a length. This is expected to be used by a data processing loop, calling capture() with the most recently-read chunk. This class handles the task of grabbing the desired region of data across potentially multiple fractional and unaligned reads. :param offset: Byte offset into the file starting the region :param length: The length of the region """ def __init__(self, offset, length): self.offset = offset self.length = length = b'' @property def complete(self): """Returns True when we have captured the desired data.""" return self.length == len(
[docs] def capture(self, chunk, current_position): """Process a chunk of data. This should be called for each chunk in the read loop, at least until complete returns True. :param chunk: A chunk of bytes in the file :param current_position: The position of the file processed by the read loop so far. Note that this will be the position in the file *after* the chunk being presented. """ read_start = current_position - len(chunk) if (read_start <= self.offset <= current_position or self.offset <= read_start <= (self.offset + self.length)): if read_start < self.offset: lead_gap = self.offset - read_start else: lead_gap = 0 += chunk[lead_gap:] =[:self.length]
[docs]class ImageFormatError(Exception): """An unrecoverable image format error that aborts the process.""" pass
[docs]class TraceDisabled(object): """A logger-like thing that swallows tracing when we do not want it."""
[docs] def debug(self, *a, **k): pass
info = debug warning = debug error = debug
[docs]class FileInspector(object): """A stream-based disk image inspector. This base class works on raw images and is subclassed for more complex types. It is to be presented with the file to be examined one chunk at a time, during read processing and will only store as much data as necessary to determine required attributes of the file. """ def __init__(self, tracing=False): self._total_count = 0 # NOTE(danms): The logging in here is extremely verbose for a reason, # but should never really be enabled at that level at runtime. To # retain all that work and assist in future debug, we have a separate # debug flag that can be passed from a manual tool to turn it on. if tracing: self._log = logging.getLogger(str(self)) else: self._log = TraceDisabled() self._capture_regions = {} def _capture(self, chunk, only=None): for name, region in self._capture_regions.items(): if only and name not in only: continue if not region.complete: region.capture(chunk, self._total_count)
[docs] def eat_chunk(self, chunk): """Call this to present chunks of the file to the inspector.""" pre_regions = set(self._capture_regions.keys()) # Increment our position-in-file counter self._total_count += len(chunk) # Run through the regions we know of to see if they want this # data self._capture(chunk) # Let the format do some post-read processing of the stream self.post_process() # Check to see if the post-read processing added new regions # which may require the current chunk. new_regions = set(self._capture_regions.keys()) - pre_regions if new_regions: self._capture(chunk, only=new_regions)
[docs] def post_process(self): """Post-read hook to process what has been read so far. This will be called after each chunk is read and potentially captured by the defined regions. If any regions are defined by this call, those regions will be presented with the current chunk in case it is within one of the new regions. """ pass
[docs] def region(self, name): """Get a CaptureRegion by name.""" return self._capture_regions[name]
[docs] def new_region(self, name, region): """Add a new CaptureRegion by name.""" if self.has_region(name): # This is a bug, we tried to add the same region twice raise ImageFormatError('Inspector re-added region %s' % name) self._capture_regions[name] = region
[docs] def has_region(self, name): """Returns True if named region has been defined.""" return name in self._capture_regions
@property def format_match(self): """Returns True if the file appears to be the expected format.""" return True @property def virtual_size(self): """Returns the virtual size of the disk image, or zero if unknown.""" return self._total_count @property def actual_size(self): """Returns the total size of the file, usually smaller than virtual_size. """ return self._total_count def __str__(self): """The string name of this file format.""" return 'raw' @property def context_info(self): """Return info on amount of data held in memory for auditing. This is a dict of region:sizeinbytes items that the inspector uses to examine the file. """ return {name: len( for name, region in self._capture_regions.items()}
# The qcow2 format consists of a big-endian 72-byte header, of which # only a small portion has information we care about: # # Dec Hex Name # 0 0x00 Magic 4-bytes 'QFI\xfb' # 4 0x04 Version (uint32_t, should always be 2 for modern files) # . . . # 24 0x18 Size in bytes (unint64_t) # #
[docs]class QcowInspector(FileInspector): """QEMU QCOW2 Format This should only require about 32 bytes of the beginning of the file to determine the virtual size. """ def __init__(self, *a, **k): super(QcowInspector, self).__init__(*a, **k) self.new_region('header', CaptureRegion(0, 512)) def _qcow_header_data(self): magic, version, bf_offset, bf_sz, cluster_bits, size = ( struct.unpack('>4sIQIIQ', self.region('header').data[:32])) return magic, size @property def virtual_size(self): if not self.region('header').complete: return 0 if not self.format_match: return 0 magic, size = self._qcow_header_data() return size @property def format_match(self): if not self.region('header').complete: return False magic, size = self._qcow_header_data() return magic == b'QFI\xFB' def __str__(self): return 'qcow2'
# The VHD (or VPC as QEMU calls it) format consists of a big-endian # 512-byte "footer" at the beginning of the file with various # information, most of which does not matter to us: # # Dec Hex Name # 0 0x00 Magic string (8-bytes, always 'conectix') # 40 0x28 Disk size (uint64_t) # #
[docs]class VHDInspector(FileInspector): """Connectix/MS VPC VHD Format This should only require about 512 bytes of the beginning of the file to determine the virtual size. """ def __init__(self, *a, **k): super(VHDInspector, self).__init__(*a, **k) self.new_region('header', CaptureRegion(0, 512)) @property def format_match(self): return self.region('header').data.startswith(b'conectix') @property def virtual_size(self): if not self.region('header').complete: return 0 if not self.format_match: return 0 return struct.unpack('>Q', self.region('header').data[40:48])[0] def __str__(self): return 'vhd'
# The VHDX format consists of a complex dynamic little-endian # structure with multiple regions of metadata and data, linked by # offsets with in the file (and within regions), identified by MSFT # GUID strings. The header is a 320KiB structure, only a few pieces of # which we actually need to capture and interpret: # # Dec Hex Name # 0 0x00000 Identity (Technically 9-bytes, padded to 64KiB, the first # 8 bytes of which are 'vhdxfile') # 196608 0x30000 The Region table (64KiB of a 32-byte header, followed # by up to 2047 36-byte region table entry structures) # # The region table header includes two items we need to read and parse, # which are: # # 196608 0x30000 4-byte signature ('regi') # 196616 0x30008 Entry count (uint32-t) # # The region table entries follow the region table header immediately # and are identified by a 16-byte GUID, and provide an offset of the # start of that region. We care about the "metadata region", identified # by the METAREGION class variable. The region table entry is (offsets # from the beginning of the entry, since it could be in multiple places): # # 0 0x00000 16-byte MSFT GUID # 16 0x00010 Offset of the actual metadata region (uint64_t) # # When we find the METAREGION table entry, we need to grab that offset # and start examining the region structure at that point. That # consists of a metadata table of structures, which point to places in # the data in an unstructured space that follows. The header is # (offsets relative to the region start): # # 0 0x00000 8-byte signature ('metadata') # . . . # 16 0x00010 2-byte entry count (up to 2047 entries max) # # This header is followed by the specified number of metadata entry # structures, identified by GUID: # # 0 0x00000 16-byte MSFT GUID # 16 0x00010 4-byte offset (uint32_t, relative to the beginning of # the metadata region) # # We need to find the "Virtual Disk Size" metadata item, identified by # the GUID in the VIRTUAL_DISK_SIZE class variable, grab the offset, # add it to the offset of the metadata region, and examine that 8-byte # chunk of data that follows. # # The "Virtual Disk Size" is a naked uint64_t which contains the size # of the virtual disk, and is our ultimate target here. # #
[docs]class VHDXInspector(FileInspector): """MS VHDX Format This requires some complex parsing of the stream. The first 256KiB of the image is stored to get the header and region information, and then we capture the first metadata region to read those records, find the location of the virtual size data and parse it. This needs to store the metadata table entries up until the VDS record, which may consist of up to 2047 32-byte entries at max. Finally, it must store a chunk of data at the offset of the actual VDS uint64. """ METAREGION = '8B7CA206-4790-4B9A-B8FE-575F050F886E' VIRTUAL_DISK_SIZE = '2FA54224-CD1B-4876-B211-5DBED83BF4B8' def __init__(self, *a, **k): super(VHDXInspector, self).__init__(*a, **k) self.new_region('ident', CaptureRegion(0, 32)) self.new_region('header', CaptureRegion(192 * 1024, 64 * 1024))
[docs] def post_process(self): # After reading a chunk, we may have the following conditions: # # 1. We may have just completed the header region, and if so, # we need to immediately read and calculate the location of # the metadata region, as it may be starting in the same # read we just did. # 2. We may have just completed the metadata region, and if so, # we need to immediately calculate the location of the # "virtual disk size" record, as it may be starting in the # same read we just did. if self.region('header').complete and not self.has_region('metadata'): region = self._find_meta_region() if region: self.new_region('metadata', region) elif self.has_region('metadata') and not self.has_region('vds'): region = self._find_meta_entry(self.VIRTUAL_DISK_SIZE) if region: self.new_region('vds', region)
@property def format_match(self): return self.region('ident').data.startswith(b'vhdxfile') @staticmethod def _guid(buf): """Format a MSFT GUID from the 16-byte input buffer.""" guid_format = '<IHHBBBBBBBB' return '%08X-%04X-%04X-%02X%02X-%02X%02X%02X%02X%02X%02X' % ( struct.unpack(guid_format, buf)) def _find_meta_region(self): # The region table entries start after a 16-byte table header region_entry_first = 16 # Parse the region table header to find the number of regions regi, cksum, count, reserved = struct.unpack( '<IIII', self.region('header').data[:16]) if regi != 0x69676572: raise ImageFormatError('Region signature not found at %x' % ( self.region('header').offset)) if count >= 2048: raise ImageFormatError('Region count is %i (limit 2047)' % count) # Process the regions until we find the metadata one; grab the # offset and return self._log.debug('Region entry first is %x', region_entry_first) self._log.debug('Region entries %i', count) meta_offset = 0 for i in range(0, count): entry_start = region_entry_first + (i * 32) entry_end = entry_start + 32 entry = self.region('header').data[entry_start:entry_end] self._log.debug('Entry offset is %x', entry_start) # GUID is the first 16 bytes guid = self._guid(entry[:16]) if guid == self.METAREGION: # This entry is the metadata region entry meta_offset, meta_len, meta_req = struct.unpack( '<QII', entry[16:]) self._log.debug('Meta entry %i specifies offset: %x', i, meta_offset) # NOTE(danms): The meta_len in the region descriptor is the # entire size of the metadata table and data. This can be # very large, so we should only capture the size required # for the maximum length of the table, which is one 32-byte # table header, plus up to 2047 32-byte entries. meta_len = 2048 * 32 return CaptureRegion(meta_offset, meta_len) self._log.warning('Did not find metadata region') return None def _find_meta_entry(self, desired_guid): meta_buffer = self.region('metadata').data if len(meta_buffer) < 32: # Not enough data yet for full header return None # Make sure we found the metadata region by checking the signature sig, reserved, count = struct.unpack('<8sHH', meta_buffer[:12]) if sig != b'metadata': raise ImageFormatError( 'Invalid signature for metadata region: %r' % sig) entries_size = 32 + (count * 32) if len(meta_buffer) < entries_size: # Not enough data yet for all metadata entries. This is not # strictly necessary as we could process whatever we have until # we find the V-D-S one, but there are only 2047 32-byte # entries max (~64k). return None if count >= 2048: raise ImageFormatError( 'Metadata item count is %i (limit 2047)' % count) for i in range(0, count): entry_offset = 32 + (i * 32) guid = self._guid(meta_buffer[entry_offset:entry_offset + 16]) if guid == desired_guid: # Found the item we are looking for by id. # Stop our region from capturing item_offset, item_length, _reserved = struct.unpack( '<III', meta_buffer[entry_offset + 16:entry_offset + 28]) self.region('metadata').length = len(meta_buffer) self._log.debug('Found entry at offset %x', item_offset) # Metadata item offset is from the beginning of the metadata # region, not the file. return CaptureRegion( self.region('metadata').offset + item_offset, item_length) self._log.warning('Did not find guid %s', desired_guid) return None @property def virtual_size(self): # Until we have found the offset and have enough metadata buffered # to read it, return "unknown" if not self.has_region('vds') or not self.region('vds').complete: return 0 size, = struct.unpack('<Q', self.region('vds').data) return size def __str__(self): return 'vhdx'
# The VMDK format comes in a large number of variations, but the # single-file 'monolithicSparse' version 4 one is mostly what we care # about. It contains a 512-byte little-endian header, followed by a # variable-length "descriptor" region of text. The header looks like: # # Dec Hex Name # 0 0x00 4-byte magic string 'KDMV' # 4 0x04 Version (uint32_t) # 8 0x08 Flags (uint32_t, unused by us) # 16 0x10 Number of 512 byte sectors in the disk (uint64_t) # 24 0x18 Granularity (uint64_t, unused by us) # 32 0x20 Descriptor offset in 512-byte sectors (uint64_t) # 40 0x28 Descriptor size in 512-byte sectors (uint64_t) # # After we have the header, we need to find the descriptor region, # which starts at the sector identified in the "descriptor offset" # field, and is "descriptor size" 512-byte sectors long. Once we have # that region, we need to parse it as text, looking for the # createType=XXX line that specifies the mechanism by which the data # extents are stored in this file. We only support the # "monolithicSparse" format, so we just need to confirm that this file # contains that specifier. # #
[docs]class VMDKInspector(FileInspector): """vmware VMDK format (monolithicSparse variant only) This needs to store the 512 byte header and the descriptor region which should be just after that. The descriptor region is some variable number of 512 byte sectors, but is just text defining the layout of the disk. """ def __init__(self, *a, **k): super(VMDKInspector, self).__init__(*a, **k) self.new_region('header', CaptureRegion(0, 512))
[docs] def post_process(self): # If we have just completed the header region, we need to calculate # the location and length of the descriptor, which should immediately # follow and may have been partially-read in this read. if not self.region('header').complete: return sig, ver, _flags, _sectors, _grain, desc_sec, desc_num = struct.unpack( '<4sIIQQQQ', self.region('header').data[:44]) if sig != b'KDMV': raise ImageFormatError('Signature KDMV not found: %r' % sig) return if ver not in (1, 2, 3): raise ImageFormatError('Unsupported format version %i' % ver) return if not self.has_region('descriptor'): self.new_region('descriptor', CaptureRegion( desc_sec * 512, desc_num * 512))
@property def format_match(self): return self.region('header').data.startswith(b'KDMV') @property def virtual_size(self): if not self.has_region('descriptor'): # Not enough data yet return 0 descriptor_rgn = self.region('descriptor') if not descriptor_rgn.complete: # Not enough data yet return 0 descriptor = type_idx = descriptor.index(b'createType="') + len(b'createType="') type_end = descriptor.find(b'"', type_idx) # Make sure we don't grab and log a huge chunk of data in a # maliciously-formatted descriptor region if type_end - type_idx < 64: vmdktype = descriptor[type_idx:type_end] else: vmdktype = b'formatnotfound' if vmdktype != b'monolithicSparse': raise ImageFormatError('Unsupported VMDK format %s' % vmdktype) return 0 # If we have the descriptor, we definitely have the header _sig, _ver, _flags, sectors, _grain, _desc_sec, _desc_num = ( struct.unpack('<IIIQQQQ', self.region('header').data[:44])) return sectors * 512 def __str__(self): return 'vmdk'
# The VirtualBox VDI format consists of a 512-byte little-endian # header, some of which we care about: # # Dec Hex Name # 64 0x40 4-byte Magic (0xbeda107f) # . . . # 368 0x170 Size in bytes (uint64_t) # #
[docs]class VDIInspector(FileInspector): """VirtualBox VDI format This only needs to store the first 512 bytes of the image. """ def __init__(self, *a, **k): super(VDIInspector, self).__init__(*a, **k) self.new_region('header', CaptureRegion(0, 512)) @property def format_match(self): if not self.region('header').complete: return False signature, = struct.unpack('<I', self.region('header').data[0x40:0x44]) return signature == 0xbeda107f @property def virtual_size(self): if not self.region('header').complete: return 0 if not self.format_match: return 0 size, = struct.unpack('<Q', self.region('header').data[0x170:0x178]) return size def __str__(self): return 'vdi'
[docs]class InfoWrapper(object): """A file-like object that wraps another and updates a format inspector. This passes chunks to the format inspector while reading. If the inspector fails, it logs the error and stops calling it, but continues proxying data from the source to its user. """ def __init__(self, source, fmt): self._source = source self._format = fmt self._error = False def __iter__(self): return self def _process_chunk(self, chunk): if not self._error: try: self._format.eat_chunk(chunk) except Exception as e: # Absolutely do not allow the format inspector to break # our streaming of the image. If we failed, just stop # trying, log and keep going. LOG.error('Format inspector failed, aborting: %s', e) self._error = True def __next__(self): try: chunk = next(self._source) except StopIteration: raise self._process_chunk(chunk) return chunk
[docs] def read(self, size): chunk = self._process_chunk(chunk) return chunk
[docs] def close(self): if hasattr(self._source, 'close'): self._source.close()
[docs]def get_inspector(format_name): """Returns a FormatInspector class based on the given name. :param format_name: The name of the disk_format (raw, qcow2, etc). :returns: A FormatInspector or None if unsupported. """ formats = { 'raw': FileInspector, 'qcow2': QcowInspector, 'vhd': VHDInspector, 'vhdx': VHDXInspector, 'vmdk': VMDKInspector, 'vdi': VDIInspector, } return formats.get(format_name)