lldb/examples/summaries/cocoa/objc_runtime.py

"""
Objective-C runtime wrapper for use by LLDB Python formatters

part of The LLVM Compiler Infrastructure
This file is distributed under the University of Illinois Open Source
License. See LICENSE.TXT for details.
"""
import lldb
import cache
import attrib_fromdict
import functools
import Logger

class Utilities:
	@staticmethod
	def read_ascii(process, pointer,max_len=128):
		logger = Logger.Logger()
		error = lldb.SBError()
		content = None
		try:
			content = process.ReadCStringFromMemory(pointer,max_len,error)
		except:
			pass
		if content == None or len(content) == 0 or error.fail == True:
			return None
		return content

	@staticmethod
	def is_valid_pointer(pointer, pointer_size, allow_tagged=False, allow_NULL=False):
		logger = Logger.Logger()
		if pointer == None:
			return False
		if pointer == 0:
			return allow_NULL
		if allow_tagged and (pointer % 2) == 1:
			return True
		return ((pointer % pointer_size) == 0)

	# Objective-C runtime has a rule that pointers in a class_t will only have bits 0 thru 46 set
	# so if any pointer has bits 47 thru 63 high we know that this is not a valid isa
	@staticmethod
	def is_allowed_pointer(pointer):
		logger = Logger.Logger()
		if pointer == None:
			return False
		return ((pointer & 0xFFFF800000000000) == 0)

	@staticmethod
	def read_child_of(valobj,offset,type):
		logger = Logger.Logger()
		child = valobj.CreateChildAtOffset("childUNK",offset,type)
		if child == None or child.IsValid() == False:
			return None;
		return child.GetValueAsUnsigned()

	@staticmethod
	def is_valid_identifier(name):
		logger = Logger.Logger()
		if name is None:
			return None
		if len(name) == 0:
			return None
		# technically, the ObjC runtime does not enforce any rules about what name a class can have
		# in practice, the commonly used byte values for a class name are the letters, digits and some
		# symbols: $, %, -, _, .
		# WARNING: this means that you cannot use this runtime implementation if you need to deal
		# with class names that use anything but what is allowed here
		ok_values = dict.fromkeys("$%_.-ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890")
		return all(c in ok_values for c in name)

	@staticmethod
	def check_is_osx_lion(target):
		logger = Logger.Logger()
		# assume the only thing that has a Foundation.framework is a Mac
		# assume anything < Lion does not even exist
		mod = target.module['Foundation']
		if mod == None or mod.IsValid() == False:
			return None
		ver = mod.GetVersion()
		if ver == None or ver == []:
			return None
		return (ver[0] < 900)

	# a utility method that factors out code common to almost all the formatters
	# takes in an SBValue and a metrics object
	# returns a class_data and a wrapper (or None, if the runtime alone can't decide on a wrapper)
	@staticmethod
	def prepare_class_detection(valobj,statistics):
		logger = Logger.Logger()
		class_data = ObjCRuntime(valobj)
		if class_data.is_valid() == False:
			statistics.metric_hit('invalid_pointer',valobj)
			wrapper = InvalidPointer_Description(valobj.GetValueAsUnsigned(0) == 0)
			return class_data,wrapper
		class_data = class_data.read_class_data()
		if class_data.is_valid() == False:
			statistics.metric_hit('invalid_isa',valobj)
			wrapper = InvalidISA_Description()
			return class_data,wrapper
		if class_data.is_kvo():
			class_data = class_data.get_superclass()
		if class_data.is_valid() == False:
			statistics.metric_hit('invalid_isa',valobj)
			wrapper = InvalidISA_Description()
			return class_data,wrapper
		return class_data,None


class RoT_Data:
	def __init__(self,rot_pointer,params):
		logger = Logger.Logger()
		if (Utilities.is_valid_pointer(rot_pointer.GetValueAsUnsigned(),params.pointer_size, allow_tagged=False)):
			self.sys_params = params
			self.valobj = rot_pointer
			#self.flags = Utilities.read_child_of(self.valobj,0,self.sys_params.uint32_t)
			#self.instanceStart = Utilities.read_child_of(self.valobj,4,self.sys_params.uint32_t)
			self.instanceSize = None # lazy fetching
			offset = 24 if self.sys_params.is_64_bit else 16
			#self.ivarLayoutPtr = Utilities.read_child_of(self.valobj,offset,self.sys_params.addr_ptr_type)
			self.namePointer = Utilities.read_child_of(self.valobj,offset,self.sys_params.types_cache.addr_ptr_type)
			self.check_valid()
		else:
			logger >> "Marking as invalid - rot is invalid"
			self.valid = False
		if self.valid:
			self.name = Utilities.read_ascii(self.valobj.GetTarget().GetProcess(),self.namePointer)
			if not(Utilities.is_valid_identifier(self.name)):
				logger >> "Marking as invalid - name is invalid"
				self.valid = False

	# perform sanity checks on the contents of this class_ro_t
	def check_valid(self):
		self.valid = True
		# misaligned pointers seem to be possible for this field
		#if not(Utilities.is_valid_pointer(self.namePointer,self.sys_params.pointer_size,allow_tagged=False)):
		#	self.valid = False
		#	pass

	def __str__(self):
		logger = Logger.Logger()
		return \
		 "instanceSize = " + hex(self.instance_size()) + "\n" + \
		 "namePointer = " + hex(self.namePointer) + " --> " + self.name

	def is_valid(self):
		return self.valid

	def instance_size(self,align=False):
		logger = Logger.Logger()
		if self.is_valid() == False:
			return None
		if self.instanceSize == None:
			self.instanceSize = Utilities.read_child_of(self.valobj,8,self.sys_params.types_cache.uint32_t)
		if align:
			unalign = self.instance_size(False)
			if self.sys_params.is_64_bit:
				return ((unalign + 7) & ~7) % 0x100000000
			else:
				return ((unalign + 3) & ~3) % 0x100000000
		else:
			return self.instanceSize

class RwT_Data:
	def __init__(self,rwt_pointer,params):
		logger = Logger.Logger()
		if (Utilities.is_valid_pointer(rwt_pointer.GetValueAsUnsigned(),params.pointer_size, allow_tagged=False)):
			self.sys_params = params
			self.valobj = rwt_pointer
			#self.flags = Utilities.read_child_of(self.valobj,0,self.sys_params.uint32_t)
			#self.version = Utilities.read_child_of(self.valobj,4,self.sys_params.uint32_t)
			self.roPointer = Utilities.read_child_of(self.valobj,8,self.sys_params.types_cache.addr_ptr_type)
			self.check_valid()
		else:
			logger >> "Marking as invalid - rwt is invald"
			self.valid = False
		if self.valid:
			self.rot = self.valobj.CreateValueFromAddress("rot",self.roPointer,self.sys_params.types_cache.addr_ptr_type).AddressOf()
			self.data = RoT_Data(self.rot,self.sys_params)

	# perform sanity checks on the contents of this class_rw_t
	def check_valid(self):
		logger = Logger.Logger()
		self.valid = True
		if not(Utilities.is_valid_pointer(self.roPointer,self.sys_params.pointer_size,allow_tagged=False)):
			logger >> "Marking as invalid - ropointer is invalid"
			self.valid = False

	def __str__(self):
		logger = Logger.Logger()
		return \
		 "roPointer = " + hex(self.roPointer)

	def is_valid(self):
		logger = Logger.Logger()
		if self.valid:
			return self.data.is_valid()
		return False

class Class_Data_V2:
	def __init__(self,isa_pointer,params):
		logger = Logger.Logger()
		if (isa_pointer != None) and (Utilities.is_valid_pointer(isa_pointer.GetValueAsUnsigned(),params.pointer_size, allow_tagged=False)):
			self.sys_params = params
			self.valobj = isa_pointer
			self.check_valid()
		else:
			logger >> "Marking as invalid - isa is invalid or None"
			self.valid = False
		if self.valid:
			self.rwt = self.valobj.CreateValueFromAddress("rwt",self.dataPointer,self.sys_params.types_cache.addr_ptr_type).AddressOf()
			self.data = RwT_Data(self.rwt,self.sys_params)

	# perform sanity checks on the contents of this class_t
	# this call tries to minimize the amount of data fetched- as soon as we have "proven"
	# that we have an invalid object, we stop reading
	def check_valid(self):
		logger = Logger.Logger()
		self.valid = True
		
		self.isaPointer = Utilities.read_child_of(self.valobj,0,self.sys_params.types_cache.addr_ptr_type)
		if not(Utilities.is_valid_pointer(self.isaPointer,self.sys_params.pointer_size,allow_tagged=False)):
			logger >> "Marking as invalid - isaPointer is invalid"
			self.valid = False
			return
		if not(Utilities.is_allowed_pointer(self.isaPointer)):
			logger >> "Marking as invalid - isaPointer is not allowed"
			self.valid = False
			return

		self.cachePointer = Utilities.read_child_of(self.valobj,2*self.sys_params.pointer_size,self.sys_params.types_cache.addr_ptr_type)
		if not(Utilities.is_valid_pointer(self.cachePointer,self.sys_params.pointer_size,allow_tagged=False)):
			logger >> "Marking as invalid - cachePointer is invalid"
			self.valid = False
			return
		if not(Utilities.is_allowed_pointer(self.cachePointer)):
			logger >> "Marking as invalid - cachePointer is not allowed"
			self.valid = False
			return

		self.vtablePointer = Utilities.read_child_of(self.valobj,3*self.sys_params.pointer_size,self.sys_params.types_cache.addr_ptr_type)
		if not(Utilities.is_valid_pointer(self.vtablePointer,self.sys_params.pointer_size,allow_tagged=False)):
			logger >> "Marking as invalid - vtablePointer is invalid"
			self.valid = False
			return
		if not(Utilities.is_allowed_pointer(self.vtablePointer)):
			logger >> "Marking as invalid - vtablePointer is not allowed"
			self.valid = False
			return

		self.dataPointer = Utilities.read_child_of(self.valobj,4*self.sys_params.pointer_size,self.sys_params.types_cache.addr_ptr_type)
		if not(Utilities.is_valid_pointer(self.dataPointer,self.sys_params.pointer_size,allow_tagged=False)):
			logger >> "Marking as invalid - dataPointer is invalid"
			self.valid = False
			return
		if not(Utilities.is_allowed_pointer(self.dataPointer)):
			logger >> "Marking as invalid - dataPointer is not allowed"
			self.valid = False
			return

		self.superclassIsaPointer = Utilities.read_child_of(self.valobj,1*self.sys_params.pointer_size,self.sys_params.types_cache.addr_ptr_type)
		if not(Utilities.is_valid_pointer(self.superclassIsaPointer,self.sys_params.pointer_size,allow_tagged=False, allow_NULL=True)):
			logger >> "Marking as invalid - superclassIsa is invalid"
			self.valid = False
			return
		if not(Utilities.is_allowed_pointer(self.superclassIsaPointer)):
			logger >> "Marking as invalid - superclassIsa is not allowed"
			self.valid = False
			return

	# in general, KVO is implemented by transparently subclassing
	# however, there could be exceptions where a class does something else
	# internally to implement the feature - this method will have no clue that a class
	# has been KVO'ed unless the standard implementation technique is used
	def is_kvo(self):
		logger = Logger.Logger()
		if self.is_valid():
			if self.class_name().startswith("NSKVONotifying_"):
				return True
		return False

	# some CF classes have a valid ObjC isa in their CFRuntimeBase
	# but instead of being class-specific this isa points to a match-'em-all class
	# which is __NSCFType (the versions without __ also exists and we are matching to it
	#                      just to be on the safe side)
	def is_cftype(self):
		logger = Logger.Logger()
		if self.is_valid():
			return self.class_name() == '__NSCFType' or self.class_name() == 'NSCFType'

	def get_superclass(self):
		logger = Logger.Logger()
		if self.is_valid():
			parent_isa_pointer = self.valobj.CreateChildAtOffset("parent_isa",
				self.sys_params.pointer_size,
				self.sys_params.addr_ptr_type)
			return Class_Data_V2(parent_isa_pointer,self.sys_params)
		else:
			return None

	def class_name(self):
		logger = Logger.Logger()
		if self.is_valid():
			return self.data.data.name
		else:
			return None

	def is_valid(self):
		logger = Logger.Logger()
		if self.valid:
			return self.data.is_valid()
		return False

	def __str__(self):
		logger = Logger.Logger()
		return 'isaPointer = ' + hex(self.isaPointer) + "\n" + \
		 "superclassIsaPointer = " + hex(self.superclassIsaPointer) + "\n" + \
		 "cachePointer = " + hex(self.cachePointer) + "\n" + \
		 "vtablePointer = " + hex(self.vtablePointer) + "\n" + \
		 "data = " + hex(self.dataPointer)

	def is_tagged(self):
		return False

	def instance_size(self,align=False):
		logger = Logger.Logger()
		if self.is_valid() == False:
			return None
		return self.rwt.rot.instance_size(align)

# runtime v1 is much less intricate than v2 and stores relevant information directly in the class_t object
class Class_Data_V1:
	def __init__(self,isa_pointer,params):
		logger = Logger.Logger()
		if (isa_pointer != None) and (Utilities.is_valid_pointer(isa_pointer.GetValueAsUnsigned(),params.pointer_size, allow_tagged=False)):
			self.valid = True
			self.sys_params = params
			self.valobj = isa_pointer
			self.check_valid()
		else:
			logger >> "Marking as invalid - isaPointer is invalid or None"
			self.valid = False
		if self.valid:
			self.name = Utilities.read_ascii(self.valobj.GetTarget().GetProcess(),self.namePointer)
			if not(Utilities.is_valid_identifier(self.name)):
				logger >> "Marking as invalid - name is not valid"
				self.valid = False

	# perform sanity checks on the contents of this class_t
	def check_valid(self):
		logger = Logger.Logger()
		self.valid = True

		self.isaPointer = Utilities.read_child_of(self.valobj,0,self.sys_params.types_cache.addr_ptr_type)
		if not(Utilities.is_valid_pointer(self.isaPointer,self.sys_params.pointer_size,allow_tagged=False)):
			logger >> "Marking as invalid - isaPointer is invalid"
			self.valid = False
			return

		self.superclassIsaPointer = Utilities.read_child_of(self.valobj,1*self.sys_params.pointer_size,self.sys_params.types_cache.addr_ptr_type)
		if not(Utilities.is_valid_pointer(self.superclassIsaPointer,self.sys_params.pointer_size,allow_tagged=False,allow_NULL=True)):
			logger >> "Marking as invalid - superclassIsa is invalid"
			self.valid = False
			return

		self.namePointer = Utilities.read_child_of(self.valobj,2*self.sys_params.pointer_size,self.sys_params.types_cache.addr_ptr_type)
		#if not(Utilities.is_valid_pointer(self.namePointer,self.sys_params.pointer_size,allow_tagged=False,allow_NULL=False)):
		#	self.valid = False
		#	return

	# in general, KVO is implemented by transparently subclassing
	# however, there could be exceptions where a class does something else
	# internally to implement the feature - this method will have no clue that a class
	# has been KVO'ed unless the standard implementation technique is used
	def is_kvo(self):
		logger = Logger.Logger()
		if self.is_valid():
			if self.class_name().startswith("NSKVONotifying_"):
				return True
		return False

	# some CF classes have a valid ObjC isa in their CFRuntimeBase
	# but instead of being class-specific this isa points to a match-'em-all class
	# which is __NSCFType (the versions without __ also exists and we are matching to it
	#                      just to be on the safe side)
	def is_cftype(self):
		logger = Logger.Logger()
		if self.is_valid():
			return self.class_name() == '__NSCFType' or self.class_name() == 'NSCFType'

	def get_superclass(self):
		logger = Logger.Logger()
		if self.is_valid():
			parent_isa_pointer = self.valobj.CreateChildAtOffset("parent_isa",
				self.sys_params.pointer_size,
				self.sys_params.addr_ptr_type)
			return Class_Data_V1(parent_isa_pointer,self.sys_params)
		else:
			return None

	def class_name(self):
		logger = Logger.Logger()
		if self.is_valid():
			return self.name
		else:
			return None

	def is_valid(self):
		return self.valid

	def __str__(self):
		logger = Logger.Logger()
		return 'isaPointer = ' + hex(self.isaPointer) + "\n" + \
		 "superclassIsaPointer = " + hex(self.superclassIsaPointer) + "\n" + \
		 "namePointer = " + hex(self.namePointer) + " --> " + self.name + \
		 "instanceSize = " + hex(self.instanceSize()) + "\n"

	def is_tagged(self):
		return False

	def instance_size(self,align=False):
		logger = Logger.Logger()
		if self.is_valid() == False:
			return None
		if self.instanceSize == None:
			self.instanceSize = Utilities.read_child_of(self.valobj,5*self.sys_params.pointer_size,self.sys_params.types_cache.addr_ptr_type)
		if align:
			unalign = self.instance_size(False)
			if self.sys_params.is_64_bit:
				return ((unalign + 7) & ~7) % 0x100000000
			else:
				return ((unalign + 3) & ~3) % 0x100000000
		else:
			return self.instanceSize

# these are the only tagged pointers values for current versions
# of OSX - they might change in future OS releases, and no-one is
# advised to rely on these values, or any of the bitmasking formulas
# in TaggedClass_Data. doing otherwise is at your own risk
TaggedClass_Values_Lion = {1 : 'NSNumber', \
                           5: 'NSManagedObject', \
                           6: 'NSDate', \
                           7: 'NSDateTS' };
TaggedClass_Values_NMOS = {0: 'NSAtom', \
                           3 : 'NSNumber', \
                           4: 'NSDateTS', \
                           5: 'NSManagedObject', \
                           6: 'NSDate' };

class TaggedClass_Data:
	def __init__(self,pointer,params):
		logger = Logger.Logger()
		global TaggedClass_Values_Lion,TaggedClass_Values_NMOS
		self.valid = True
		self.name = None
		self.sys_params = params
		self.valobj = pointer
		self.val = (pointer & ~0x0000000000000000FF) >> 8
		self.class_bits = (pointer & 0xE) >> 1
		self.i_bits = (pointer & 0xF0) >> 4
		
		if self.sys_params.is_lion:
			if self.class_bits in TaggedClass_Values_Lion:
				self.name = TaggedClass_Values_Lion[self.class_bits]
			else:
				logger >> "Marking as invalid - not a good tagged pointer for Lion"
				self.valid = False
		else:
			if self.class_bits in TaggedClass_Values_NMOS:
				self.name = TaggedClass_Values_NMOS[self.class_bits]
			else:
				logger >> "Marking as invalid - not a good tagged pointer for NMOS"
				self.valid = False


	def is_valid(self):
		return self.valid

	def class_name(self):
		logger = Logger.Logger()
		if self.is_valid():
			return self.name
		else:
			return False

	def value(self):
		return self.val if self.is_valid() else None

	def info_bits(self):
		return self.i_bits if self.is_valid() else None

	def is_kvo(self):
		return False

	def is_cftype(self):
		return False

	# we would need to go around looking for the superclass or ask the runtime
	# for now, we seem not to require support for this operation so we will merrily
	# pretend to be at a root point in the hierarchy
	def get_superclass(self):
		return None

	# anything that is handled here is tagged
	def is_tagged(self):
		return True

	# it seems reasonable to say that a tagged pointer is the size of a pointer
	def instance_size(self,align=False):
		logger = Logger.Logger()
		if self.is_valid() == False:
			return None
		return self.sys_params.pointer_size


class InvalidClass_Data:
	def __init__(self):
		pass
	def is_valid(self):
		return False

@functools.total_ordering
class Version:
	def __init__(self, major, minor, release, build_string):
		self._major = major
		self._minor = minor
		self._release = release
		self._build_string = build_string

	def get_major(self):
		return self._major
	def get_minor(self):
		return self._minor
	def get_release(self):
		return self._release
	def get_build_string(self):
		return self._build_string

	major = property(get_major,None)
	minor = property(get_minor,None)
	release = property(get_release,None)
	build_string = property(get_build_string,None)

	def __lt__(self,other):
		if (self.major < other.major):
			return True
		if (self.minor < other.minor):
			return True
		if (self.release < other.release):
			return True
		# build strings are not compared since they are heavily platform-dependent and might not always
		# be available
		return False

	def __eq__(self,other):
		return (self.major == other.major) and \
		       (self.minor == other.minor) and \
		       (self.release == other.release) and \
		       (self.build_string == other.build_string)

runtime_version = cache.Cache()
os_version = cache.Cache()
types_caches = cache.Cache()
isa_caches = cache.Cache()

class SystemParameters:
	def __init__(self,valobj):
		logger = Logger.Logger()
		self.adjust_for_architecture(valobj)
		self.adjust_for_process(valobj)

	def adjust_for_process(self, valobj):
		logger = Logger.Logger()
		global runtime_version
		global os_version
		global types_caches
		global isa_caches
		
		process = valobj.GetTarget().GetProcess()
		self.pid = process.GetProcessID()
		
		if runtime_version.look_for_key(self.pid):
			self.runtime_version = runtime_version.get_value(self.pid)
		else:
			self.runtime_version = ObjCRuntime.runtime_version(process)
			runtime_version.add_item(self.pid,self.runtime_version)
		
		if os_version.look_for_key(self.pid):
			self.is_lion = os_version.get_value(self.pid)
		else:
			self.is_lion = Utilities.check_is_osx_lion(valobj.GetTarget())
			os_version.add_item(self.pid,self.is_lion)
		
		if types_caches.look_for_key(self.pid):
			self.types_cache = types_caches.get_value(self.pid)
		else:
			self.types_cache = attrib_fromdict.AttributesDictionary(allow_reset=False)
			self.types_cache.addr_type = valobj.GetType().GetBasicType(lldb.eBasicTypeUnsignedLong)
			self.types_cache.addr_ptr_type = self.types_cache.addr_type.GetPointerType()
			self.types_cache.uint32_t = valobj.GetType().GetBasicType(lldb.eBasicTypeUnsignedInt)
			types_caches.add_item(self.pid,self.types_cache)
		
		if isa_caches.look_for_key(self.pid):
			self.isa_cache = isa_caches.get_value(self.pid)
		else:
			self.isa_cache = cache.Cache()
			isa_caches.add_item(self.pid,self.isa_cache)

	def adjust_for_architecture(self,valobj):
		process = valobj.GetTarget().GetProcess()
		self.pointer_size = process.GetAddressByteSize()
		self.is_64_bit = (self.pointer_size == 8)
		self.is_little = (process.GetByteOrder() == lldb.eByteOrderLittle)
		self.cfruntime_size = 16 if self.is_64_bit else 8

	# a simple helper function that makes it more explicit that one is calculating
	# an offset that is made up of X pointers and Y bytes of additional data
	# taking into account pointer size - if you know there is going to be some padding
	# you can pass that in and it will be taken into account (since padding may be different between
	# 32 and 64 bit versions, you can pass padding value for both, the right one will be used)
	def calculate_offset(self, num_pointers = 0, bytes_count = 0, padding32 = 0, padding64 = 0):
		value = bytes_count + num_pointers*self.pointer_size
		return value + padding64 if self.is_64_bit else value + padding32

class ObjCRuntime:

	# the ObjC runtime has no explicit "version" field that we can use
	# instead, we discriminate v1 from v2 by looking for the presence
	# of a well-known section only present in v1
	@staticmethod
	def runtime_version(process):
		logger = Logger.Logger()
		if process.IsValid() == False:
			logger >> "No process - bailing out"
			return None
		target = process.GetTarget()
		num_modules = target.GetNumModules()
		module_objc = None
		for idx in range(num_modules):
			module = target.GetModuleAtIndex(idx)
			if module.GetFileSpec().GetFilename() == 'libobjc.A.dylib':
				module_objc = module
				break
		if module_objc == None or module_objc.IsValid() == False:
			logger >> "no libobjc - bailing out"
			return None
		num_sections = module.GetNumSections()
		section_objc = None
		for idx in range(num_sections):
			section = module.GetSectionAtIndex(idx)
			if section.GetName() == '__OBJC':
				section_objc = section
				break
		if section_objc != None and section_objc.IsValid():
			logger >> "found __OBJC: v1"
			return 1
		logger >> "no __OBJC: v2"
		return 2

	@staticmethod
	def runtime_from_isa(isa):
		logger = Logger.Logger()
		runtime = ObjCRuntime(isa)
		runtime.isa = isa
		return runtime

	def __init__(self,valobj):
		logger = Logger.Logger()
		self.valobj = valobj
		self.adjust_for_architecture()
		self.sys_params = SystemParameters(self.valobj)
		self.unsigned_value = self.valobj.GetValueAsUnsigned()
		self.isa_value = None

	def adjust_for_architecture(self):
		pass

# an ObjC pointer can either be tagged or must be aligned
	def is_tagged(self):
		logger = Logger.Logger()
		if self.valobj is None:
			return False
		return (Utilities.is_valid_pointer(self.unsigned_value,self.sys_params.pointer_size, allow_tagged=True) and \
		not(Utilities.is_valid_pointer(self.unsigned_value,self.sys_params.pointer_size, allow_tagged=False)))

	def is_valid(self):
		logger = Logger.Logger()
		if self.valobj is None:
			return False
		if self.valobj.IsInScope() == False:
			return False
		return Utilities.is_valid_pointer(self.unsigned_value,self.sys_params.pointer_size, allow_tagged=True)

	def is_nil(self):
		return self.unsigned_value == 0

	def read_isa(self):
		logger = Logger.Logger()
		if self.isa_value != None:
			logger >> "using cached isa"
			return self.isa_value
		isa_pointer = self.valobj.CreateChildAtOffset("cfisa",
			0,
			self.sys_params.types_cache.addr_ptr_type)
		if isa_pointer == None or isa_pointer.IsValid() == False:
			logger >> "invalid isa - bailing out"
			return None;
		if isa_pointer.GetValueAsUnsigned(1) == 1:
			logger >> "invalid isa value - bailing out"
			return None;
		self.isa_value = isa_pointer
		return isa_pointer

	def read_class_data(self):
		logger = Logger.Logger()
		global isa_cache
		if self.is_tagged():
			# tagged pointers only exist in ObjC v2
			if self.sys_params.runtime_version == 2:
				logger >> "on v2 and tagged - maybe"
				# not every odd-valued pointer is actually tagged. most are just plain wrong
				# we could try and predetect this before even creating a TaggedClass_Data object
				# but unless performance requires it, this seems a cleaner way to tackle the task
				tentative_tagged = TaggedClass_Data(self.unsigned_value,self.sys_params)
				if tentative_tagged.is_valid():
					logger >> "truly tagged"
					return tentative_tagged
				else:
					logger >> "not tagged - error"
					return InvalidClass_Data()
			else:
				logger >> "on v1 and tagged - error"
				return InvalidClass_Data()
		if self.is_valid() == False:
			return InvalidClass_Data()
		isa = self.read_isa()
		if isa == None:
			return InvalidClass_Data()
		isa_value = isa.GetValueAsUnsigned(1)
		if isa_value == 1:
			return InvalidClass_Data()
		data = self.sys_params.isa_cache.get_value(isa_value,default=None)
		if data != None:
			return data
		if self.sys_params.runtime_version == 2:
			data = Class_Data_V2(isa,self.sys_params)
		else:
			data = Class_Data_V1(isa,self.sys_params)
		if data == None:
			return InvalidClass_Data()
		if data.is_valid():
			self.sys_params.isa_cache.add_item(isa_value,data,ok_to_replace=True)
		return data

# these classes below can be used by the data formatters to provide a consistent message that describes a given runtime-generated situation
class SpecialSituation_Description:
	def message(self):
		return ''

class InvalidPointer_Description(SpecialSituation_Description):

	def __init__(self,nil):
		self.is_nil = nil

	def message(self):
		if self.is_nil:
			return '@"<nil>"'
		else:
			return '<invalid pointer>'

class InvalidISA_Description(SpecialSituation_Description):

	def __init__(self):
		pass

	def message(self):
		return '<not an Objective-C object>'