xref: /xnu-12377.41.6/tools/lldbmacros/core/pointer.py (revision bbb1b6f9e71b8cdde6e5cd6f4841f207dee3d828)

"""
Custom pointer support

This module provides support for special pointer types that are not native to the
language used by the target being debugged. Such pointers may be represented as a struct
or class (for example IOKit's shared pointers).

A custom pointer class must subclass the PointerPolicy class and implement all of its
abstract methods. The MetaPointerPolicy metaclass ensures that all known subclasses are
registered in a global list (wherever they are located in the lldb macro sources).

A client can obtain a PointerPolicy instance by calling the match method with an SBValue
instance as an argument. The returned value is one of:

    * None - the match was unsuccessful and this SBValue instance is not a pointer.
    * Concrete instance - An instance of the concrete PointerPolicy class that will handle
      pointer operations for the given SBValue.

Concrete policy instances implement an API that allows a client to operate on a value
like a native pointer (for example unwrapping a native pointer from a smart pointer).

Example:

    # Obtain an SBValue instance.
    val = kern.global.GlobalVariable.GetSBValue()

    # Try to match the pointer policy for the given value.
    policy = PointerPolicy.match(val)

    # Unwrap the pointer SBValue.
    if policy:
        val = policy.GetPointerSBValue(val)

    ... Operate on val as usual.
"""
from operator import methodcaller
from abc import ABCMeta, abstractmethod

import lldb

from .caching import cache_statically


class MetaPointerPolicy(ABCMeta):
    """ Register a custom pointer policy in global list. """

    classes = []

    def __new__(cls, clsname, bases, args):
        newcls = super(MetaPointerPolicy, cls).__new__(cls, clsname, bases, args)
        cls.classes.append(newcls)
        return newcls


class Singleton(MetaPointerPolicy):
    """ Meta class for creation of singleton instances. """

    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]


class PointerPolicy(object, metaclass=ABCMeta):
    """ Abstract base class common to every custom pointer policy. """

    @classmethod
    def match(cls, sbvalue):
        """ Match pointer representation based on given SBValue. """
        matching = filter(bool, map(methodcaller('match', sbvalue), MetaPointerPolicy.classes))
        return next(matching, None)

    @abstractmethod
    def GetPointerSBValue(self, sbvalue):
        """ Returns pointer value that debugger should operate on. """


# Pointers need to have their TBI byte stripped if in use. TBI KASan,
# for instance, tags pointers to detect improper memory accesses. Reading
# values from such tagged pointers fails.
#
# Stripping the pointers requires to learn whether TBI is in use or not.
# We do that by checking presence of 'kasan_tbi_enabled' symbol which only
# exists on the TBI KASan variant. Since KASan is one of more TBI
# consumers (along with PAC or Sandbox) this is not an ideal approach.
# Inspecting respective CPU state would be more appropriate.


class NativePointer(PointerPolicy, metaclass=Singleton):
    """ Policy for native pointers.

        Strips top bits of a pointer if TBI is in use. Otherwise
        pointer is used as-is.

        Native pointers do not have any per-pointer attributes so this policy
        can be singleton instance.
    """

    @staticmethod
    @cache_statically
    def isTagged(target=None):
        """ Pointer stripping isn't required as of recent lldb changes that fixed ignoring
        non-addresable bits.
        Due to performance degredation on relevant coredumps, stripping is being
        as a quick remedy.
        Depending on future debugging needs, it'll be removed completely, or improved
        to have acceptable performance.
        """
        is_tagged = False

        """ Returns true on TBI KASan targets, false otherwise. """
        # is_tagged = target.FindFirstGlobalVariable('kasan_tbi_enabled').IsValid()
        return is_tagged

    def __init__(self):
        if self.isTagged():
            self._stripPtr = self.stripPtr
        else:
            self._stripPtr = lambda val: val

    @classmethod
    def match(cls, sbvalue):
        return cls() if sbvalue.GetType().IsPointerType() else None

    @staticmethod
    def stripPtr(sbvalue: lldb.SBValue):
        """ Strips the TBI byte value. Since the value is not a plain value but
            represents a value of a variable, a register or an expression the
            conversion is performed by (re-)creating the value through expression.
        """
        if sbvalue.GetValueAsAddress() != sbvalue.GetValueAsUnsigned():
            addr = sbvalue.GetValueAsAddress()
            sbv_new = sbvalue.CreateValueFromExpression(None, '(void *)' + str(addr))
            return sbv_new.Cast(sbvalue.GetType())

        return sbvalue

    def GetPointerSBValue(self, sbvalue):
        return self._stripPtr(sbvalue)