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Close #14205: dict lookup raises a RuntimeError if the dict is modified during

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a lookup.

"if you want to make a sandbox on top of CPython, you have to fix segfaults"
so let's fix segfaults!
This commit is contained in:
Victor Stinner 2012-03-06 01:03:13 +01:00
parent b2c7cca0cf
commit 198b291df7
5 changed files with 31 additions and 352 deletions
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47
Lib/test/crashers/nasty_eq_vs_dict.py
View file

@ -1,47 +0,0 @@
# from http://mail.python.org/pipermail/python-dev/2001-June/015239.html
# if you keep changing a dictionary while looking up a key, you can
# provoke an infinite recursion in C
# At the time neither Tim nor Michael could be bothered to think of a
# way to fix it.
class Yuck:
def __init__(self):
self.i = 0
def make_dangerous(self):
self.i = 1
def __hash__(self):
# direct to slot 4 in table of size 8; slot 12 when size 16
return 4 + 8
def __eq__(self, other):
if self.i == 0:
# leave dict alone
pass
elif self.i == 1:
# fiddle to 16 slots
self.__fill_dict(6)
self.i = 2
else:
# fiddle to 8 slots
self.__fill_dict(4)
self.i = 1
return 1
def __fill_dict(self, n):
self.i = 0
dict.clear()
for i in range(n):
dict[i] = i
dict[self] = "OK!"
y = Yuck()
dict = {y: "OK!"}
z = Yuck()
y.make_dangerous()
print(dict[z])

22
Lib/test/test_dict.py
View file

@ -379,7 +379,7 @@ class DictTest(unittest.TestCase):
x.fail = True x.fail = True
self.assertRaises(Exc, d.pop, x) self.assertRaises(Exc, d.pop, x)
def test_mutatingiteration(self): def test_mutating_iteration(self):
# changing dict size during iteration # changing dict size during iteration
d = {} d = {}
d[1] = 1 d[1] = 1
@ -387,6 +387,26 @@ class DictTest(unittest.TestCase):
for i in d: for i in d:
d[i+1] = 1 d[i+1] = 1
def test_mutating_lookup(self):
# changing dict during a lookup
class NastyKey:
mutate_dict = None
def __hash__(self):
# hash collision!
return 1
def __eq__(self, other):
if self.mutate_dict:
self.mutate_dict[self] = 1
return self == other
d = {}
d[NastyKey()] = 0
NastyKey.mutate_dict = d
with self.assertRaises(RuntimeError):
d[NastyKey()] = None
def test_repr(self): def test_repr(self):
d = {} d = {}
self.assertEqual(repr(d), '{}') self.assertEqual(repr(d), '{}')

291
Lib/test/test_mutants.py
View file

@ -1,291 +0,0 @@
from test.support import verbose, TESTFN
import random
import os
# From SF bug #422121: Insecurities in dict comparison.
# Safety of code doing comparisons has been an historical Python weak spot.
# The problem is that comparison of structures written in C *naturally*
# wants to hold on to things like the size of the container, or "the
# biggest" containee so far, across a traversal of the container; but
# code to do containee comparisons can call back into Python and mutate
# the container in arbitrary ways while the C loop is in midstream. If the
# C code isn't extremely paranoid about digging things out of memory on
# each trip, and artificially boosting refcounts for the duration, anything
# from infinite loops to OS crashes can result (yes, I use Windows <wink>).
#
# The other problem is that code designed to provoke a weakness is usually
# white-box code, and so catches only the particular vulnerabilities the
# author knew to protect against. For example, Python's list.sort() code
# went thru many iterations as one "new" vulnerability after another was
# discovered.
#
# So the dict comparison test here uses a black-box approach instead,
# generating dicts of various sizes at random, and performing random
# mutations on them at random times. This proved very effective,
# triggering at least six distinct failure modes the first 20 times I
# ran it. Indeed, at the start, the driver never got beyond 6 iterations
# before the test died.
# The dicts are global to make it easy to mutate tham from within functions.
dict1 = {}
dict2 = {}
# The current set of keys in dict1 and dict2. These are materialized as
# lists to make it easy to pick a dict key at random.
dict1keys = []
dict2keys = []
# Global flag telling maybe_mutate() whether to *consider* mutating.
mutate = 0
# If global mutate is true, consider mutating a dict. May or may not
# mutate a dict even if mutate is true. If it does decide to mutate a
# dict, it picks one of {dict1, dict2} at random, and deletes a random
# entry from it; or, more rarely, adds a random element.
def maybe_mutate():
global mutate
if not mutate:
return
if random.random() < 0.5:
return
if random.random() < 0.5:
target, keys = dict1, dict1keys
else:
target, keys = dict2, dict2keys
if random.random() < 0.2:
# Insert a new key.
mutate = 0 # disable mutation until key inserted
while 1:
newkey = Horrid(random.randrange(100))
if newkey not in target:
break
target[newkey] = Horrid(random.randrange(100))
keys.append(newkey)
mutate = 1
elif keys:
# Delete a key at random.
mutate = 0 # disable mutation until key deleted
i = random.randrange(len(keys))
key = keys[i]
del target[key]
del keys[i]
mutate = 1
# A horrid class that triggers random mutations of dict1 and dict2 when
# instances are compared.
class Horrid:
def __init__(self, i):
# Comparison outcomes are determined by the value of i.
self.i = i
# An artificial hashcode is selected at random so that we don't
# have any systematic relationship between comparison outcomes
# (based on self.i and other.i) and relative position within the
# hash vector (based on hashcode).
# XXX This is no longer effective.
##self.hashcode = random.randrange(1000000000)
def __hash__(self):
return 42
return self.hashcode
def __eq__(self, other):
maybe_mutate() # The point of the test.
return self.i == other.i
def __ne__(self, other):
raise RuntimeError("I didn't expect some kind of Spanish inquisition!")
__lt__ = __le__ = __gt__ = __ge__ = __ne__
def __repr__(self):
return "Horrid(%d)" % self.i
# Fill dict d with numentries (Horrid(i), Horrid(j)) key-value pairs,
# where i and j are selected at random from the candidates list.
# Return d.keys() after filling.
def fill_dict(d, candidates, numentries):
d.clear()
for i in range(numentries):
d[Horrid(random.choice(candidates))] = \
Horrid(random.choice(candidates))
return list(d.keys())
# Test one pair of randomly generated dicts, each with n entries.
# Note that dict comparison is trivial if they don't have the same number
# of entires (then the "shorter" dict is instantly considered to be the
# smaller one, without even looking at the entries).
def test_one(n):
global mutate, dict1, dict2, dict1keys, dict2keys
# Fill the dicts without mutating them.
mutate = 0
dict1keys = fill_dict(dict1, range(n), n)
dict2keys = fill_dict(dict2, range(n), n)
# Enable mutation, then compare the dicts so long as they have the
# same size.
mutate = 1
if verbose:
print("trying w/ lengths", len(dict1), len(dict2), end=' ')
while dict1 and len(dict1) == len(dict2):
if verbose:
print(".", end=' ')
c = dict1 == dict2
if verbose:
print()
# Run test_one n times. At the start (before the bugs were fixed), 20
# consecutive runs of this test each blew up on or before the sixth time
# test_one was run. So n doesn't have to be large to get an interesting
# test.
# OTOH, calling with large n is also interesting, to ensure that the fixed
# code doesn't hold on to refcounts *too* long (in which case memory would
# leak).
def test(n):
for i in range(n):
test_one(random.randrange(1, 100))
# See last comment block for clues about good values for n.
test(100)
##########################################################################
# Another segfault bug, distilled by Michael Hudson from a c.l.py post.
class Child:
def __init__(self, parent):
self.__dict__['parent'] = parent
def __getattr__(self, attr):
self.parent.a = 1
self.parent.b = 1
self.parent.c = 1
self.parent.d = 1
self.parent.e = 1
self.parent.f = 1
self.parent.g = 1
self.parent.h = 1
self.parent.i = 1
return getattr(self.parent, attr)
class Parent:
def __init__(self):
self.a = Child(self)
# Hard to say what this will print! May vary from time to time. But
# we're specifically trying to test the tp_print slot here, and this is
# the clearest way to do it. We print the result to a temp file so that
# the expected-output file doesn't need to change.
f = open(TESTFN, "w")
print(Parent().__dict__, file=f)
f.close()
os.unlink(TESTFN)
##########################################################################
# And another core-dumper from Michael Hudson.
dict = {}
# Force dict to malloc its table.
for i in range(1, 10):
dict[i] = i
f = open(TESTFN, "w")
class Machiavelli:
def __repr__(self):
dict.clear()
# Michael sez: "doesn't crash without this. don't know why."
# Tim sez: "luck of the draw; crashes with or without for me."
print(file=f)
return repr("machiavelli")
def __hash__(self):
return 0
dict[Machiavelli()] = Machiavelli()
print(str(dict), file=f)
f.close()
os.unlink(TESTFN)
del f, dict
##########################################################################
# And another core-dumper from Michael Hudson.
dict = {}
# let's force dict to malloc its table
for i in range(1, 10):
dict[i] = i
class Machiavelli2:
def __eq__(self, other):
dict.clear()
return 1
def __hash__(self):
return 0
dict[Machiavelli2()] = Machiavelli2()
try:
dict[Machiavelli2()]
except KeyError:
pass
del dict
##########################################################################
# And another core-dumper from Michael Hudson.
dict = {}
# let's force dict to malloc its table
for i in range(1, 10):
dict[i] = i
class Machiavelli3:
def __init__(self, id):
self.id = id
def __eq__(self, other):
if self.id == other.id:
dict.clear()
return 1
else:
return 0
def __repr__(self):
return "%s(%s)"%(self.__class__.__name__, self.id)
def __hash__(self):
return 0
dict[Machiavelli3(1)] = Machiavelli3(0)
dict[Machiavelli3(2)] = Machiavelli3(0)
f = open(TESTFN, "w")
try:
try:
print(dict[Machiavelli3(2)], file=f)
except KeyError:
pass
finally:
f.close()
os.unlink(TESTFN)
del dict
del dict1, dict2, dict1keys, dict2keys

3
Misc/NEWS
View file

@ -10,6 +10,9 @@ What's New in Python 3.3.0 Alpha 2?
Core and Builtins Core and Builtins
----------------- -----------------
- Issue #14205: dict lookup raises a RuntimeError if the dict is modified
during a lookup.
Library Library
------- -------

18
Objects/dictobject.c
View file

@ -347,12 +347,9 @@ lookdict(PyDictObject *mp, PyObject *key, register Py_hash_t hash)
return ep; return ep;
} }
else { else {
/* The compare did major nasty stuff to the PyErr_SetString(PyExc_RuntimeError,
* dict: start over. "dictionary changed size during lookup");
* XXX A clever adversary could prevent this return NULL;
* XXX from terminating.
*/
return lookdict(mp, key, hash);
} }
} }
freeslot = NULL; freeslot = NULL;
@ -379,12 +376,9 @@ lookdict(PyDictObject *mp, PyObject *key, register Py_hash_t hash)
return ep; return ep;
} }
else { else {
/* The compare did major nasty stuff to the PyErr_SetString(PyExc_RuntimeError,
* dict: start over. "dictionary changed size during lookup");
* XXX A clever adversary could prevent this return NULL;
* XXX from terminating.
*/
return lookdict(mp, key, hash);
} }
} }
else if (ep->me_key == dummy && freeslot == NULL) else if (ep->me_key == dummy && freeslot == NULL)