Issue 1658799: Handle requests to intern string subtype instances (original) (raw)
Created on 2007-02-13 10:34 by hniksic, last changed 2022-04-11 14:56 by admin. This issue is now closed.
Messages (7)
Author: Hrvoje Nikšić (hniksic) *
Date: 2007-02-13 10:34
This patch implements a small modification of PyString_InternInPlace that allows for safe interning of string subtype instances. The change should be fully backward compatible -- for a rationale and discussion, see:
http://mail.python.org/pipermail/python-dev/2007-February/070973.html
Author: Armin Rigo (arigo) * 
Date: 2007-02-13 22:36
I think that this opens an attack (untested, though): it allows a previously-interned string to be removed from the dictionary. This might lead to a crash because the old string still thinks it is interned. Try something along the lines of:
s1 = "hel" s1 = intern(s1 + "lo")
class S(str): def hash(self): return hash(s1) def eq(self, other): return other == s1
s = S("world") intern(s) del s1
Author: Hrvoje Nikšić (hniksic) *
Date: 2007-02-14 08:31
I don't think an attack is possible. This patch retains the property that only exact strings are interned. If you create a pathological string subtype that hashes like a different string instance (one that has already been interned), all you'll achieve is that "interning" will return the other instance. As far as I can tell, no string is actually removed from the interned dictionary (until it becomes unreachable, that is.)
What is the expected result of your test code? I tried it and it ran without error.
Author: Armin Rigo (arigo) *
Date: 2007-02-14 12:13
Ah, the code was the wrong way around. The following causes an Fatal Python error in a debug build:
s1 = "hel" s1 = intern(s1 + "lo")
class S(str): def hash(self): return 0 def eq(self, other): return False
s = S(s1) s2 = intern(s) del s1
Author: Armin Rigo (arigo) *
Date: 2007-02-14 12:21
Btw, any reason why you cannot simply say in your Python program: intern(str(s)) ?
Author: Hrvoje Nikšić (hniksic) *
Date: 2007-03-20 14:25
Thanks for the detailed analysis. I missed this case and I retract the patch until I think of a way to resolve this problem. The obvious possibility is to always copy subtype instances before attempting to intern them, but right now I don't have the time to investigate if this slows things down.
As for using s = intern(str(s)) in Python, it's a start, but it does somewhat more than I'd like -- for example, it "interns" all kinds of objects, which is not desirable. My patch was written with the intention of making PyString_InternInPlace more robust wrt string subtype instances, so that all the code in core and extensions that simply calls PyString_InternInPlace keeps working without modification.
In the long run, the interface of PyString_InternInPlace is a bit too undeterministic for my taste. It has no error reporting, so it silently ignores some kinds of errors (not enough memory), throws fatal error on others (non-string being passed), and also completely ignores string subtypes. In my code I use this utility function:
int intern_force(PyObject **s) { if (PyString_CheckExact(s)) / Most likely case: we're passed an exact string. */ PyString_InternInPlace(s); else if (PyString_Check(s)) { / The case we're covering with this function: we got a string subtype. Intern a copy. */ PyObject *copy; copy = PyString_FromStringAndSize(PyString_AS_STRING(*s), PyString_GET_SIZE(*s)); if (!copy) return -1; Py_DECREF(*s); *s = copy; PyString_InternInPlace(s); } else { PyErr_SetString(PyExc_TypeError, "intern_force passed a non-string"); return -1; }
if (!PyString_CHECK_INTERNED(s)) { /* PyString_InternInPlace failed and cleared the exception, most likely due to insufficient memory. */ PyErr_Format(PyExc_RuntimeError, "failed to intern string '%s'", PyString_AS_STRING(*s)); return -1; } return 0; }
I don't expect a function like this one to become a part of Python, but PyString_InternInPlace could be usefully improved even without breaking compatibility.
Author: Daniel Diniz (ajaksu2) * 
Date: 2009-03-30 21:24
Closing as the patch was retracted. Please reopen if you disagree.
History
Date
User
Action
Args
2022-04-11 14:56:22
admin
set
github: 44576
2009-03-30 21:24:51
ajaksu2
set
status: open -> closed
type: enhancement
nosy: + ajaksu2
messages: +
resolution: not a bug
stage: resolved
2007-02-13 10:34:01
hniksic
create