From 401fdf9c851eb61229250ebffa942adca99b36d1 Mon Sep 17 00:00:00 2001 From: Mark Dickinson Date: Sat, 21 Jan 2023 10:23:59 +0000 Subject: [PATCH] gh-101037: Fix potential memory underallocation for zeros of int subtypes (#101038) This PR fixes object allocation in long_subtype_new to ensure that there's at least one digit in all cases, and makes sure that the value of that digit is copied over from the source long. Needs backport to 3.11, but not any further: the change to require at least one digit was only introduced for Python 3.11. Fixes #101037. --- Include/cpython/longintrepr.h | 3 +++ .../2023-01-14-17-03-08.gh-issue-101037.9ATNuf.rst | 2 ++ Objects/longobject.c | 5 +++++ 3 files changed, 10 insertions(+) create mode 100644 Misc/NEWS.d/next/Core and Builtins/2023-01-14-17-03-08.gh-issue-101037.9ATNuf.rst diff --git a/Include/cpython/longintrepr.h b/Include/cpython/longintrepr.h index 68dbf9c4382dc5..6d52427508b5fe 100644 --- a/Include/cpython/longintrepr.h +++ b/Include/cpython/longintrepr.h @@ -71,6 +71,9 @@ typedef long stwodigits; /* signed variant of twodigits */ 0 <= ob_digit[i] <= MASK. The allocation function takes care of allocating extra memory so that ob_digit[0] ... ob_digit[abs(ob_size)-1] are actually available. + We always allocate memory for at least one digit, so accessing ob_digit[0] + is always safe. However, in the case ob_size == 0, the contents of + ob_digit[0] may be undefined. CAUTION: Generic code manipulating subtypes of PyVarObject has to aware that ints abuse ob_size's sign bit. diff --git a/Misc/NEWS.d/next/Core and Builtins/2023-01-14-17-03-08.gh-issue-101037.9ATNuf.rst b/Misc/NEWS.d/next/Core and Builtins/2023-01-14-17-03-08.gh-issue-101037.9ATNuf.rst new file mode 100644 index 00000000000000..a48756657a29d3 --- /dev/null +++ b/Misc/NEWS.d/next/Core and Builtins/2023-01-14-17-03-08.gh-issue-101037.9ATNuf.rst @@ -0,0 +1,2 @@ +Fix potential memory underallocation issue for instances of :class:`int` +subclasses with value zero. diff --git a/Objects/longobject.c b/Objects/longobject.c index 1db4ca418e066e..51ac86961c9954 100644 --- a/Objects/longobject.c +++ b/Objects/longobject.c @@ -5638,6 +5638,11 @@ long_subtype_new(PyTypeObject *type, PyObject *x, PyObject *obase) n = Py_SIZE(tmp); if (n < 0) n = -n; + /* Fast operations for single digit integers (including zero) + * assume that there is always at least one digit present. */ + if (n == 0) { + n = 1; + } newobj = (PyLongObject *)type->tp_alloc(type, n); if (newobj == NULL) { Py_DECREF(tmp);