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gh-120950: Fix overflow in math.log() with large int-like argument (GH-121011)

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Handling of arbitrary large int-like argument is now consistent with
handling arbitrary large int arguments.
This commit is contained in:
Serhiy Storchaka 2025-11-12 00:27:13 +02:00 • committed by GitHub
parent 9e7340cd3b
commit 4359706ac8
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3 changed files with 111 additions and 30 deletions
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80
Modules/mathmodule.c
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@ -57,6 +57,7 @@ raised for division by zero and mod by zero.
#endif
#include "Python.h"
#include "pycore_abstract.h" // _PyNumber_Index()
#include "pycore_bitutils.h" // _Py_bit_length()
#include "pycore_call.h" // _PyObject_CallNoArgs()
#include "pycore_import.h" // _PyImport_SetModuleString()
@ -1577,44 +1578,63 @@ math_modf_impl(PyObject *module, double x)
However, intermediate overflow is possible for an int if the number of bits
in that int is larger than PY_SSIZE_T_MAX. */
static PyObject*
loghelper_int(PyObject* arg, double (*func)(double))
{
/* If it is int, do it ourselves. */
double x, result;
int64_t e;
/* Negative or zero inputs give a ValueError. */
if (!_PyLong_IsPositive((PyLongObject *)arg)) {
PyErr_SetString(PyExc_ValueError,
"expected a positive input");
return NULL;
}
x = PyLong_AsDouble(arg);
if (x == -1.0 && PyErr_Occurred()) {
if (!PyErr_ExceptionMatches(PyExc_OverflowError))
return NULL;
/* Here the conversion to double overflowed, but it's possible
to compute the log anyway. Clear the exception and continue. */
PyErr_Clear();
x = _PyLong_Frexp((PyLongObject *)arg, &e);
assert(!PyErr_Occurred());
/* Value is ~= x * 2**e, so the log ~= log(x) + log(2) * e. */
result = fma(func(2.0), (double)e, func(x));
}
else
/* Successfully converted x to a double. */
result = func(x);
return PyFloat_FromDouble(result);
}
static PyObject*
loghelper(PyObject* arg, double (*func)(double))
{
/* If it is int, do it ourselves. */
if (PyLong_Check(arg)) {
double x, result;
int64_t e;
/* Negative or zero inputs give a ValueError. */
if (!_PyLong_IsPositive((PyLongObject *)arg)) {
/* The input can be an arbitrary large integer, so we
don't include it's value in the error message. */
PyErr_SetString(PyExc_ValueError,
"expected a positive input");
return loghelper_int(arg, func);
}
/* Else let libm handle it by itself. */
PyObject *res = math_1(arg, func, 0, "expected a positive input, got %s");
if (res == NULL &&
PyErr_ExceptionMatches(PyExc_OverflowError) &&
PyIndex_Check(arg))
{
/* Here the conversion to double overflowed, but it's possible
to compute the log anyway. Clear the exception, convert to
integer and continue. */
PyErr_Clear();
arg = _PyNumber_Index(arg);
if (arg == NULL) {
return NULL;
}
x = PyLong_AsDouble(arg);
if (x == -1.0 && PyErr_Occurred()) {
if (!PyErr_ExceptionMatches(PyExc_OverflowError))
return NULL;
/* Here the conversion to double overflowed, but it's possible
to compute the log anyway. Clear the exception and continue. */
PyErr_Clear();
x = _PyLong_Frexp((PyLongObject *)arg, &e);
assert(e >= 0);
assert(!PyErr_Occurred());
/* Value is ~= x * 2**e, so the log ~= log(x) + log(2) * e. */
result = fma(func(2.0), (double)e, func(x));
}
else
/* Successfully converted x to a double. */
result = func(x);
return PyFloat_FromDouble(result);
res = loghelper_int(arg, func);
Py_DECREF(arg);
}
/* Else let libm handle it by itself. */
return math_1(arg, func, 0, "expected a positive input, got %s");
return res;
}