Issue #18137: Detect integer overflow on precision in float.__format__()

and complex.__format__().

Lib/test/test_format.py

@@ -302,6 +302,23 @@ class FormatTest(unittest.TestCase):
 def test_main():
     test_support.run_unittest(FormatTest)
 
+    def test_precision(self):
+        INT_MAX = 2147483647
+
+        f = 1.2
+        self.assertEqual(format(f, ".0f"), "1")
+        self.assertEqual(format(f, ".3f"), "1.200")
+        with self.assertRaises(ValueError) as cm:
+            format(f, ".%sf" % (INT_MAX + 1))
+        self.assertEqual(str(cm.exception), "precision too big")
+
+        c = complex(f)
+        self.assertEqual(format(f, ".0f"), "1")
+        self.assertEqual(format(f, ".3f"), "1.200")
+        with self.assertRaises(ValueError) as cm:
+            format(f, ".%sf" % (INT_MAX + 1))
+        self.assertEqual(str(cm.exception), "precision too big")
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS

@@ -9,6 +9,9 @@ What's New in Python 2.7.6?
 Core and Builtins
 -----------------
 
+- Issue #18137: Detect integer overflow on precision in float.__format__()
+  and complex.__format__().
+
 - Issue #18038: SyntaxError raised during compilation sources with illegal
   encoding now always contains an encoding name.
 

Objects/stringlib/formatter.h

@@ -928,7 +928,7 @@ format_float_internal(PyObject *value,
     Py_ssize_t n_total;
     int has_decimal;
     double val;
-    Py_ssize_t precision = format->precision;
+    Py_ssize_t precision;
     Py_ssize_t default_precision = 6;
     STRINGLIB_CHAR type = format->type;
     int add_pct = 0;
@@ -947,6 +947,12 @@ format_float_internal(PyObject *value,
        from a hard-code pseudo-locale */
     LocaleInfo locale;
 
+    if (format->precision > INT_MAX) {
+        PyErr_SetString(PyExc_ValueError, "precision too big");
+        goto done;
+    }
+    precision = (int)format->precision;
+
     /* Alternate is not allowed on floats. */
     if (format->alternate) {
         PyErr_SetString(PyExc_ValueError,
@@ -1078,7 +1084,7 @@ format_complex_internal(PyObject *value,
     Py_ssize_t n_im_total;
     int re_has_decimal;
     int im_has_decimal;
-    Py_ssize_t precision = format->precision;
+    Py_ssize_t precision;
     Py_ssize_t default_precision = 6;
     STRINGLIB_CHAR type = format->type;
     STRINGLIB_CHAR *p_re;
@@ -1107,6 +1113,12 @@ format_complex_internal(PyObject *value,
        from a hard-code pseudo-locale */
     LocaleInfo locale;
 
+    if (format->precision > INT_MAX) {
+        PyErr_SetString(PyExc_ValueError, "precision too big");
+        goto done;
+    }
+    precision = (int)format->precision;
+
     /* Alternate is not allowed on complex. */
     if (format->alternate) {
         PyErr_SetString(PyExc_ValueError,