Issue #14700: Fix buggy overflow checks for large precision and width in new-style and old-style formatting.

4 files changed, 17 insertions, 22 deletions

diff --git a/Objects/stringlib/formatter.h b/Objects/stringlib/formatter.h
index c49a104..6b28224 100644
--- a/Objects/stringlib/formatter.h
+++ b/Objects/stringlib/formatter.h
@@ -73,7 +73,7 @@ static int
 get_integer(STRINGLIB_CHAR **ptr, STRINGLIB_CHAR *end,
                   Py_ssize_t *result)
 {
-    Py_ssize_t accumulator, digitval, oldaccumulator;
+    Py_ssize_t accumulator, digitval;
     int numdigits;
     accumulator = numdigits = 0;
     for (;;(*ptr)++, numdigits++) {
@@ -83,19 +83,17 @@ get_integer(STRINGLIB_CHAR **ptr, STRINGLIB_CHAR *end,
         if (digitval < 0)
             break;
         /*
-           This trick was copied from old Unicode format code.  It's cute,
-           but would really suck on an old machine with a slow divide
-           implementation.  Fortunately, in the normal case we do not
-           expect too many digits.
+           Detect possible overflow before it happens:
+
+              accumulator * 10 + digitval > PY_SSIZE_T_MAX if and only if
+              accumulator > (PY_SSIZE_T_MAX - digitval) / 10.
         */
-        oldaccumulator = accumulator;
-        accumulator *= 10;
-        if ((accumulator+10)/10 != oldaccumulator+1) {
+        if (accumulator > (PY_SSIZE_T_MAX - digitval) / 10) {
             PyErr_Format(PyExc_ValueError,
                          "Too many decimal digits in format string");
             return -1;
         }
-        accumulator += digitval;
+        accumulator = accumulator * 10 + digitval;
     }
     *result = accumulator;
     return numdigits;
diff --git a/Objects/stringlib/string_format.h b/Objects/stringlib/string_format.h
index 075fa1d..965e1ad 100644
--- a/Objects/stringlib/string_format.h
+++ b/Objects/stringlib/string_format.h
@@ -197,7 +197,6 @@ get_integer(const SubString *str)
 {
     Py_ssize_t accumulator = 0;
     Py_ssize_t digitval;
-    Py_ssize_t oldaccumulator;
     STRINGLIB_CHAR *p;
 
     /* empty string is an error */
@@ -209,19 +208,17 @@ get_integer(const SubString *str)
         if (digitval < 0)
             return -1;
         /*
-           This trick was copied from old Unicode format code.  It's cute,
-           but would really suck on an old machine with a slow divide
-           implementation.  Fortunately, in the normal case we do not
-           expect too many digits.
+           Detect possible overflow before it happens:
+
+              accumulator * 10 + digitval > PY_SSIZE_T_MAX if and only if
+              accumulator > (PY_SSIZE_T_MAX - digitval) / 10.
         */
-        oldaccumulator = accumulator;
-        accumulator *= 10;
-        if ((accumulator+10)/10 != oldaccumulator+1) {
+        if (accumulator > (PY_SSIZE_T_MAX - digitval) / 10) {
             PyErr_Format(PyExc_ValueError,
                          "Too many decimal digits in format string");
             return -1;
         }
-        accumulator += digitval;
+        accumulator = accumulator * 10 + digitval;
     }
     return accumulator;
 }
diff --git a/Objects/stringobject.c b/Objects/stringobject.c
index 39fa740..152ea21 100644
--- a/Objects/stringobject.c
+++ b/Objects/stringobject.c
@@ -4369,7 +4369,7 @@ PyString_Format(PyObject *format, PyObject *args)
                     c = Py_CHARMASK(*fmt++);
                     if (!isdigit(c))
                         break;
-                    if ((width*10) / 10 != width) {
+                    if (width > (PY_SSIZE_T_MAX - ((int)c - '0')) / 10) {
                         PyErr_SetString(
                             PyExc_ValueError,
                             "width too big");
@@ -4404,7 +4404,7 @@ PyString_Format(PyObject *format, PyObject *args)
                         c = Py_CHARMASK(*fmt++);
                         if (!isdigit(c))
                             break;
-                        if ((prec*10) / 10 != prec) {
+                        if (prec > (INT_MAX - ((int)c - '0')) / 10) {
                             PyErr_SetString(
                                 PyExc_ValueError,
                                 "prec too big");
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index e3c2cb1..79b87df 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -8394,7 +8394,7 @@ PyObject *PyUnicode_Format(PyObject *format,
                     c = *fmt++;
                     if (c < '0' || c > '9')
                         break;
-                    if ((width*10) / 10 != width) {
+                    if (width > (PY_SSIZE_T_MAX - ((int)c - '0')) / 10) {
                         PyErr_SetString(PyExc_ValueError,
                                         "width too big");
                         goto onError;
@@ -8427,7 +8427,7 @@ PyObject *PyUnicode_Format(PyObject *format,
                         c = *fmt++;
                         if (c < '0' || c > '9')
                             break;
-                        if ((prec*10) / 10 != prec) {
+                        if (prec > (INT_MAX - ((int)c - '0')) / 10) {
                             PyErr_SetString(PyExc_ValueError,
                                             "prec too big");
                             goto onError;