Fix for possible signed overflow: the behaviour of -LONG_MIN is

undefined in ANSI C.

diff --git a/Objects/longobject.c b/Objects/longobject.c
index 2e13d6141f17f83164b345a30ec8a6e006fab054..4ab8f1e3847c7d814b434791bc5f1662cb77cc4b 100644 (file)
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -99,20 +99,27 @@ PyObject *
 PyLong_FromLong(long ival)
 {
        PyLongObject *v;
+        unsigned long abs_ival;
        unsigned long t;  /* unsigned so >> doesn't propagate sign bit */
        int ndigits = 0;
        int negative = 0;
 
        if (ival < 0) {
-               ival = -ival;
+               /* if LONG_MIN == -LONG_MAX-1 (true on most platforms) then
+                  ANSI C says that the result of -ival is undefined when ival
+                  == LONG_MIN.  Hence the following workaround. */
+               abs_ival = (unsigned long)(-1-ival) + 1;
                negative = 1;
        }
+       else {
+               abs_ival = (unsigned long)ival;
+       }
 
        /* Count the number of Python digits.
           We used to pick 5 ("big enough for anything"), but that's a
           waste of time and space given that 5*15 = 75 bits are rarely
           needed. */
-       t = (unsigned long)ival;
+       t = abs_ival;
        while (t) {
                ++ndigits;
                t >>= PyLong_SHIFT;
@@ -121,7 +128,7 @@ PyLong_FromLong(long ival)
        if (v != NULL) {
                digit *p = v->ob_digit;
                v->ob_size = negative ? -ndigits : ndigits;
-               t = (unsigned long)ival;
+               t = abs_ival;
                while (t) {
                        *p++ = (digit)(t & PyLong_MASK);
                        t >>= PyLong_SHIFT;
@@ -830,20 +837,26 @@ PyObject *
 PyLong_FromLongLong(PY_LONG_LONG ival)
 {
        PyLongObject *v;
+       unsigned PY_LONG_LONG abs_ival;
        unsigned PY_LONG_LONG t;  /* unsigned so >> doesn't propagate sign bit */
        int ndigits = 0;
        int negative = 0;
 
        if (ival < 0) {
-               ival = -ival;
+               /* avoid signed overflow on negation;  see comments
+                  in PyLong_FromLong above. */
+               abs_ival = (unsigned PY_LONG_LONG)(-1-ival) + 1;
                negative = 1;
        }
+       else {
+               abs_ival = (unsigned PY_LONG_LONG)ival;
+       }
 
        /* Count the number of Python digits.
           We used to pick 5 ("big enough for anything"), but that's a
           waste of time and space given that 5*15 = 75 bits are rarely
           needed. */
-       t = (unsigned PY_LONG_LONG)ival;
+       t = abs_ival;
        while (t) {
                ++ndigits;
                t >>= PyLong_SHIFT;
@@ -852,7 +865,7 @@ PyLong_FromLongLong(PY_LONG_LONG ival)
        if (v != NULL) {
                digit *p = v->ob_digit;
                Py_SIZE(v) = negative ? -ndigits : ndigits;
-               t = (unsigned PY_LONG_LONG)ival;
+               t = abs_ival;
                while (t) {
                        *p++ = (digit)(t & PyLong_MASK);
                        t >>= PyLong_SHIFT;