[PATCH] signed integer overflow undefined behavior

[Eric Lubin]

From 1a52c994df8ca00fb22c8d3be9860f3996c83440 Mon Sep 17 00:00:00 2001
From: Eric Lubin <address@hidden>
Date: Thu, 12 Dec 2013 21:47:21 -0800
Subject: [PATCH 1/1] Fixed undefined integer overflow checks that were being
 optimized out by the compiler

The code in question shows that the developer clearly spent some time thinking 
of all the ways
the time could overflow. But he disregards the fact that integer overflow is 
undefined in C
and therefore he can't rely on checking for ((t1 < t0) ^ (d1 < 0)) which gets 
simplified to:

((t0 + d0< t0) ^ (d1 < 0))

which goes to:

((d0 < 0) ^ (d1 < 0))

because it assumes no integer overflow.

which finally evaluated to false. This occurs for all the overflow checks, 
thereby invalidating the checks altogether.
---
 lib/parse-datetime.y |   14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/lib/parse-datetime.y b/lib/parse-datetime.y
index 4dce7fa..c0e57a2 100644
--- a/lib/parse-datetime.y
+++ b/lib/parse-datetime.y
@@ -1560,13 +1560,13 @@ parse_datetime (struct timespec *result, char const *p,
         long_time_t t4 = t3 + d4;
         time_t t5 = t4;
 
-        if ((d1 / (60 * 60) ^ pc.rel.hour)
-            | (d2 / 60 ^ pc.rel.minutes)
-            | ((t1 < t0) ^ (d1 < 0))
-            | ((t2 < t1) ^ (d2 < 0))
-            | ((t3 < t2) ^ (d3 < 0))
-            | ((t4 < t3) ^ (d4 < 0))
-            | (t5 != t4))
+        if ((LONG_MAX / (60 * 60) < pc.rel.hour)  /* verify d1 doesn't 
overflow */
+            | (LONG_MAX / 60 < pc.rel.minutes)    /* verify d2 doesn't 
overflow */
+            | ((d1 > LONG_MAX - t0) ^ (d1 < 0))  /* verify t1 doesn't overflow 
*/
+            | ((d2 > LONG_MAX - t1) ^ (d2 < 0))  /* verify t2 doesn't overflow 
*/
+            | ((d3 > LONG_MAX - t2) ^ (d3 < 0))  /* verify t3 doesn't overflow 
*/
+            | ((d4 > LONG_MAX - t3) ^ (d4 < 0))  /* verify t4 doesn't overflow 
*/
+            | (t4 > LONG_MAX))  /* verify t4 won't overflow in a time_t */
           goto fail;
 
         result->tv_sec = t5;
-- 
1.7.9.5