net: codel: Avoid undefined behavior from signed overflow

As described in commit 5a581b367 (jiffies: Avoid undefined
behavior from signed overflow), according to the C standard
3.4.3p3, overflow of a signed integer results in undefined
behavior.

To fix this, do as the above commit, and do an unsigned
subtraction, and interpreting the result as a signed
two's-complement number.  This is based on the theory from
RFC 1982 and is nicely described in wikipedia here:
 https://en.wikipedia.org/wiki/Serial_number_arithmetic#General_Solution

A side-note, I have seen practical issues with the previous logic
when dealing with 16-bit, on a 64-bit machine (gcc version
4.4.5). This were 32-bit, which I have not observed issues with.

Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Jesper Dangaard Brouer <netoptimizer@brouer.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/codel.h

@@ -72,10 +72,21 @@ static inline codel_time_t codel_get_time(void)
 	return ns >> CODEL_SHIFT;
 }
 
-#define codel_time_after(a, b)		((s32)(a) - (s32)(b) > 0)
-#define codel_time_after_eq(a, b)	((s32)(a) - (s32)(b) >= 0)
-#define codel_time_before(a, b)		((s32)(a) - (s32)(b) < 0)
-#define codel_time_before_eq(a, b)	((s32)(a) - (s32)(b) <= 0)
+/* Dealing with timer wrapping, according to RFC 1982, as desc in wikipedia:
+ *  https://en.wikipedia.org/wiki/Serial_number_arithmetic#General_Solution
+ * codel_time_after(a,b) returns true if the time a is after time b.
+ */
+#define codel_time_after(a, b)						\
+	(typecheck(codel_time_t, a) &&					\
+	 typecheck(codel_time_t, b) &&					\
+	 ((s32)((a) - (b)) > 0))
+#define codel_time_before(a, b) 	codel_time_after(b, a)
+
+#define codel_time_after_eq(a, b)					\
+	(typecheck(codel_time_t, a) &&					\
+	 typecheck(codel_time_t, b) &&					\
+	 ((s32)((a) - (b)) >= 0))
+#define codel_time_before_eq(a, b)	codel_time_after_eq(b, a)
 
 /* Qdiscs using codel plugin must use codel_skb_cb in their own cb[] */
 struct codel_skb_cb {