netdev - [net-next PATCH] net: codel: Avoid undefined behavior from signed overflow

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Message-ID: <20131030172341.19203.93490.stgit@dragon>
Date:	Wed, 30 Oct 2013 18:23:42 +0100
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	netdev@...r.kernel.org
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dave Taht <dave.taht@...il.com>,
	Jesper Dangaard Brouer <brouer@...hat.com>
Subject: [net-next PATCH] net: codel: Avoid undefined behavior from signed
	overflow

From: Jesper Dangaard Brouer <netoptimizer@...uer.com>

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@...ux.vnet.ibm.com>
Signed-off-by: Jesper Dangaard Brouer <netoptimizer@...uer.com>
---

 include/net/codel.h |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/net/codel.h b/include/net/codel.h
index 389cf62..700fcdf 100644
--- a/include/net/codel.h
+++ b/include/net/codel.h
@@ -72,10 +72,10 @@ 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)
+#define codel_time_after(a, b)		((s32)((a) - (b)) > 0)
+#define codel_time_after_eq(a, b)	((s32)((a) - (b)) >= 0)
+#define codel_time_before(a, b) 	((s32)((a) - (b)) < 0)
+#define codel_time_before_eq(a, b)	((s32)((a) - (b)) <= 0)
 
 /* Qdiscs using codel plugin must use codel_skb_cb in their own cb[] */
 struct codel_skb_cb {

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