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authorJesper Dangaard Brouer <netoptimizer@brouer.com>2013-10-31 22:10:55 +0100
committerDavid S. Miller <davem@davemloft.net>2013-11-04 20:01:29 -0500
commit1ba3aab3033b464f352659720824d7124c21dbf9 (patch)
tree3308ce1f1b158b507f802561cda5fde8866bf8e7
parent13521a57974635384db51d65135cc0267c6ae771 (diff)
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>
-rw-r--r--include/net/codel.h19
1 files changed, 15 insertions, 4 deletions
diff --git a/include/net/codel.h b/include/net/codel.h
index 389cf621161d..3b04ff5f6f8d 100644
--- a/include/net/codel.h
+++ b/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 {