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net: codel: Avoid undefined behavior from signed overflow

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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>
This commit is contained in:
Jesper Dangaard Brouer 2013-10-31 22:10:55 +01:00 committed by David S. Miller
parent 13521a5797
commit 1ba3aab303
1 changed files with 15 additions and 4 deletions
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19
include/net/codel.h
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@ -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 {