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Commit 1ba3aab3 authored by Jesper Dangaard Brouer's avatar Jesper Dangaard Brouer Committed by David S. Miller
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net: codel: Avoid undefined behavior from signed overflow 

As described in commit 5a581b36 (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: default avatarJesper Dangaard Brouer <netoptimizer@brouer.com>
Acked-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 13521a57

include/net/codel.h

+15 −4
Original line number Diff line number Diff line
@@ -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 {