net: codel: Avoid undefined behavior from signed overflow (1ba3aab3) · Commits · 戴 / test

As described in commit (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

+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 {

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