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| author | Jesper Dangaard Brouer <netoptimizer@brouer.com> | 2013-10-31 22:10:55 +0100 |
|---|---|---|
| committer | David S. Miller <davem@davemloft.net> | 2013-11-04 20:01:29 -0500 |
| commit | 1ba3aab3033b464f352659720824d7124c21dbf9 (patch) | |
| tree | 3308ce1f1b158b507f802561cda5fde8866bf8e7 /include/net/codel.h | |
| parent | 13521a57974635384db51d65135cc0267c6ae771 (diff) | |
| download | linux-1ba3aab3033b464f352659720824d7124c21dbf9.tar.gz linux-1ba3aab3033b464f352659720824d7124c21dbf9.tar.bz2 linux-1ba3aab3033b464f352659720824d7124c21dbf9.zip | |
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>
Diffstat (limited to 'include/net/codel.h')
| -rw-r--r-- | include/net/codel.h | 19 |
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 { |