Re: [net-next PATCH] net: codel: Avoid undefined behavior from signed overflow
From: Eric Dumazet <hidden>
Date: 2013-10-30 18:01:46
On Wed, 2013-10-30 at 18:23 +0100, Jesper Dangaard Brouer wrote:
quoted hunk ↗ jump to hunk
From: Jesper Dangaard Brouer <redacted> 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 <redacted> Signed-off-by: Jesper Dangaard Brouer <redacted> --- include/net/codel.h | 8 ++++---- 1 files changed, 4 insertions(+), 4 deletions(-)diff --git a/include/net/codel.h b/include/net/codel.h index 389cf62..700fcdf 100644 --- a/include/net/codel.h +++ b/include/net/codel.h@@ -72,10 +72,10 @@ 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) +#define codel_time_after(a, b) ((s32)((a) - (b)) > 0) +#define codel_time_after_eq(a, b) ((s32)((a) - (b)) >= 0) +#define codel_time_before(a, b) ((s32)((a) - (b)) < 0) +#define codel_time_before_eq(a, b) ((s32)((a) - (b)) <= 0)
I see nothing enforcing an unsigned subtraction as claimed in your changelog. a / b could be signed. Paul commit 5a581b367b5 was OK because of existing typecheck(unsigned long, ....)