Re: [PATCH] net: sched: taprio: Fix potential integer overflow in taprio_set_picos_per_byte

[Eric Dumazet]

On 9/3/19 3:08 AM, Gustavo A. R. Silva wrote:
> Add suffix LL to constant 1000 in order to avoid a potential integer
> overflow and give the compiler complete information about the proper
> arithmetic to use. Notice that this constant is being used in a context
> that expects an expression of type s64, but it's currently evaluated
> using 32-bit arithmetic.
> 
> Addresses-Coverity-ID: 1453459 ("Unintentional integer overflow")
> Fixes: f04b514c0ce2 ("taprio: Set default link speed to 10 Mbps in taprio_set_picos_per_byte")
> Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> ---
>  net/sched/sch_taprio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> index 8d8bc2ec5cd6..956f837436ea 100644
> --- a/net/sched/sch_taprio.c
> +++ b/net/sched/sch_taprio.c
> @@ -966,7 +966,7 @@ static void taprio_set_picos_per_byte(struct net_device *dev,
>  
>  skip:
>  	picos_per_byte = div64_s64(NSEC_PER_SEC * 1000LL * 8,
> -				   speed * 1000 * 1000);
> +				   speed * 1000LL * 1000);
>  
>  	atomic64_set(&q->picos_per_byte, picos_per_byte);
>  	netdev_dbg(dev, "taprio: set %s's picos_per_byte to: %lld, linkspeed: %d\n",
> 

But, why even multiplying by 1,000,000 in the first place, this seems silly,
a standard 32 bit divide could be used instead.

->

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 8d8bc2ec5cd6281d811fd5d8a5c5211ebb0edd73..944b1af3215668e927d486b6c6c65c4599fb9539 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -965,8 +965,7 @@ static void taprio_set_picos_per_byte(struct net_device *dev,
                speed = ecmd.base.speed;
 
 skip:
-       picos_per_byte = div64_s64(NSEC_PER_SEC * 1000LL * 8,
-                                  speed * 1000 * 1000);
+       picos_per_byte = (USEC_PER_SEC * 8) / speed;
 
        atomic64_set(&q->picos_per_byte, picos_per_byte);
        netdev_dbg(dev, "taprio: set %s's picos_per_byte to: %lld, linkspeed: %d\n",