netdev - Re: [PATCH net] net/sched: taprio: fix duration_to

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Message-ID: <20240524153948.57ueybbqeyb33lxj@skbuf>
Date: Fri, 24 May 2024 18:39:48 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>,
	netdev@...r.kernel.org, eric.dumazet@...il.com,
	syzbot <syzkaller@...glegroups.com>,
	Vinicius Costa Gomes <vinicius.gomes@...el.com>
Subject: Re: [PATCH net] net/sched: taprio: fix duration_to_length()

On Thu, May 23, 2024 at 01:45:49PM +0000, Eric Dumazet wrote:
> duration_to_length() is incorrectly using div_u64()
> instead of div64_u64().
> ---
>  net/sched/sch_taprio.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> index 1ab17e8a72605385280fad9b7f656a6771236acc..827fb81fc63a098304bad198fadd4aed55d1fec4 100644
> --- a/net/sched/sch_taprio.c
> +++ b/net/sched/sch_taprio.c
> @@ -256,7 +256,8 @@ static int length_to_duration(struct taprio_sched *q, int len)
>  
>  static int duration_to_length(struct taprio_sched *q, u64 duration)
>  {
> -	return div_u64(duration * PSEC_PER_NSEC, atomic64_read(&q->picos_per_byte));
> +	return div64_u64(duration * PSEC_PER_NSEC,
> +			 atomic64_read(&q->picos_per_byte));
>  }

There's a netdev_dbg() in taprio_set_picos_per_byte(). Could you turn
that on? I'm curious what was the q->picos_per_byte value that triggered
the 64-bit division fault. There are a few weird things about
q->picos_per_byte's representation and use as an atomic64_t (s64) type.