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

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Date: Fri, 24 May 2024 17:52:17 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Vladimir Oltean <vladimir.oltean@....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 Fri, May 24, 2024 at 5:50 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Fri, May 24, 2024 at 5:39 PM Vladimir Oltean <vladimir.oltean@....com> wrote:
> >
> > 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.
>
>
> No repro yet.
>
> Anything with 32 low order bits cleared would trigger a divide by 0.
>
> (1ULL << 32) picoseconds is only 4.294 ms

BTW, just a reminder, div_u64() is a divide by a 32bit value...

static inline u64 div_u64(u64 dividend, u32 divisor)
...