Re: [PATCH net] net/sched: taprio: fix duration_to_length()

[Vladimir Oltean <vladimir.oltean@nxp.com>]

On Fri, May 24, 2024 at 05:52:17PM +0200, Eric Dumazet wrote:
> On Fri, May 24, 2024 at 5:50 PM Eric Dumazet <edumazet@google.com> wrote:
> >
> > On Fri, May 24, 2024 at 5:39 PM Vladimir Oltean <vladimir.oltean@nxp.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)
> ...

The thing is that I don't see how q->picos_per_byte could take any sane
value of either 0 or a multiple of 2^32. Its formula is "(USEC_PER_SEC * 8) / speed"
where "speed" is the link speed: 10, 100, 1000 etc. The special cases
of speed=0 and speed=SPEED_UNKNOWN are handled by falling back to SPEED_10
in the picos_per_byte calculation.

For q->picos_per_byte to be larger than 2^32, "speed" would have to be
smaller than 8000000 / U32_MAX (0.001862645).

For q->picos_per_byte to be exactly 0, "speed" would have to be larger
than 8000000. But the largest defined speed in include/uapi/linux/ethtool.h
is precisely SPEED_800000, leading to an expected q->picos_per_byte of 1.