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Date: Mon, 27 May 2024 10:07:31 +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 6:07 PM Vladimir Oltean <vladimir.oltean@....com> wrote:
>
> 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@...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)
> > ...
>
> 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.
This suggests q->picos_per_byte should be a mere u32, and that
taprio_set_picos_per_byte()
should make sure to not set 0 in q->picos_per_byte
Presumably some devices must get a speed bigger than SPEED_800000
team driver could do that, according to team_ethtool_get_link_ksettings()
diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 1ab17e8a72605385280fad9b7f656a6771236acc..71087a53630362863cc6c5e462b29dbef8cd5d74
100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -89,9 +89,9 @@ struct taprio_sched {
bool offloaded;
bool detected_mqprio;
bool broken_mqprio;
- atomic64_t picos_per_byte; /* Using picoseconds because for 10Gbps+
- * speeds it's sub-nanoseconds per byte
- */
+ atomic_t picos_per_byte; /* Using picoseconds because for 10Gbps+
+ * speeds it's sub-nanoseconds per byte
+ */
/* Protects the update side of the RCU protected current_entry */
spinlock_t current_entry_lock;
@@ -251,12 +251,12 @@ static ktime_t get_interval_end_time(struct
sched_gate_list *sched,
static int length_to_duration(struct taprio_sched *q, int len)
{
- return div_u64(len * atomic64_read(&q->picos_per_byte), PSEC_PER_NSEC);
+ return div_u64((u64)len * atomic_read(&q->picos_per_byte),
PSEC_PER_NSEC);
}
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 div_u64(duration * PSEC_PER_NSEC,
atomic_read(&q->picos_per_byte));
}
/* Sets sched->max_sdu[] and sched->max_frm_len[] to the minimum between the
@@ -666,8 +666,8 @@ static void taprio_set_budgets(struct taprio_sched *q,
if (entry->gate_duration[tc] == sched->cycle_time)
budget = INT_MAX;
else
- budget =
div64_u64((u64)entry->gate_duration[tc] * PSEC_PER_NSEC,
- atomic64_read(&q->picos_per_byte));
+ budget = div_u64((u64)entry->gate_duration[tc]
* PSEC_PER_NSEC,
+ atomic_read(&q->picos_per_byte));
atomic_set(&entry->budget[tc], budget);
}
@@ -1291,7 +1291,7 @@ static void taprio_set_picos_per_byte(struct
net_device *dev,
{
struct ethtool_link_ksettings ecmd;
int speed = SPEED_10;
- int picos_per_byte;
+ u32 picos_per_byte;
int err;
err = __ethtool_get_link_ksettings(dev, &ecmd);
@@ -1303,11 +1303,11 @@ static void taprio_set_picos_per_byte(struct
net_device *dev,
skip:
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",
- dev->name, (long long)atomic64_read(&q->picos_per_byte),
- ecmd.base.speed);
+ if (!picos_per_byte)
+ picos_per_byte = 1U;
+ atomic_set(&q->picos_per_byte, picos_per_byte);
+ netdev_dbg(dev, "taprio: set %s's picos_per_byte to: %u,
linkspeed: %d\n",
+ dev->name, picos_per_byte, ecmd.base.speed);
}
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