Re: [PATCH] net/sched: cbs: perform rate conversions in signed 64-bit arithmetic

On Thu, Aug 6, 2026 at 11:53 AM Jack Wang <[email protected]> wrote:
>
> cbs_set_port_rate() and cbs_change() multiply link rates and slope values
> by BYTES_PER_KBIT, an unsigned long constant.  On 32-bit architectures,
> the multiplications therefore take place in 32-bit unsigned arithmetic
> before the results are assigned to s64 fields.
>
> For port rates above approximately 34.36 Gbit/s this wraps port_rate.
> The same conversion turns a negative sendslope into a large positive
> value, reversing the CBS credit adjustment.  This affects software CBS;
> port_rate is also refreshed on NETDEV_UP and NETDEV_CHANGE notifications.
>
> Cast the first operand of each multiplication to s64 so all intermediate
> operations use signed 64-bit arithmetic and preserve the value's sign on
> every architecture.
>

Check what the sashikos are saying please  - i agree with their view.
I believe we need a v2:
I'd suggest a v2 that addresses both (and optionally validates the
idleslope sign and a negative idleslope can arm the watchdog in the
past and busy-loop).

For completeness, the pre-existing timediff_to_credits() s64 overflow
that gemini sashiko flagged is a separate follow-up, not part of this
patch.

cheers,
jamal

> Fixes: 585d763af09c ("net/sched: Introduce Credit Based Shaper (CBS) qdisc")
> Fixes: 397006ba5d918 ("net/sched: cbs: Fix integer overflow in cbs_set_port_rate()")
>
> Signed-off-by: Jack Wang <[email protected]>
> ---
>  net/sched/sch_cbs.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/net/sched/sch_cbs.c b/net/sched/sch_cbs.c
> index 1c93469c56e3..9ef31ecdaa05 100644
> --- a/net/sched/sch_cbs.c
> +++ b/net/sched/sch_cbs.c
> @@ -335,7 +335,7 @@ static void cbs_set_port_rate(struct net_device *dev, struct cbs_sched_data *q)
>                 speed = ecmd.base.speed;
>
>  skip:
> -       port_rate = speed * 1000 * BYTES_PER_KBIT;
> +       port_rate = (s64)speed * 1000 * BYTES_PER_KBIT;
>
>         atomic64_set(&q->port_rate, port_rate);
>         netdev_dbg(dev, "cbs: set %s's port_rate to: %lld, linkspeed: %d\n",
> @@ -405,8 +405,8 @@ static int cbs_change(struct Qdisc *sch, struct nlattr *opt,
>         /* Everything went OK, save the parameters used. */
>         WRITE_ONCE(q->hicredit, qopt->hicredit);
>         WRITE_ONCE(q->locredit, qopt->locredit);
> -       WRITE_ONCE(q->idleslope, qopt->idleslope * BYTES_PER_KBIT);
> -       WRITE_ONCE(q->sendslope, qopt->sendslope * BYTES_PER_KBIT);
> +       WRITE_ONCE(q->idleslope, (s64)qopt->idleslope * BYTES_PER_KBIT);
> +       WRITE_ONCE(q->sendslope, (s64)qopt->sendslope * BYTES_PER_KBIT);
>         WRITE_ONCE(q->offload, qopt->offload);
>
>         return 0;
> --
> 2.53.0
>