Re: [PATCH net v2] net/sched: cbs: perform rate conversions in signed 64-bit arithmetic

[Victor Nogueira]

Hi!

On 12/08/2026 07:00, Jack Wang 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
>
> Also reject a non-positive idleslope. A negative idleslope can arm the
> watchdog in the past and busy-loop.
>
> 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 <163wangjack@xxxxxxxxx>
> ---
> v2:
>    - Reject a non-positive idleslope to prevent scheduling the watchdog in
>      the past.
> [...]
>   net/sched/sch_cbs.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/net/sched/sch_cbs.c b/net/sched/sch_cbs.c
> index 1c93469c56e3..e960c8e01ef4 100644
> --- a/net/sched/sch_cbs.c
> +++ b/net/sched/sch_cbs.c
> [...]
> @@ -392,6 +392,10 @@ static int cbs_change(struct Qdisc *sch, struct nlattr *opt,
>   	}
>   
>   	qopt = nla_data(tb[TCA_CBS_PARMS]);
> +	if (qopt->idleslope <= 0) {
> +		NL_SET_ERR_MSG(extack, "Idleslope must be greater than zero");
> +		return -EINVAL;
> +	}

Unfortunately this is breaking tdc because it's forbidding an idleslope 
of 0:

# Test 1820: Create CBS with default setting
# exit: 2
# exit: 0
# Error: Idleslope must be greater than zero.

Please don't forget to run tdc before sending tc-related patches.

cheers,
Victor