Re: [PATCH v3 net] net/sched: taprio: enforce minimum value for picos_per_byte

[Cong Wang <xiyou.wangcong@gmail.com>]

On Tue, Jul 29, 2025 at 02:31:49AM +0900, Takamitsu Iwai wrote:
> Syzbot reported a WARNING in taprio_get_start_time().
> 
> When link speed is 470,589 or greater, q->picos_per_byte becomes too
> small, causing length_to_duration(q, ETH_ZLEN) to return zero.
> 
> This zero value leads to validation failures in fill_sched_entry() and
> parse_taprio_schedule(), allowing arbitrary values to be assigned to
> entry->interval and cycle_time. As a result, sched->cycle can become zero.
> 
> Since SPEED_800000 is the largest defined speed in
> include/uapi/linux/ethtool.h, this issue can occur in realistic scenarios.
> 
> To ensure length_to_duration() returns a non-zero value for minimum-sized
> Ethernet frames (ETH_ZLEN = 60), picos_per_byte must be at least 17
> (60 * 17 > PSEC_PER_NSEC which is 1000).
> 
> This patch enforces a minimum value of 17 for picos_per_byte when the
> calculated value would be lower, and adds a warning message to inform
> users that scheduling accuracy may be affected at very high link speeds.

Is it possible to reproduce this with a selftest? If so, please consider
adding one.

Thanks.