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

[Vinicius Costa Gomes <vinicius.gomes@intel.com>]

Cong Wang <xiyou.wangcong@gmail.com> writes:

> 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.

Good idea. From a quick look, it seems that netdevsim doesn't have
support for .{get,set}_link_ksettings(), so I guess it would be
something for later.


Cheers,
-- 
Vinicius