Re: [PATCH net v8 4/6] net/sched: netem: validate slot configuration

[Paolo Abeni]

On 4/21/26 3:10 PM, Simon Horman wrote:
> On Fri, Apr 17, 2026 at 08:19:42PM -0700, Stephen Hemminger wrote:
>> Reject slot configurations that have no defensible meaning:
>>
>>   - negative min_delay or max_delay
>>   - min_delay greater than max_delay
>>   - negative dist_delay or dist_jitter
>>   - negative max_packets or max_bytes
>>
>> Negative or out-of-order delays underflow in get_slot_next(),
>> producing garbage intervals. Negative limits trip the per-slot
>> accounting (packets_left/bytes_left <= 0) on the first packet of
>> every slot, defeating the rate-limiting half of the slot feature.
>>
>> Note that dist_jitter has been silently coerced to its absolute
>> value by get_slot() since the feature was introduced; rejecting
>> negatives here converts that silent coercion into -EINVAL. The
>> abs() can be removed in a follow-up.
>>
>> Fixes: 836af83b54e3 ("netem: support delivering packets in delayed time slots")
>> Signed-off-by: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>
> 
> Reviewed-by: Simon Horman <horms@xxxxxxxxxx>
> 
> I acknowledge that Sashiko has provided feedback on this patch.
> 
> 1. "Does rejecting negative dist_jitter values with -EINVAL cause a
>     regression in userspace ABI backward compatibility?  Since the kernel
>     previously accepted these values and silently coerced them using abs(),
>     existing userspace tools or scripts that happen to pass negative values
>     might start failing to configure the qdisc."
> 
> This is intended and explicitly explained in the cover letter.
Jamal, given the uAPI implication, could you please double check that
the change is fine?

Thanks,

Paolo