Re: [PATCH net v4 1/2] net/sched: Restrict conditions for adding duplicating netems to qdisc tree

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

On Sat, Jun 28, 2025 at 05:25:25PM -0400, Jamal Hadi Salim wrote:
> your approach was to overwrite the netem specific cb which is exposed
> via the cb ->data that can be overwritten for example by a trivial
> ebpf program attach to any level of the hierarchy. This specific
> variant from Cong is not accessible to ebpf but as i expressed my view
> in other email i feel it is not a good solution.
> 
> https://lore.kernel.org/netdev/CAM0EoMk4dxOFoN_=3yOy+XrtU=yvjJXAw3fVTmN9=M=R=vtbxA@mail.gmail.com/

Hi Jamal,

I have two concerns regarding your/Will's proposal:

1) I am not sure whether disallowing such case is safe. From my
understanding this case is not obviously or logically wrong. So if we
disallow it, we may have a chance to break some application.

2) Singling out this case looks not elegant to me. Even _if_ we really
want to disallow such case, we still need to find a better and more
elegant way to do so, for example, adding a new ops for netem and calling
it in sch_api.c. Something like below:

// Implement netem_avoid_duplicate()
// ...

static struct Qdisc_ops netem_qdisc_ops __read_mostly = {
  .avoid_duplicate = netem_avoid_duplicate,
};

// In sch_api.c
// traverse the Qdisch hierarch and call ->avoid_duplicate()

What do you think?

Thanks.