netdev - Re: [PATCH net 1/1 v3] net: sched: Disallow replacing of child qdisc from one parent to another

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Message-ID: <CAM0EoMnYi3JBPS7KyPoW5-St-xAaJ8Xa1tEp8JH9483Z5k8cLg@mail.gmail.com>
Date: Wed, 15 Jan 2025 09:15:31 -0500
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: netdev@...r.kernel.org, jiri@...nulli.us, xiyou.wangcong@...il.com, 
	davem@...emloft.net, edumazet@...gle.com, security@...nel.org, 
	nnamrec@...il.com
Subject: Re: [PATCH net 1/1 v3] net: sched: Disallow replacing of child qdisc
 from one parent to another

On Tue, Jan 14, 2025 at 8:26 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Sat, 11 Jan 2025 10:14:55 -0500 Jamal Hadi Salim wrote:
> > The semantics of "replace" is for a del/add _on the same node_ and not
> > a delete from one node(3:1) and add to another node (1:3) as in step10.
> > While we could "fix" with a more complex approach there could be
> > consequences to expectations so the patch takes the preventive approach of
> > "disallow such config".
>
> Your explanation reads like you want to prevent a qdisc changing
> from one parent to another.
>

Yes.

> > +                             if (leaf_q && leaf_q->parent != q->parent) {
> > +                                     NL_SET_ERR_MSG(extack, "Invalid Parent for operation");
> > +                                     return -EINVAL;
> > +                             }
>
> But this test looks at the full parent path, not the major.
> So the only case you allow is replacing the node.. with itself?
>

Yes.

> Did you mean to wrap these in TC_H_MAJ() || the parent comparison
> is redundant || I misunderstand?

I may be missing something - what does TC_H_MAJ() provide?
The 3:1 and 1:3 in that example are both descendants of the same
parent. It could have been 1:3 vs 1:2 and the same rules would apply.

cheers,
jamal