netdev - Re: [Patch net 1/2] netem: Fix skb duplication logic to prevent infinite loops

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Message-ID: <CAM0EoMkZ=X08L2D_3=FiXUDjg4ver-_ahuCv7TCMF3ExDi4HaA@mail.gmail.com>
Date: Tue, 8 Jul 2025 09:18:36 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: William Liu <will@...lsroot.io>, Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org, 
	stephen@...workplumber.org, Savino Dicanosa <savy@...t3mfailure.io>
Subject: Re: [Patch net 1/2] netem: Fix skb duplication logic to prevent
 infinite loops

On Mon, Jul 7, 2025 at 5:26 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 7 Jul 2025 16:49:46 -0400 Jamal Hadi Salim wrote:
> > > The tc_skb_ext approach has a problem... the config option that
> > > enables it is NET_TC_SKB_EXT. I assumed this is a generic name for
> > > skb extensions in the tc subsystem, but unfortunately this is
> > > hardcoded for NET_CLS_ACT recirculation support.
> > >
> > > So what this means is we have the following choices:
> > > 1. Make SCH_NETEM depend on NET_CLS_ACT and NET_TC_SKB_EXT
> > > 2. Add "|| IS_ENABLED(CONFIG_SCH_NETEM)" next to
> > > "IS_ENABLED(CONFIG_NET_TC_SKB_EXT)" 3. Separate NET_TC_SKB_EXT and
> > > the idea of recirculation support. But I'm not sure how people feel
> > > about renaming config options. And this would require a small
> > > change to the Mellanox driver subsystem.
> > >
> > > None of these sound too nice to do, and I'm not sure which approach
> > > to take. In an ideal world, 3 would be best, but I'm not sure how
> > > others would feel about all that just to account for a netem edge
> > > case.
> >
> > I think you should just create a new field/type, add it here:
> > include/linux/skbuff.h around line 4814 and make netem just select
> > CONFIG_SKB_EXTENSIONS kconfig
> > It's not the best solution but we are grasping for straws at this
> > point.
>
> Did someone report a real user of nested duplication?
>

None i have seen - regardless, if there was one it would be a strange use case.

> Let's go ahead with the patch preventing such configurations and worry
> about supporting them IIF someone actually asks.

That was my view as well.

cheers,
jamal