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

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Message-ID: <BsjpBIR8vpl-4AQId33pvJNPXsIqk6I32ejPJYaq0n0bqYuyjvURK7hlny4mawL5siHXHBUx9eSV44XrIMCRAMXcB6tuMJU3VH1vwVviEHA=@willsroot.io>
Date: Tue, 01 Jul 2025 18:46:38 +0000
From: William Liu <will@...lsroot.io>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Stephen Hemminger <stephen@...workplumber.org>, Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org, jhs@...atatu.com, victor@...atatu.com, pctammela@...atatu.com, pabeni@...hat.com, kuba@...nel.org, dcaratti@...hat.com, savy@...t3mfailure.io, jiri@...nulli.us, davem@...emloft.net, horms@...nel.org
Subject: Re: [PATCH net v4 1/2] net/sched: Restrict conditions for adding duplicating netems to qdisc tree

On Tuesday, July 1st, 2025 at 6:11 PM, Eric Dumazet <edumazet@...gle.com> wrote:

> 
> 
> On Sat, Jun 28, 2025 at 8:15 AM Stephen Hemminger
> stephen@...workplumber.org wrote:
> 
> > Why a whole u32 for one flag?
> > 
> > This increases qdisc_skb_cb from 28 bytes to 32 bytes.
> > So still ok, but there should be a build check that it is less than
> > space in skb->cb.
> 
> 
> I am pretty sure this will break some configs. This has been discussed
> in the past.
> 
> I would guess drivers/net/amt.c would need some tweaks.
> 
> commit bec161add35b478a7746bf58bcdea6faa19129ef
> Author: Taehee Yoo ap420073@...il.com
> 
> Date: Sun Jan 7 14:42:41 2024 +0000
> 
> amt: do not use overwrapped cb area
> 
> Also net/core/filter.c:9740 would complain.

In that case, maybe it is safer to just break a config very few (if any) people use than risk spreading more breakage? I doubt people are stacking netems together as they would pretty easily encounter the OOM loop issue - the only legitimate use case is having multiple duplicating netems as children in separate paths of the tree. 

Of course, this can all be solved if people would be happy to add a bit to sk_buff. From some brief testing, this did not add a byte, and even if it did, it shouldn't matter memory wise since we are backed by a slab allocator. And if we take that route, we might as well bring back the ttl fields Jamal was referring to in [1] and prevent potential future loop based DOS bugs.

I just hope this can be resolved soon, this bug report has been going on for quite a while (since May 6th).

Best,
Will

[1] https://lwn.net/Articles/719297/