netdev - Re: Routing loops & TTL tracking with tunnel devices

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Message-ID: <CAHmME9rt_fGfgQSL12Q8CnNdh0Fc-8Z9CBEM9iSNjGCQ_En6Ow@mail.gmail.com>
Date:   Sat, 30 Apr 2022 00:09:27 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: Routing loops & TTL tracking with tunnel devices

Hi Eric,

On Sat, Apr 30, 2022 at 12:05 AM Eric Dumazet <edumazet@...gle.com> wrote:
> I assume you add encap headers to the skb ?

Yes; it's encapsulated in UDP, and under that some short header.
However, everything under that is encrypted. So,

> You could check if the wireguard header is there already, or if the
> amount of headers is crazy.

so it's not quite possible to peer down further to see.

> You also can take a look at CONFIG_SKB_EXTENSIONS infrastructure.

Blech, this involves some kind of per-packet allocation, right? I was
hoping there might be some 6 or 7 or 8 bit field in sk_buff that's not
used anywhere on the TX path that maybe I could overload for this
purpose...

Jason