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

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Message-ID: <CANn89iLLmTQ6XZG3BsP29E2r0Qix0SBVff0RNz=EUULOAQK88A@mail.gmail.com>
Date:   Fri, 29 Apr 2022 15:46:21 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     "Jason A. Donenfeld" <Jason@...c4.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

On Fri, Apr 29, 2022 at 3:09 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> 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...
>

Maybe reuse redirected / tc_skip_classify (used in drivers/net/ifb.c)