linux-kernel - Re: Routing loops & TTL tracking with tunnel devices

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Date:   Fri, 29 Apr 2022 13:54:27 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, kuba@...nel.org,
        hannes@...essinduktion.org, edumazet@...gle.com
Subject: Re: Routing loops & TTL tracking with tunnel devices


On 4/28/22 17:37, Jason A. Donenfeld wrote:
> Hey Eric,
>
> On Tue, Nov 17, 2015 at 03:41:35AM +0100, Jason A. Donenfeld wrote:
>> On Mon, Nov 16, 2015 at 11:28 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>>> There is very little chance we'll accept a new member in sk_buff, unless
>>> proven needed.
>> I actually have no intention of doing this! I'm wondering if there
>> already is a member in sk_buff that moonlights as my desired ttl
>> counter, or if there's another mechanism for avoiding routing loops. I
>> want to work with what's already there, rather than meddling with the
>> innards of important and memory sensitive structures such as sk_buff.
> Well, 7 years later... Maybe you have a better idea now of what I was
> working on then. :)
>
> As an update on this issue, it's still quasi problematic. To review, I
> can't use the TTL value, because the outer packet always must get the
> TTL of the route to the outer destination, not the inner packet minus
> one. I can't rely on reaching MTU size, because people want this to work
> with fragmentation (see [1] for my attempt to disallow fragmentation for
> this issue, which resulted in hoots and hollers). I can't use the
> per-cpu xmit_recursion variable, because I use threads.
>
> What I can sort of use is taking advantage of what looks like a bug in
> pskb expansion, such that it always allocates too much, and pretty
> quickly fails allocations after a few loops. Only powerpc64 and s390x
> don't appear to have this bug. See [2] for a description of this in
> depth I wrote a few months ago to you.


Hmm, I will take a look later I think. Thanks for the reminder.


>
> Anyway, it'd be nice if there were a free u8 somewhere in sk_buff that I
> could use for tracking times through the stack. Other kernels have this
> but afaict Linux still does not. I looked into trying to overload some
> existing fields -- tstamp/skb_mstamp_ns or queue_mapping -- which I was
> thinking might be totally unused on TX?


if skbs are stored in some internal wireguard queue, can not you use 
skb->cb[],

like many other layers do ?


>
> Any ideas about this?
>
> Thanks,
> Jason
>
> [1] https://lore.kernel.org/wireguard/CAHmME9rNnBiNvBstb7MPwK-7AmAN0sOfnhdR=eeLrowWcKxaaQ@mail.gmail.com/
> [2] https://lore.kernel.org/netdev/CAHmME9pv1x6C4TNdL6648HydD8r+txpV4hTUXOBVkrapBXH4QQ@mail.gmail.com/