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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Fri, 29 Apr 2022 23:53:44 +0200
Message-ID: <CAHmME9qXC-4OPc5xRbC6CQJcpzb96EXzNWAist5A8momYxvVUA@mail.gmail.com>
Subject: Re: Routing loops & TTL tracking with tunnel devices
To:     Eric Dumazet <eric.dumazet@gmail.com>
Cc:     Netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Hannes Frederic Sowa <hannes@stressinduktion.org>,
        Eric Dumazet <edumazet@google.com>
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Hi Eric,

On Fri, Apr 29, 2022 at 11:14 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>
>
> On 4/29/22 14:07, Jason A. Donenfeld wrote:
>
> Hi Eric,
>
> On Fri, Apr 29, 2022 at 01:54:27PM -0700, Eric Dumazet wrote:
>
> 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 ?
>
> This isn't for some internal wireguard queue. The packets get sent out
> of udp_tunnel_xmit_skb(), so they leave wireguard's queues.
>
>
> OK, where is the queue then ?
>
> dev_xmit_recursion() is supposed to catch long chains of virtual devices.

This is the long-chain-of-virtual-devices case indeed. But
dev_xmit_recursion() does not help here, since that's just a per-cpu
increment, assuming that the packet actually gets xmit'd in its
ndo_start_xmit function. But in reality, wireguard queues up the
packet, encrypts it in some worker later, and eventually transmits it
with udp_tunnel_xmit_skb(). All the while ndo_start_xmit() has long
returned. So no help from dev_xmit_recursion().

Jason