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

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Message-ID: <CAHmME9odyPcXSv5Vc5LCYiMANZKjhZ3+WbNdh8wFmG2BVdkHoA@mail.gmail.com>
Date:	Tue, 17 Nov 2015 03:57:10 +0100
From:	"Jason A. Donenfeld" <Jason@...c4.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Jiri Benc <jbenc@...hat.com>, therbert@...gle.com,
	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Routing loops & TTL tracking with tunnel devices

On Mon, Nov 16, 2015 at 11:25 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> Have a look at __dev_queue_xmit and the per_cpu recursion limits
> implemented there:
>
>                         if (__this_cpu_read(xmit_recursion) >
>                         RECURSION_LIMIT)
>                                 goto recursion_alert;

Ahh, thanks for pointing that out. So this works with virtual devices
with no queue. As of some recent changes, that now applies to what I'm
doing.

Unfortunately, I get a complete hard crash, with the blinking
keyboard. The only thing written to serial before it dies is:
[  171.347446] Dead loop on virtual device wg0, fix it urgently!
This means it did hit that recursion condition, which is good.
I assume the recursion limit is just too high, and this has something
to do with me overflowing the stack. I'll test this hypothesis and see
if I can add a similar check inside my driver to make it lower. If
this works, I'm satisfied.

Thanks a lot for the pointer here.
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