netdev - Re: [syzbot] [btrfs?] [netfilter?] BUG: MAX_LOCKDEP_CHAIN

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Date: Thu, 20 Jul 2023 09:18:44 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Cc: Florian Westphal <fw@...len.de>, Aleksandr Nogikh <nogikh@...gle.com>, 
 syzbot <syzbot+9bbbacfbf1e04d5221f7@...kaller.appspotmail.com>,
 dsterba@...e.cz,  bakmitopiacibubur@...a.indosterling.com, clm@...com,
 davem@...emloft.net,  dsahern@...nel.org, dsterba@...e.com,
 gregkh@...uxfoundation.org,  jirislaby@...nel.org, josef@...icpanda.com,
 kadlec@...filter.org,  linux-btrfs@...r.kernel.org,
 linux-fsdevel@...r.kernel.org,  linux-kernel@...r.kernel.org,
 linux-serial@...r.kernel.org,  linux@...linux.org.uk,
 netfilter-devel@...r.kernel.org, pablo@...filter.org, 
 syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [btrfs?] [netfilter?] BUG: MAX_LOCKDEP_CHAIN_HLOCKS
 too low! (2)

On Wed, 2023-07-19 at 20:30 -0700, Jakub Kicinski wrote:
> On Thu, 20 Jul 2023 01:12:07 +0200 Florian Westphal wrote:
> > I don't see any netfilter involvement here.
> > 
> > The repro just creates a massive amount of team devices.
> > 
> > At the time it hits the LOCKDEP limits on my test vm it has
> > created ~2k team devices, system load is at +14 because udev
> > is also busy spawing hotplug scripts for the new devices.
> > 
> > After reboot and suspending the running reproducer after about 1500
> > devices (before hitting lockdep limits), followed by 'ip link del' for
> > the team devices gets the lockdep entries down to ~8k (from 40k),
> > which is in the range that it has on this VM after a fresh boot.
> > 
> > So as far as I can see this workload is just pushing lockdep
> > past what it can handle with the configured settings and is
> > not triggering any actual bug.
> 
> The lockdep splat because of netdevice stacking is one of our top
> reports from syzbot. Is anyone else feeling like we should add 
> an artificial but very high limit on netdev stacking? :(

We already have a similar limit for xmit: XMIT_RECURSION_LIMIT. I guess
stacking more then such devices will be quite useless/non functional.
We could use such value to limit the device stacking, too.

Cheers,

Paolo