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Date:   Mon, 15 Jul 2019 15:46:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Theodore Ts'o <tytso@....edu>,
        syzbot <syzbot+4bfbbf28a2e50ab07368@...kaller.appspotmail.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        David Miller <davem@...emloft.net>, eladr@...lanox.com,
        Ido Schimmel <idosch@...lanox.com>,
        Jiri Pirko <jiri@...lanox.com>,
        John Stultz <john.stultz@...aro.org>,
        linux-ext4@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: INFO: rcu detected stall in ext4_write_checks

On Mon, Jul 15, 2019 at 03:33:11PM +0200, Dmitry Vyukov wrote:
> On Mon, Jul 15, 2019 at 3:29 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Sun, Jul 14, 2019 at 11:49:15AM -0700, Paul E. McKenney wrote:
> > > On Sun, Jul 14, 2019 at 05:48:00PM +0300, Dmitry Vyukov wrote:
> > > > But short term I don't see any other solution than stop testing
> > > > sched_setattr because it does not check arguments enough to prevent
> > > > system misbehavior. Which is a pity because syzkaller has found some
> > > > bad misconfigurations that were oversight on checking side.
> > > > Any other suggestions?
> > >
> > > Keep the times down to a few seconds?  Of course, that might also
> > > fail to find interesting bugs.
> >
> > Right, if syzcaller can put a limit on the period/deadline parameters
> > (and make sure to not write "-1" to
> > /proc/sys/kernel/sched_rt_runtime_us) then per the in-kernel
> > access-control should not allow these things to happen.
> 
> Since we are racing with emails, could you suggest a 100% safe
> parameters? Because I only hear people saying "safe", "sane",
> "well-behaving" :)
> If we move the check to user-space, it does not mean that we can get
> away without actually defining what that means.

Right, well, that's part of the problem. I think Paul just did the
reverse math and figured that 95% of X must not be larger than my
watchdog timeout and landed on 14 seconds.

I'm thinking 4 seconds (or rather 4.294967296) would be a very nice
number.

> Now thinking of this, if we come up with some simple criteria, could
> we have something like a sysctl that would allow only really "safe"
> parameters?

I suppose we could do that, something like:
sysctl_deadline_period_{min,max}. I'll have to dig back a bit on where
we last talked about that and what the problems where.

For one, setting the min is a lot harder, but I suppose we can start at
TICK_NSEC or something.

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