linux-kernel - Re: Status of reverted Linux patch "tty: Fix ldisc crash on reopened tty", Linux 4.9 kernel frequent crashes

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Date:   Thu, 31 Aug 2017 15:22:05 +1000
From:   Michael Neuling <mikey@...ling.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Pasi Kärkkäinen <pasik@....fi>
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Peter Hurley <peter@...leysoftware.com>,
        Nathan March <nathan@...net>, centos-virt@...tos.org,
        linux-kernel@...r.kernel.org
Subject: Re: Status of reverted Linux patch "tty: Fix ldisc crash on
 reopened tty", Linux 4.9 kernel frequent crashes

On Thu, 2017-08-31 at 06:36 +0200, Greg Kroah-Hartman wrote:
> On Wed, Aug 30, 2017 at 11:10:14PM +0300, Pasi Kärkkäinen wrote:
> > Hello everyone,
> > 
> > Recently Nathan March reported on centos-virt list he's getting frequent
> > Linux kernel crashes with Linux 4.9 LTS kernel because of the missing patch
> > "tty: Fix ldisc crash on reopened tty".
> 
> Crashes with "normal" operation, or crashes when running a fuzzer or
> other type of program?

For me it crashed on boot.

> 
> > The patch was already merged upstream here:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i
> > d=71472fa9c52b1da27663c275d416d8654b905f05
> > 
> > but then reverted here:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i
> > d=896d81fefe5d1919537db2c2150ab6384e4a6610
> > 
> > Nathan confirmed if he applies the patch from
> > 71472fa9c52b1da27663c275d416d8654b905f05 to his Linux 4.9 LTS kernel the
> > bug/problem goes away, so the patch (or similar fix) is still needed, at
> > least for 4.9 LTS kernel.
> > 
> > 
> > Mikulas reported he's able to trigger the same crash on Linux 4.10:
> > https://www.spinics.net/lists/kernel/msg2440637.html
> > https://lists.gt.net/linux/kernel/2664604?search_string=ldisc%20reopened;#26
> > 64604
> > 
> > Michael Neuling reported he's able to trigger the bug on PowerPC:
> > https://lkml.org/lkml/2017/3/10/1582
> > 
> > 
> > So now the question is.. is anyone currently working on getting this patch
> > fixed and applied upstream? I think one of the problems earlier was being
> > able to reliable reproduce the crash.. Nathan says he's able to reproduce it
> > many times per week on his environment on x86_64.
> 
> I don't know of anyone working on it, want to do it yourself?

I'm not anymore. We found it was only triggered on a bogus CONFIG option
combination.  Once we removed that, it no longer happened.

The underlying bug was still there though.

Mikey