From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-it0-f67.google.com ([209.85.214.67]:40808 "EHLO mail-it0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755630AbeEaQ4Q (ORCPT ); Thu, 31 May 2018 12:56:16 -0400 Received: by mail-it0-f67.google.com with SMTP id j186-v6so28864831ita.5 for ; Thu, 31 May 2018 09:56:16 -0700 (PDT) Subject: Re: general protection fault in wb_workfn (2) To: Jan Kara , Tetsuo Handa Cc: syzbot , syzkaller-bugs@googlegroups.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, viro@zeniv.linux.org.uk, tj@kernel.org, david@fromorbit.com, linux-block@vger.kernel.org, Dmitry Vyukov References: <000000000000cbd959056d1851ca@google.com> <0c7c5dea-7312-8a59-9d1b-5467f69719bf@I-love.SAKURA.ne.jp> <20180528133503.awomzj6djozbo5bv@quack2.suse.cz> <2dda7a11-3f6b-bdba-a68a-7c0694806cc4@I-love.SAKURA.ne.jp> <20180531114228.6zx27xxurmz27pke@quack2.suse.cz> <20180531134212.oo4ra4ypuruab3xp@quack2.suse.cz> From: Jens Axboe Message-ID: Date: Thu, 31 May 2018 10:56:13 -0600 MIME-Version: 1.0 In-Reply-To: <20180531134212.oo4ra4ypuruab3xp@quack2.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On 5/31/18 7:42 AM, Jan Kara wrote: > On Thu 31-05-18 22:19:44, Tetsuo Handa wrote: >> On 2018/05/31 20:42, Jan Kara wrote: >>> On Thu 31-05-18 01:00:08, Tetsuo Handa wrote: >>>> So, we have no idea what is happening... >>>> Then, what about starting from temporary debug printk() patch shown below? >>>> >>>> >From 4f70f72ad3c9ae6ce1678024ef740aca4958e5b0 Mon Sep 17 00:00:00 2001 >>>> From: Tetsuo Handa >>>> Date: Wed, 30 May 2018 09:57:10 +0900 >>>> Subject: [PATCH] bdi: Add temporary config for debugging wb_workfn() versus >>>> bdi_unregister() race bug. >>>> >>>> syzbot is hitting NULL pointer dereference at wb_workfn() [1]. But due to >>>> limitations that syzbot cannot find reproducer for this bug (frequency is >>>> once or twice per a day) nor we can't capture vmcore in the environment >>>> which syzbot is using, for now we need to rely on printk() debugging. >>>> >>>> [1] https://syzkaller.appspot.com/bug?id=e0818ccb7e46190b3f1038b0c794299208ed4206 >>>> >>>> Signed-off-by: Tetsuo Handa >>> >>> Hum a bit ugly solution but if others are fine with this, I can live with >>> it for a while as well. Or would it be possible for syzkaller to just test >>> some git tree where this patch is included? Then we would not even have to >>> have the extra config option... >> >> If syzbot can reproduce this bug that way. While it is possible to add/remove >> git trees syzbot tests, frequently adding/removing trees is bothering. >> >> syzbot can enable extra config option. Maybe the config name should be >> something like CONFIG_DEBUG_FOR_SYZBOT rather than individual topic. >> >> I think that syzbot is using many VM instances. I don't know how many >> instances will be needed for reproducing this bug within reasonable period. >> More git trees syzbot tests, (I assume that) longer period will be needed >> for reproducing this bug. The most reliable way is to use the shared part >> of all trees (i.e. linux.git). > > I understand this, I'd be just a bit reluctant to merge temporary debug > patches like this to Linus' tree only to revert them later just because > syzkaller... What do others think? I guess I don't understand why having it in Linus's tree would make a difference to syzkaller? If there is a compelling reason why that absolutely has to be done, I don't think it should be accompanied by a special Kconfig option for it. It should just be on unconditionally, with the intent to remove it before release. -- Jens Axboe