From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from www262.sakura.ne.jp ([202.181.97.72]:56485 "EHLO www262.sakura.ne.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751750AbeFENqf (ORCPT ); Tue, 5 Jun 2018 09:46:35 -0400 Subject: Re: general protection fault in wb_workfn (2) To: Dmitry Vyukov Cc: Jens Axboe , Jan Kara , 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, Linus Torvalds 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: Tetsuo Handa Message-ID: <95865cab-e12f-d45b-b6e3-465b624862ba@i-love.sakura.ne.jp> Date: Tue, 5 Jun 2018 22:45:31 +0900 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Dmitry, can you assign VM resources for a git tree for this bug? This bug wants to fight against https://github.com/google/syzkaller/blob/master/docs/syzbot.md#no-custom-patches ... On 2018/06/01 1:56, Jens Axboe wrote: > 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. >