On Tue, Jun 5, 2018 at 3:45 PM, Tetsuo Handa
[off-list ref] wrote:

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 ...

Hi Tetsuo,

Most of the reasons for not doing it still stand. A syzkaller instance
will produce not just this bug, it will produce hundreds of different
bugs. Then the question is: what to do with these bugs? Report all to
mailing lists?
I think the solution here is just to run syzkaller instance locally.
It's just a program anybody can run it on any kernel with any custom
patches. Moreover for local instance it's also possible to limit set
of tested syscalls to increase probability of hitting this bug and at
the same time filter out most of other bugs.

Do we have any idea about the guilty subsystem? You mentioned
bdi_unregister, why? What would be the set of syscalls to concentrate
on?
I will do a custom run when I get around to it, if nobody else beats me to it.

On 2018/06/01 1:56, Jens Axboe wrote:

quoted

On 5/31/18 7:42 AM, Jan Kara wrote:

quoted

On Thu 31-05-18 22:19:44, Tetsuo Handa wrote:

quoted

On 2018/05/31 20:42, Jan Kara wrote:

quoted

On Thu 31-05-18 01:00:08, Tetsuo Handa wrote:

quoted

So, we have no idea what is happening...
Then, what about starting from temporary debug printk() patch shown below?

quoted

From 4f70f72ad3c9ae6ce1678024ef740aca4958e5b0 Mon Sep 17 00:00:00 2001

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
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 <penguin-kernel@I-love.SAKURA.ne.jp>

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.