linux-kernel - Re: [syzbot] [bcachefs?] [rcu?] BUG: unable to handle kernel NULL pointer dereference in rcu

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Message-Id: <112773d5-4056-4c9c-8a13-95056ed48c98@app.fastmail.com>
Date: Thu, 12 Jun 2025 10:20:09 -0700
From: "Boqun Feng" <boqun.feng@...il.com>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>,
 "Aleksandr Nogikh" <nogikh@...gle.com>
Cc: syzbot <syzbot+80e5d6f453f14a53383a@...kaller.appspotmail.com>,
 "Andrew Morton" <akpm@...ux-foundation.org>,
 "Josh Triplett" <josh@...htriplett.org>, kent.overstreet@...ux.dev,
 linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, "Paul E. McKenney" <paulmck@...nel.org>,
 rcu@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [bcachefs?] [rcu?] BUG: unable to handle kernel NULL pointer
 dereference in rcu_core (3)

On Thu, Jun 12, 2025, at 2:37 AM, Uladzislau Rezki wrote:
> On Thu, Jun 12, 2025 at 09:42:32AM +0200, Aleksandr Nogikh wrote:
>> On Wed, Jun 11, 2025 at 10:58 PM Boqun Feng <boqun.feng@...il.com> wrote:
>> >
>> > On Wed, Jun 11, 2025 at 12:57:04PM -0700, syzbot wrote:
>> > > Hello,
>> > >
>> > > syzbot has tested the proposed patch and the reproducer did not trigger any issue:
>> > >
>> > > Reported-by: syzbot+80e5d6f453f14a53383a@...kaller.appspotmail.com
>> > > Tested-by: syzbot+80e5d6f453f14a53383a@...kaller.appspotmail.com
>> > >
>> > > Tested on:
>> > >
>> > > commit:         488ef356 KEYS: Invert FINAL_PUT bit
>> > > git tree:       upstream
>> > > console output: https://syzkaller.appspot.com/x/log.txt?x=129a660c580000
>> >
>> > Is there a way to see the whole console output? If Ulad's patch fixes
>> > the exact issue, we should be able to see a WARN_ON_ONCE() triggered.
>> 
>> If WARN_ON_ONCE() were triggered, the associated kernel panic output
>> would have been at the end of this log.
>> 
>> >
>> > Regards,
>> > Boqun
>> >
>> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=713d218acd33d94
>> > > dashboard link: https://syzkaller.appspot.com/bug?extid=80e5d6f453f14a53383a
>> 
>> FWIW the last time the bug was observed on syzbot was 100 days ago, so
>> it has likely been fixed since then or has become much harder to
>> reproduce.
>> 
> That is even worse, if it is last for 100 days already.
>

My understanding is that the evidence shows that the
issue that directly caused null-ptr-derek the has been
fixed 100 days ago.

Regards,
Boqun

> --
> Uladzislau Rezki