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

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Message-ID: <CANp29Y4d3de3cc9weuqHCw2sDMdtArOFz5B=783KZ4419Ux=Ag@mail.gmail.com>
Date: Thu, 12 Jun 2025 09:42:32 +0200
From: Aleksandr Nogikh <nogikh@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: syzbot <syzbot+80e5d6f453f14a53383a@...kaller.appspotmail.com>, 
	akpm@...ux-foundation.org, josh@...htriplett.org, kent.overstreet@...ux.dev, 
	linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, paulmck@...nel.org, rcu@...r.kernel.org, 
	syzkaller-bugs@...glegroups.com, urezki@...il.com
Subject: Re: [syzbot] [bcachefs?] [rcu?] BUG: unable to handle kernel NULL
 pointer dereference in rcu_core (3)

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.

> > compiler:       Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6
> > patch:          https://syzkaller.appspot.com/x/patch.diff?x=170e460c580000
> >
> > Note: testing is done by a robot and is best-effort only.
> >
>

-- 
Aleksandr