linux-kernel - Re: [PATCH v2] lockdep: Allow tuning tracing capacity constants.

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Message-ID: <CACT4Y+bjPr=64Lq1-ARD6T=K9LmC_Aor4BRXPcZVtUU8vF0oGg@mail.gmail.com>
Date:   Wed, 16 Sep 2020 13:28:19 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH v2] lockdep: Allow tuning tracing capacity constants.

On Fri, Sep 4, 2020 at 6:05 PM Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
>
> Hello. Can we apply this patch?
>
> This patch addresses top crashers for syzbot, and applying this patch
> will help utilizing syzbot's resource for finding other bugs.

Acked-by: Dmitry Vyukov <dvyukov@...gle.com>

Peter, do you still have concerns with this?

Both MAX_LOCKDEP_ENTRIES and MAX_LOCKDEP_CHAINS still fire on syzbot a
lot and harm ability to test whole kernel:
https://syzkaller.appspot.com/bug?id=3d97ba93fb3566000c1c59691ea427370d33ea1b
https://syzkaller.appspot.com/bug?id=63fc8d0501c39609dd2f268e4190ec9a72619563

I hate disabling lockdep entirely as it also finds lots of useful things.


> On 2020/08/28 0:20, Tetsuo Handa wrote:
> > Since syzkaller continues various test cases until the kernel crashes,
> > syzkaller tends to examine more locking dependencies than normal systems.
> > As a result, syzbot is reporting that the fuzz testing was terminated
> > due to hitting upper limits lockdep can track [1] [2] [3].
> >
> > Let's allow individually tuning upper limits via kernel config options
> > (based on an assumption that there is no inter-dependency among these
> > constants).
> >
> > [1] https://syzkaller.appspot.com/bug?id=3d97ba93fb3566000c1c59691ea427370d33ea1b
> > [2] https://syzkaller.appspot.com/bug?id=381cb436fe60dc03d7fd2a092b46d7f09542a72a
> > [3] https://syzkaller.appspot.com/bug?id=a588183ac34c1437fc0785e8f220e88282e5a29f
> >
> > Reported-by: syzbot <syzbot+cd0ec5211ac07c18c049@...kaller.appspotmail.com>
> > Reported-by: syzbot <syzbot+91fd909b6e62ebe06131@...kaller.appspotmail.com>
> > Reported-by: syzbot <syzbot+62ebe501c1ce9a91f68c@...kaller.appspotmail.com>
> > Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> > ---
> >  kernel/locking/lockdep.c           |  2 +-
> >  kernel/locking/lockdep_internals.h |  8 +++---
> >  lib/Kconfig.debug                  | 40 ++++++++++++++++++++++++++++++
> >  3 files changed, 45 insertions(+), 5 deletions(-)
> >