linux-kernel - Re: [PATCH v3] lockdep: Allow tuning tracing capacity constants.

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Message-ID: <6933e938-f219-5e13-aee6-fe4de87eb43e@i-love.sakura.ne.jp>
Date:   Sun, 18 Oct 2020 22:02:19 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3] lockdep: Allow tuning tracing capacity constants.

Peter, what do you think? Can we apply this patch?

A potential for-now workaround for syzkaller would be to allow syzkaller
not to encounter the BUG: message (by masking BUG: message on the kernel
side) when hitting these limits, for continue testing until the kernel
crashes (due to other bugs like UAF) would be to some degree useful.

On 2020/10/10 21:58, 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].
> 
> Peter Zijlstra does not want to allow tuning these limits via kernel
> config options, for such change discourages thinking. But currently we
> are not actionable, for lockdep does not report the culprit for hitting
> these limits [4].
> 
> Therefore, I propose this patch again, with a caveat that this patch is
> expected to be reverted after lockdep becomes capable of reporting the
> culprit, for I consider that "postpone fixing lock related problems in
> existing code" is less painful than "not detecting lock related problems
> introduced by new patches".
> 
> [1] https://syzkaller.appspot.com/bug?id=3d97ba93fb3566000c1c59691ea427370d33ea1b
> [2] https://syzkaller.appspot.com/bug?id=381cb436fe60dc03d7fd2a092b46d7f09542a72a
> [3] https://syzkaller.appspot.com/bug?id=a588183ac34c1437fc0785e8f220e88282e5a29f
> [4] https://lkml.kernel.org/r/CACT4Y+agTiEF-1i9LbAgp-q_02oYF0kAPZGAAJ==-wx2Xh7xzQ@mail.gmail.com
> 
> 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>
> Acked-by: Dmitry Vyukov <dvyukov@...gle.com>
> ---
>  kernel/locking/lockdep.c           |  2 +-
>  kernel/locking/lockdep_internals.h |  8 +++---
>  lib/Kconfig.debug                  | 40 ++++++++++++++++++++++++++++++
>  3 files changed, 45 insertions(+), 5 deletions(-)