linux-kernel - Re: [syzbot] BUG: MAX_LOCKDEP

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Date:   Wed, 19 May 2021 12:57:58 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+a70a6358abd2c3f9550f@...kaller.appspotmail.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        WireGuard mailing list <wireguard@...ts.zx2c4.com>
Subject: Re: [syzbot] BUG: MAX_LOCKDEP_KEYS too low! (2)

On 5/19/21 12:48 PM, Dmitry Vyukov wrote:
> On Wed, May 19, 2021 at 7:35 PM syzbot
> <syzbot+a70a6358abd2c3f9550f@...kaller.appspotmail.com> wrote:
>>
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    b81ac784 net: cdc_eem: fix URL to CDC EEM 1.0 spec
>> git tree:       net
>> console output: https://syzkaller.appspot.com/x/log.txt?x=15a257c3d00000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=5b86a12e0d1933b5
>> dashboard link: https://syzkaller.appspot.com/bug?extid=a70a6358abd2c3f9550f
>>
>> Unfortunately, I don't have any reproducer for this issue yet.
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+a70a6358abd2c3f9550f@...kaller.appspotmail.com
>>
>> BUG: MAX_LOCKDEP_KEYS too low!
> 

include/linux/lockdep.h

#define MAX_LOCKDEP_KEYS_BITS		13
#define MAX_LOCKDEP_KEYS		(1UL << MAX_LOCKDEP_KEYS_BITS)

Documentation/locking/lockdep-design.rst:

Troubleshooting:
----------------

The validator tracks a maximum of MAX_LOCKDEP_KEYS number of lock classes.
Exceeding this number will trigger the following lockdep warning::

	(DEBUG_LOCKS_WARN_ON(id >= MAX_LOCKDEP_KEYS))

By default, MAX_LOCKDEP_KEYS is currently set to 8191, and typical
desktop systems have less than 1,000 lock classes, so this warning
normally results from lock-class leakage or failure to properly
initialize locks.  These two problems are illustrated below:

> 
> What config controls this? I don't see "MAX_LOCKDEP_KEYS too low" in
> any of the config descriptions...
> Here is what syzbot used:
> 
> CONFIG_LOCKDEP=y
> CONFIG_LOCKDEP_BITS=16
> CONFIG_LOCKDEP_CHAINS_BITS=17
> CONFIG_LOCKDEP_STACK_TRACE_BITS=20
> CONFIG_LOCKDEP_STACK_TRACE_HASH_BITS=14
> CONFIG_LOCKDEP_CIRCULAR_QUEUE_BITS=12
> 
> We already bumped most of these.
> The log contains dump of the lockdep debug files, is there any offender?
> 
> Also looking at the log I noticed a memory safety bug in lockdep implementation:

...

-- 
~Randy