Re: [syzbot] BUG: MAX_LOCKDEP_KEYS too low! (2)

[Dmitry Vyukov <dvyukov@google.com>]

On Wed, May 19, 2021 at 9:58 PM Randy Dunlap <rdunlap@infradead.org> wrote:
>
> On 5/19/21 12:48 PM, Dmitry Vyukov wrote:
> > On Wed, May 19, 2021 at 7:35 PM syzbot
> > <syzbot+a70a6358abd2c3f9550f@syzkaller.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@syzkaller.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)

Ouch, so it's not configurable yet :(
Unless, of course, we identify the offender that produced thousands of
lock classes in the log and fix it.


> 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
>
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