linux-kernel - Re: WARNING: bad unlock balance in ovl

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Date:   Mon, 2 Mar 2020 12:17:34 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     syzbot <syzbot+66a9752fa927f745385e@...kaller.appspotmail.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: WARNING: bad unlock balance in ovl_llseek

On Mon, Mar 2, 2020 at 12:10 PM Amir Goldstein <amir73il@...il.com> wrote:

> > =====================================
> > WARNING: bad unlock balance detected!
> > 5.6.0-rc3-syzkaller #0 Not tainted
> > -------------------------------------
> > syz-executor194/8947 is trying to release lock (&ovl_i_lock_key[depth]) at:
> > [<ffffffff828b7835>] ovl_inode_unlock fs/overlayfs/overlayfs.h:328 [inline]
> > [<ffffffff828b7835>] ovl_llseek+0x215/0x2c0 fs/overlayfs/file.c:193
> > but there are no more locks to release!
> >
>
> This is strange. I don't see how that can happen nor how my change would
> have caused this regression. If anything, the lock chance may have brought
> a bug in stack file ops to light, but don't see the bug.

The bug is that ovl_inode_lock() is interruptible and that the caller
doesn't check for error.

I think the fix is to make this lock uninterruptible (probably rename
the current helper to _interruptible and use the current name as the
uninterruptible version).

Thanks,
Miklos