Re: [PATCH v2] ovl: fix dentry leak in ovl_get_redirect

[Liangyan]

This is the race scenario based on call trace we captured which cause 
the dentry leak.


     CPU 0                                CPU 1
ovl_set_redirect                       lookup_fast
  ovl_get_redirect                       __d_lookup
    dget_dlock
      //no lock protection here            spin_lock(&dentry->d_lock)
      dentry->d_lockref.count++            dentry->d_lockref.count++


If we use dget_parent instead, we may have this race.


     CPU 0                                    CPU 1
ovl_set_redirect                           lookup_fast
  ovl_get_redirect                           __d_lookup
    dget_parent
      raw_seqcount_begin(&dentry->d_seq)      spin_lock(&dentry->d_lock)
      lockref_get_not_zero(&ret->d_lockref)   dentry->d_lockref.count++ 



On 20/12/21 下午8:11, Al Viro wrote:
> On Mon, Dec 21, 2020 at 07:14:44PM +0800, Joseph Qi wrote:
> > Hi Viro,
> >
> > On 12/21/20 2:26 PM, Al Viro wrote:
> > > On Sun, Dec 20, 2020 at 08:09:27PM +0800, Liangyan wrote:
> > >
> > > > +++ b/fs/overlayfs/dir.c
> > > > @@ -973,6 +973,7 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect)
> > > >   	for (d = dget(dentry); !IS_ROOT(d);) {
> > > >   		const char *name;
> > > >   		int thislen;
> > > > +		struct dentry *parent = NULL;
> > > >   
> > > >   		spin_lock(&d->d_lock);
> > > >   		name = ovl_dentry_get_redirect(d);
> > > > @@ -992,7 +993,22 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect)
> > > >   
> > > >   		buflen -= thislen;
> > > >   		memcpy(&buf[buflen], name, thislen);
> > > > -		tmp = dget_dlock(d->d_parent);
> > > > +		parent = d->d_parent;
> > > > +		if (unlikely(!spin_trylock(&parent->d_lock))) {
> > > > +			rcu_read_lock();
> > > > +			spin_unlock(&d->d_lock);
> > > > +again:
> > > > +			parent = READ_ONCE(d->d_parent);
> > > > +			spin_lock(&parent->d_lock);
> > > > +			if (unlikely(parent != d->d_parent)) {
> > > > +				spin_unlock(&parent->d_lock);
> > > > +				goto again;
> > > > +			}
> > > > +			rcu_read_unlock();
> > > > +			spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);
> > > > +		}
> > > > +		tmp = dget_dlock(parent);
> > > > +		spin_unlock(&parent->d_lock);
> > > >   		spin_unlock(&d->d_lock);
> > >
> > > Yecchhhh....  What's wrong with just doing
> > > 		spin_unlock(&d->d_lock);
> > > 		parent = dget_parent(d);
> > > 		dput(d);
> > > 		d = parent;
> > > instead of that?
> >
> > Now race happens on non-RCU path in lookup_fast(), I'm afraid d_seq can
> > not close the race window.
>
> Explain, please.  What exactly are you observing?