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

[Al Viro]

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?