linux-kernel - Re: [PATCH 3/3] VFS: close race between getcwd() and d

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Date:   Tue, 21 Nov 2017 23:50:58 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     NeilBrown <neilb@...e.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] VFS: close race between getcwd() and d_move()

On Fri, Nov 10, 2017 at 08:53:28PM +0000, Al Viro wrote:
> On Fri, Nov 10, 2017 at 03:45:41PM +1100, NeilBrown wrote:
> > -void __d_drop(struct dentry *dentry)
> > +static void ___d_drop(struct dentry *dentry)
> >  {
> >  	if (!d_unhashed(dentry)) {
> >  		struct hlist_bl_head *b;
> > @@ -486,12 +488,15 @@ void __d_drop(struct dentry *dentry)
> >  
> >  		hlist_bl_lock(b);
> >  		__hlist_bl_del(&dentry->d_hash);
> > -		dentry->d_hash.pprev = NULL;
> >  		hlist_bl_unlock(b);
> >  		/* After this call, in-progress rcu-walk path lookup will fail. */
> >  		write_seqcount_invalidate(&dentry->d_seq);
> >  	}
> >  }
> > +void __d_drop(struct dentry *dentry) {
> > +	___d_drop(dentry);
> > +	dentry->d_hash.pprev = NULL;
> 
> Umm...  That reordering (unhashed vs. ->d_seq) might be a problem
> on the RCU side.  I'm not sure it is, we might get away with that,
> actually, but I want to finish digging through the pathwalk-related
> code.  Cursing it for being too subtle for its own good, as usual...

OK, I believe that it's survivable, but I'd prefer to keep in -next
for a while and give it more testing.