Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 29 Apr 2020 11:25:08 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Markus Elfring <Markus.Elfring@web.de>
Cc:     Waiman Long <longman@redhat.com>, Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Changbin Du <changbin.du@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] mm/slub: Fix incorrect interpretation of s->offset
Message-ID: <20200429182508.GU29705@bombadil.infradead.org>
References: <20200429135328.26976-1-longman@redhat.com>
 <af19e771-062f-6ed7-0b9a-e8cd8d0fdb6a@web.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <af19e771-062f-6ed7-0b9a-e8cd8d0fdb6a@web.de>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Apr 29, 2020 at 06:42:55PM +0200, Markus Elfring wrote:
> > In a couple of places in the slub memory allocator, the code uses
> > "s->offset" as a check to see if the free pointer is put right after the
> > object. That check is no longer true with commit 3202fa62fb43 ("slub:
> > relocate freelist pointer to middle of object").
> 
> Will any further collateral evolution become interesting?

What do you mean by this question?

> > +static inline unsigned int get_info_end(struct kmem_cache *s)
> > +{
> > +	if (freeptr_outside_object(s))
> > +		return s->inuse + sizeof(void *);
> > +	else
> > +		return s->inuse;
> > +}
> 
> How do you think about the following source code variants?
> 
> +	return freeptr_outside_object(s)
> +	       ? s->inuse + sizeof(void *)
> +	       : s->inuse;

That is less clear than the version Wayman posted.

> >  static struct track *get_track(struct kmem_cache *s, void *object,
> >  	enum track_item alloc)
> >  {
> >  	struct track *p;
> >
> > -	if (s->offset)
> > -		p = object + s->offset + sizeof(void *);
> > -	else
> > -		p = object + s->inuse;
> > +	p = object + get_info_end(s);
> >
> >  	return p + alloc;
> >  }
> 
> +	struct track *p = object + get_info_end(s);
> 
>  	return p + alloc;

Yes, I think that's an improvement.