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Date:   Tue, 11 Jul 2023 09:21:36 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Roman Gushchin <roman.gushchin@linux.dev>, linux-mm@kvack.org,
        patches@lists.linux.dev, linux-kernel@vger.kernel.org,
        Matteo Rizzo <matteorizzo@google.com>,
        Jann Horn <jannh@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Marco Elver <elver@google.com>,
        Alexander Potapenko <glider@google.com>,
        kasan-dev@googlegroups.com, linux-hardening@vger.kernel.org
Subject: Re: [PATCH 2/2] mm/slub: remove freelist_dereference()
Message-ID: <202307110917.DEED145F0@keescook>
References: <20230711134623.12695-3-vbabka@suse.cz>
 <20230711134623.12695-4-vbabka@suse.cz>
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On Tue, Jul 11, 2023 at 03:46:25PM +0200, Vlastimil Babka wrote:
> freelist_dereference() is a one-liner only used from get_freepointer().
> Remove it and make get_freepointer() call freelist_ptr_decode()
> directly to make the code easier to follow.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/slub.c | 16 ++++++----------
>  1 file changed, 6 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 07edad305512..c4556a5dab4b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -397,18 +397,14 @@ static inline void *freelist_ptr_decode(const struct kmem_cache *s,
>  	return decoded;
>  }
>  
> -/* Returns the freelist pointer recorded at location ptr_addr. */
> -static inline void *freelist_dereference(const struct kmem_cache *s,
> -					 void *ptr_addr)
> -{
> -	return freelist_ptr_decode(s, *(freeptr_t *)(ptr_addr),
> -			    (unsigned long)ptr_addr);
> -}
> -
>  static inline void *get_freepointer(struct kmem_cache *s, void *object)
>  {
> -	object = kasan_reset_tag(object);
> -	return freelist_dereference(s, (freeptr_t *)(object + s->offset));
> +	unsigned long ptr_addr;
> +	freeptr_t p;
> +
> +	ptr_addr = ((unsigned long)kasan_reset_tag(object)) + s->offset;
> +	p = *(freeptr_t *)(ptr_addr);
> +	return freelist_ptr_decode(s, p, ptr_addr);
>  }
>  
>  #ifndef CONFIG_SLUB_TINY
> -- 
> 2.41.0
> 

I like reducing the complexity here, but I find dropping the "object"
reassignment makes this a bit harder to read. What about:

	object = kasan_reset_tag(object);
	unsigned long ptr_addr = (unsigned long)object + s->offset;
	freeptr_t p = *(freeptr_t *)(ptr_addr);

	return freelist_ptr_decode(s, p, ptr_addr);

?

They're the same result, so either way:

Acked-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook