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Subject: Re: [PATCH v3] mm/slub: Fix incorrect interpretation of s->offset
To:     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>,
        Matthew Wilcox <willy@infradead.org>, linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org
References: <20200429135328.26976-1-longman@redhat.com>
From:   Markus Elfring <Markus.Elfring@web.de>
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Message-ID: <af19e771-062f-6ed7-0b9a-e8cd8d0fdb6a@web.de>
Date:   Wed, 29 Apr 2020 18:42:55 +0200
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> 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?


=E2=80=A6
> +++ b/mm/slub.c
> @@ -551,15 +551,32 @@ static void print_section(char *level, char *text,=
 u8 *addr,
=E2=80=A6
> +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;


>  static struct track *get_track(struct kmem_cache *s, void *object,
>  	enum track_item alloc)
>  {
>  	struct track *p;
>
> -	if (s->offset)
> -		p =3D object + s->offset + sizeof(void *);
> -	else
> -		p =3D object + s->inuse;
> +	p =3D object + get_info_end(s);
>
>  	return p + alloc;
>  }

+	struct track *p =3D object + get_info_end(s);

 	return p + alloc;


Regards,
Markus