linux-kernel - Re: [PATCH v3] mm/slub: Fix incorrect interpretation of s->offset

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Date:   Wed, 29 Apr 2020 18:42:55 +0200
From:   Markus Elfring <Markus.Elfring@....de>
To:     Waiman Long <longman@...hat.com>, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Changbin Du <changbin.du@...il.com>,
        Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/slub: Fix incorrect interpretation of s->offset

> 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?


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


Regards,
Markus