linux-kernel - Re: [PATCH] kasan, slub: fix handling of kasan_slab

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Date:   Fri, 2 Mar 2018 15:10:21 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrey Konovalov <andreyknvl@...gle.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>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com
Cc:     Kostya Serebryany <kcc@...gle.com>
Subject: Re: [PATCH] kasan, slub: fix handling of kasan_slab_free hook

On 02/23/2018 06:53 PM, Andrey Konovalov wrote:
> The kasan_slab_free hook's return value denotes whether the reuse of a
> slab object must be delayed (e.g. when the object is put into memory
> qurantine).
> 
> The current way SLUB handles this hook is by ignoring its return value
> and hardcoding checks similar (but not exactly the same) to the ones
> performed in kasan_slab_free, which is prone to making mistakes.
> 

What are those differences exactly? And what problems do they cause?
Answers to these questions should be in the changelog.


> This patch changes the way SLUB handles this by:
> 1. taking into account the return value of kasan_slab_free for each of
>    the objects, that are being freed;
> 2. reconstructing the freelist of objects to exclude the ones, whose
>    reuse must be delayed.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> ---




>  
> @@ -2965,14 +2974,13 @@ static __always_inline void slab_free(struct kmem_cache *s, struct page *page,
>  				      void *head, void *tail, int cnt,
>  				      unsigned long addr)
>  {
> -	slab_free_freelist_hook(s, head, tail);
>  	/*
> -	 * slab_free_freelist_hook() could have put the items into quarantine.
> -	 * If so, no need to free them.
> +	 * With KASAN enabled slab_free_freelist_hook modifies the freelist
> +	 * to remove objects, whose reuse must be delayed.
>  	 */
> -	if (s->flags & SLAB_KASAN && !(s->flags & SLAB_TYPESAFE_BY_RCU))
> -		return;
> -	do_slab_free(s, page, head, tail, cnt, addr);
> +	slab_free_freelist_hook(s, &head, &tail);
> +	if (head != NULL)

That's an additional branch in non-debug fast-path. Find a way to avoid this.


> +		do_slab_free(s, page, head, tail, cnt, addr);
>  }
>  
>  #ifdef CONFIG_KASAN
>