Re: [PATCH] kasan, slub: fix handling of kasan_slab_free hook

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

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@google.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
>