Re: [PATCH 4/5] kasan, slub: move kasan_poison_slab hook before page_address

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 11 Feb 2019 22:59:53 +0100 Andrey Konovalov <andreyknvl@google.com> wrote:

> With tag based KASAN page_address() looks at the page flags to see
> whether the resulting pointer needs to have a tag set. Since we don't
> want to set a tag when page_address() is called on SLAB pages, we call
> page_kasan_tag_reset() in kasan_poison_slab(). However in allocate_slab()
> page_address() is called before kasan_poison_slab(). Fix it by changing
> the order.
> 
> ...
>
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1642,12 +1642,15 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>  	if (page_is_pfmemalloc(page))
>  		SetPageSlabPfmemalloc(page);
>  
> +	kasan_poison_slab(page);
> +
>  	start = page_address(page);
>  
> -	if (unlikely(s->flags & SLAB_POISON))
> +	if (unlikely(s->flags & SLAB_POISON)) {
> +		metadata_access_enable();
>  		memset(start, POISON_INUSE, PAGE_SIZE << order);
> -
> -	kasan_poison_slab(page);
> +		metadata_access_disable();
> +	}
>  
>  	shuffle = shuffle_freelist(s, page);

This doesn't compile when CONFIG_SLUB_DEBUG=n.  Please review carefully:

--- a/mm/slub.c~kasan-slub-move-kasan_poison_slab-hook-before-page_address-fix
+++ a/mm/slub.c
@@ -1357,6 +1357,14 @@ slab_flags_t kmem_cache_flags(unsigned i
 
 #define disable_higher_order_debug 0
 
+static inline void metadata_access_enable(void)
+{
+}
+
+static inline void metadata_access_disable(void)
+{
+}
+
 static inline unsigned long slabs_node(struct kmem_cache *s, int node)
 							{ return 0; }
 static inline unsigned long node_nr_slabs(struct kmem_cache_node *n)
_