Re: [PATCH v2 16/20] kasan: simplify assign_tag and set_tag calls

[Marco Elver <elver@google.com>]

On Tue, Nov 10, 2020 at 11:20PM +0100, Andrey Konovalov wrote:
> set_tag() already ignores the tag for the generic mode, so just call it
> as is. Add a check for the generic mode to assign_tag(), and simplify its
> call in ____kasan_kmalloc().
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> Link: https://linux-review.googlesource.com/id/I18905ca78fb4a3d60e1a34a4ca00247272480438
> ---
>  mm/kasan/common.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)

Reviewed-by: Marco Elver <elver@google.com>

> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 69ab880abacc..40ff3ce07a76 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -238,6 +238,9 @@ void __kasan_poison_object_data(struct kmem_cache *cache, void *object)
>  static u8 assign_tag(struct kmem_cache *cache, const void *object,
>  			bool init, bool keep_tag)
>  {
> +	if (IS_ENABLED(CONFIG_KASAN_GENERIC))
> +		return 0xff;
> +

Hopefully the compiler is clever enough to start inlining this function.

>  	/*
>  	 * 1. When an object is kmalloc()'ed, two hooks are called:
>  	 *    kasan_slab_alloc() and kasan_kmalloc(). We assign the
> @@ -280,8 +283,8 @@ void * __must_check __kasan_init_slab_obj(struct kmem_cache *cache,
>  		__memset(alloc_meta, 0, sizeof(*alloc_meta));
>  	}
>  
> -	if (IS_ENABLED(CONFIG_KASAN_SW_TAGS) || IS_ENABLED(CONFIG_KASAN_HW_TAGS))
> -		object = set_tag(object, assign_tag(cache, object, true, false));
> +	/* Tag is ignored in set_tag() without CONFIG_KASAN_SW/HW_TAGS */
> +	object = set_tag(object, assign_tag(cache, object, true, false));
>  
>  	return (void *)object;
>  }
> @@ -362,9 +365,7 @@ static void *____kasan_kmalloc(struct kmem_cache *cache, const void *object,
>  				KASAN_GRANULE_SIZE);
>  	redzone_end = round_up((unsigned long)object + cache->object_size,
>  				KASAN_GRANULE_SIZE);
> -
> -	if (IS_ENABLED(CONFIG_KASAN_SW_TAGS) || IS_ENABLED(CONFIG_KASAN_HW_TAGS))
> -		tag = assign_tag(cache, object, false, keep_tag);
> +	tag = assign_tag(cache, object, false, keep_tag);
>  

The definition of 'tag' at the start of ____kasan_kmalloc() no longer
needs an initializer.

>  	/* Tag is ignored in set_tag without CONFIG_KASAN_SW/HW_TAGS */
>  	kasan_unpoison_memory(set_tag(object, tag), size);