Re: [RFC PATCH 2/2] kasan: hw_tags: Add boot option to elide free time poisoning

[Isaac Manjarres <isaacmanjarres@google.com>]

On Fri, Jun 12, 2026 at 04:44:24AM +0000, Dev Jain wrote:
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index fc9169a547662..4fa8abb312faa 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
>  #ifdef CONFIG_KASAN_GENERIC
> @@ -478,6 +489,16 @@ static inline u8 kasan_random_tag(void) { return 0; }
>  
>  static inline void kasan_poison(const void *addr, size_t size, u8 value, bool init)
>  {
> +	if (kasan_tag_only_on_alloc_enabled()) {
> +		if ((value != KASAN_SLAB_REDZONE) && (value != KASAN_PAGE_REDZONE)) {
> +			if (init)
> +				memset((void *)kasan_reset_tag(addr), 0, size);
> +			return;
> +		}
> +	}
> +
> +	value |= 0xF0;
> +

I wonder if it would make more sense to have this as:

if (kasan_tag_only_on_alloc_enabled() && (value == KASAN_SLAB_FREE ||
    value == KASAN_PAGE_FREE)) {
	if (init)
		memset((void *)kasan_reset_tag(addr), 0, size);
	return;
}

That seems a bit clearer to me as to what it is that you're doing, and
also makes it so that you don't have to do any bit manipulation
on the value when you're filling in the redzones.

Thanks,
Isaac