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

[Dev Jain <dev.jain@arm.com>]

On 13/06/26 5:53 am, Isaac Manjarres wrote:
> 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.

Ah so you mean, we can define KASAN_SLAB_FREE and KASAN_PAGE_FREE to be
different values, leaving KASAN_SLAB_REDZONE and KASAN_PAGE_REDZONE to
be 0xFE, the poison value. Yep I'll do that.
> 
> Thanks,
> Isaac