Re: [PATCH v4 29/39] arm64: mte: Switch GCR_EL1 in kernel entry and exit

[Vincenzo Frascino]

On 10/9/20 11:16 AM, Catalin Marinas wrote:
> On Fri, Oct 09, 2020 at 10:56:02AM +0100, Vincenzo Frascino wrote:
>> On 10/9/20 9:11 AM, Catalin Marinas wrote:
>>> On Thu, Oct 08, 2020 at 07:24:12PM +0100, Vincenzo Frascino wrote:
>>>> On 10/2/20 3:06 PM, Catalin Marinas wrote:
>>>>> On Fri, Oct 02, 2020 at 01:10:30AM +0200, Andrey Konovalov wrote:
>>>>>> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
>>>>>> index 7c67ac6f08df..d1847f29f59b 100644
>>>>>> --- a/arch/arm64/kernel/mte.c
>>>>>> +++ b/arch/arm64/kernel/mte.c
>>>>>> @@ -23,6 +23,8 @@
>>>>>>  #include <asm/ptrace.h>
>>>>>>  #include <asm/sysreg.h>
>>>>>>  
>>>>>> +u64 gcr_kernel_excl __ro_after_init;
>>>>>> +
>>>>>>  static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
>>>>>>  {
>>>>>>  	pte_t old_pte = READ_ONCE(*ptep);
>>>>>> @@ -120,6 +122,13 @@ void *mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
>>>>>>  	return ptr;
>>>>>>  }
>>>>>>  
>>>>>> +void mte_init_tags(u64 max_tag)
>>>>>> +{
>>>>>> +	u64 incl = GENMASK(max_tag & MTE_TAG_MAX, 0);
>>>>>
>>>>> Nitpick: it's not obvious that MTE_TAG_MAX is a mask, so better write
>>>>> this as GENMASK(min(max_tag, MTE_TAG_MAX), 0).
>>>>
>>>> The two things do not seem equivalent because the format of the tags in KASAN is
>>>> 0xFF and in MTE is 0xF, hence if extract the minimum whatever is the tag passed
>>>> by KASAN it will always be MTE_TAG_MAX.
>>>>
>>>> To make it cleaner I propose: GENMASK(FIELD_GET(MTE_TAG_MAX, max_tag), 0);
>>>
>>> I don't think that's any clearer since FIELD_GET still assumes that
>>> MTE_TAG_MAX is a mask. I think it's better to add a comment on why this
>>> is needed, as you explained above that the KASAN tags go to 0xff.
>>>
>>> If you want to get rid of MTE_TAG_MAX altogether, just do a
>>>
>>> 	max_tag &= (1 << MAX_TAG_SIZE) - 1;
>>>
>>> before setting incl (a comment is still useful).
>>>
>>
>> Agree, but still think we should use FIELD_GET here since it is common language
>> in the kernel.
>>
>> How about we get rid of MTE_TAG_MAX and we do something like:
>>
>> GENMASK(FIELD_GET(MTE_TAG_MASK >> MTE_TAG_SHIFT, max_tag), 0);
> 
> It works for me and you can drop the MTE_TAG_MAX definition (I think
> it's only used here).
> 

Yes indeed, I will drop it.

-- 
Regards,
Vincenzo