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

[Vincenzo Frascino]

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);

Obviously with a comment ;)

-- 
Regards,
Vincenzo