Re: [PATCH v3 2/4] prtcl: introduce PR_MTE_STORE_ONLY

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Apr 24, 2025 at 10:34:57PM +0200, David Hildenbrand wrote:
> On 10.04.25 10:07, Yeoreum Yun wrote:
> > PR_MTE_STORE_ONLY is used to restrict the MTE tag check for store
> > opeartion only.
> > 
> > Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> > ---
> >   include/uapi/linux/prctl.h | 2 ++
> >   1 file changed, 2 insertions(+)
> > 
> > diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> > index 15c18ef4eb11..83ac566251d8 100644
> > --- a/include/uapi/linux/prctl.h
> > +++ b/include/uapi/linux/prctl.h
> > @@ -244,6 +244,8 @@ struct prctl_mm_map {
> >   # define PR_MTE_TAG_MASK		(0xffffUL << PR_MTE_TAG_SHIFT)
> >   /* Unused; kept only for source compatibility */
> >   # define PR_MTE_TCF_SHIFT		1
> > +/* MTE tag check store only */
> > +# define PR_MTE_STORE_ONLY		(1UL << 19)
> 
> That is the next available bit after PR_MTE_TAG_MASK, correct?
> 
> Would we want to leave some space to grow PR_MTE_TAG_MASK in the future
> (could that happen?)?

The current mask covers 16 tags (bits 59:56 of a pointer) and given the
reluctance to have a tag storage of 4 bits per 16 bytes (3% of RAM), I
doubt we'd ever grow this.

However, you have a good point, we could indeed leave 32 bits for the
tag mask, just in case MTE gets so much traction that someone wants 8
bits per tag (and likely a bigger granule than 16 bytes). It doesn't
cost us anything to add additional bits from (PR_MTE_TAG_SHIFT + 32).

Thanks.

-- 
Catalin