Thanks Soby. I agree, SRAM-based BL31 won't benefit from MTE.

> So, given the above situation, value in enabling MTE2 for BL31 seems limited.

Sure. Do we still want to keep MTE1 around? Is there a real benefit of MTE1 without MTE2?

On Wed, Apr 13, 2022 at 4:59 PM Soby Mathew <Soby.Mathew@arm.com> wrote:
Hi Okash
My 2 cents on this..
Static analyzers can flag use-after-free stack object issues although it is not guaranteed to catch all issues.

Another factor to consider is that BL31 typically runs out of SRAM and it does seem unlikely the hardware implements tag memory for SRAM. So it could be that MTE2 for BL31 would remain an FVP only feature or only for those platforms which allows BL31 to run out of DDR.

So, given the above situation, value in enabling MTE2 for BL31 seems limited.

Best Regards
Soby Mathew

> -----Original Message-----
> From: Okash Khawaja via TF-A <tf-a@lists.trustedfirmware.org>
> Sent: 07 April 2022 11:41
> To: Manish Badarkhe <Manish.Badarkhe@arm.com>
> Cc: tf-a@lists.trustedfirmware.org
> Subject: [TF-A] Re: Status of MTE for bl31
>
> Hi Manish,
>
> Thanks for confirming. Regarding benefit of MTE in BL31, it will basically add
> guards against stack overflows and use-after-free bugs related to objects
> allocated on stack. Regarding stack overflow, TF-A already has stack canaries.
> MTE protection will be stronger than canaries. So MTE adds a marginal benefit
> here. Regarding use-after-free of stack objects, I'm not sure if TF-A has
> adequate protections. On the down side, MTE adds performance overhead
> and increases code size. We probably need some numbers to see the
> overhead of MTE.
>
> It will be good to know what the community thinks are the benefits of MTE are
> on balance.
>
> Thanks,
> Okash
>
>
> On Thu, Apr 7, 2022 at 9:21 AM Manish Badarkhe
> <Manish.Badarkhe@arm.com> wrote:
> >
> > Hi Okash,
> >
> >
> > Introduced build option [1] limits to MTE1 that supports MTE related
> > instructions, but does not offer any functional usage in terms of
> > detecting tag check faults. That's why memory is not configured  as
> > normal tagged (which is an MTE2 feature) in BL31, and support for
> > MTE2 for BL31 is a topic for technical debt.
> >
> >
> >
> > [1]:
> > https://review.trustedfirmware.org/plugins/gitiles/TF-A/trusted-firmwa
> > re-a/+/fd32deeed9914a33341c821e18a1ced29bbe2185
> >
> >
> >
> > Thanks,
> > Manish Badarkhe
> >
> >
> >
> > From: Okash Khawaja <okash@google.com>
> > Date: Tuesday, 5 April 2022 at 16:13
> > To: Manish Badarkhe <Manish.Badarkhe@arm.com>
> > Cc: tf-a@lists.trustedfirmware.org <tf-a@lists.trustedfirmware.org>
> > Subject: Re: [TF-A] Status of MTE for bl31
> >
> > Hi Manish,
> >
> >
> >
> > That's another question I wanted to follow up with. Is there any real benefit
> of MTE for BL31 given that it doesn't allocate memory dynamically. I guess
> stack tagging is the only use of MTE in BL31?
> >
> >
> >
> > The concern in the original email is whether the support is incomplete. Can
> you confirm?
> >
> >
> >
> > Thanks,
> >
> > Okash
> >
> >
> >
> > On Tue, Apr 5, 2022 at 3:27 PM Manish Badarkhe
> <Manish.Badarkhe@arm.com> wrote:
> >
> > Hi Okash,
> >
> >
> >
> > Could you please let us know the real use case/requirement where you want
> MTE to be available at EL3?
> >
> >
> >
> > Thanks,
> > Manish Badarkhe
> >
> >
> >
> > From: Okash Khawaja via TF-A <tf-a@lists.trustedfirmware.org>
> > Date: Friday, 1 April 2022 at 19:51
> > To: tf-a@lists.trustedfirmware.org <tf-a@lists.trustedfirmware.org>
> > Subject: [TF-A] Status of MTE for bl31
> >
> > Hi,
> >
> > I wanted to check the status of MTE support for bl31 itself. It seems
> > like the support was added [1] for clang and armclang but I couldn't
> > find the memory attribute to map pages as tag checked [2]. Is there
> > something I missed?
> >
> > Thanks,
> > Okash
> >
> > [1]
> > https://review.trustedfirmware.org/plugins/gitiles/TF-A/trusted-firmwa
> > re-a/+/fd32deeed9914a33341c821e18a1ced29bbe2185
> >
> > [2]
> > https://review.trustedfirmware.org/plugins/gitiles/TF-A/trusted-firmwa
> > re-a/+/refs/tags/v2.6/lib/xlat_tables_v2/aarch64/xlat_tables_arch.c#25
> > 4
> > --
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