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From: Catalin Marinas <catalin.marinas@arm.com>
To: Evgenii Stepanov <eugenis@google.com>
Cc: Mark Brown <broonie@kernel.org>, Will Deacon <will@kernel.org>,
	Joey Gouly <joey.gouly@arm.com>,
	Branislav Rankov <branislav.rankov@arm.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v1 4/4] arm64/mte: Add userspace interface for enabling asymmetric mode
Date: Thu, 3 Mar 2022 10:34:34 +0000	[thread overview]
Message-ID: <YiCZuogxJ7KfH63W@arm.com> (raw)
In-Reply-To: <CAFKCwrgqDp1POGvnJHBkbab=1hQWGn=4u3kE62NX1GGntbwQmQ@mail.gmail.com>

On Wed, Mar 02, 2022 at 12:58:48PM -0800, Evgenii Stepanov wrote:
> On Wed, Mar 2, 2022 at 11:33 AM Mark Brown <broonie@kernel.org> wrote:
> > On Wed, Mar 02, 2022 at 10:44:31AM -0800, Evgenii Stepanov wrote:
> > > On Wed, Mar 2, 2022 at 5:10 AM Mark Brown <broonie@kernel.org> wrote:
> > > > On Wed, Mar 02, 2022 at 11:44:53AM +0000, Catalin Marinas wrote:
> > > > > On Tue, Mar 01, 2022 at 04:52:01PM -0800, Evgenii Stepanov wrote:
> >
> > > > > > Extending PR_MTE_TCF_MASK seems bad for backward compatibility. User
> > > > > > code may do "flags =& ~PR_MTE_TCF_MASK" to disable MTE; when compiled
> > > > > > against an old version of the header this would fail to remove the ASYMM
> > > > > > bit.
> >
> > > > > But if the app is compiled against an old version, it wouldn't set
> > > > > MTE_CTRL_TCF_ASYMM either as it doesn't have the definition.
> >
> > > Libraries within a single process can be built against different
> > > header versions. In our case, this is libc vs the app: we expect to
> > > set all 3 mode bits when an app asks for "async" to enable the
> > > mte_tcf_preferred logic. Even if the app is built against an older NDK
> > > and unaware of the Asymm mode existence!
> >
> > I can't see how we can resolve that case in the kernel except by adding
> > a specific call to disable all MTE modes which would obviously only be
> > useful for future proofing given that no existing applications would
> > support it.
> 
> One option would be to introduce a new, future-proofed prctl with a
> wider mask, and throw in a few extra reserved bits just in case. Then
> have the legacy prctl always clear MTE_CTRL_TCF_ASYMM.

If this problem is real, we can easily tweak the current proposal so the
that ASYMM can only be set *if* both ASYNC and SYNC are set. IOW, it's
not a specific mode an app requests on its own but rather something the
kernel may choose via the preferred mode if the app opted in. We still
introduce a new bit for ASYMM but not change the mask.

-- 
Catalin

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  reply	other threads:[~2022-03-03 10:36 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-27 19:57 [PATCH v1 0/4] arm64/mte: Asymmetric MTE support in userspace Mark Brown
2022-01-27 19:57 ` [PATCH v1 1/4] arm64/mte: Document ABI for asymmetric mode Mark Brown
2022-01-28 16:43   ` Catalin Marinas
2022-01-28 17:23     ` Mark Brown
2022-01-28 16:55   ` Vincenzo Frascino
2022-02-15 18:14   ` Will Deacon
2022-02-16 16:36     ` Mark Brown
2022-02-16 17:06       ` Will Deacon
2022-01-27 19:57 ` [PATCH v1 2/4] arm64/mte: Add a little bit of documentation for mte_update_sctlr_user() Mark Brown
2022-01-28 16:45   ` Catalin Marinas
2022-01-28 16:57   ` Vincenzo Frascino
2022-01-27 19:57 ` [PATCH v1 3/4] arm64/mte: Add hwcap for asymmetric mode Mark Brown
2022-01-28 16:51   ` Catalin Marinas
2022-01-28 17:00   ` Vincenzo Frascino
2022-01-27 19:57 ` [PATCH v1 4/4] arm64/mte: Add userspace interface for enabling " Mark Brown
2022-01-28 17:12   ` Vincenzo Frascino
2022-01-28 17:15   ` Catalin Marinas
2022-03-02  0:52   ` Evgenii Stepanov
2022-03-02 11:44     ` Catalin Marinas
2022-03-02 13:10       ` Mark Brown
2022-03-02 18:44         ` Evgenii Stepanov
2022-03-02 19:33           ` Mark Brown
2022-03-02 20:58             ` Evgenii Stepanov
2022-03-03 10:34               ` Catalin Marinas [this message]
2022-03-03 15:44                 ` Mark Brown
2022-03-03 22:47                   ` Evgenii Stepanov
2022-03-04 21:09                 ` Peter Collingbourne
2022-03-07 15:36                   ` Catalin Marinas
2022-03-07 19:10                     ` Mark Brown
2022-03-07 20:55                       ` Peter Collingbourne
2022-03-08 13:34                         ` Mark Brown
2022-03-07 20:55                     ` Peter Collingbourne
2022-03-08 18:26                       ` Catalin Marinas
2022-02-10 21:43 ` [PATCH v1 0/4] arm64/mte: Asymmetric MTE support in userspace Branislav Rankov

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