linux-kernel - Re: [PATCH v4 03/36] arm64/gcs: Document the ABI for Guarded Control Stacks

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Message-ID: <aaea542c-929c-4c9b-8caa-ca67e0eb9c1e@sirena.org.uk>
Date:   Fri, 18 Aug 2023 20:38:02 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Arnd Bergmann <arnd@...db.de>, Oleg Nesterov <oleg@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Shuah Khan <shuah@...nel.org>,
        "Rick P. Edgecombe" <rick.p.edgecombe@...el.com>,
        Deepak Gupta <debug@...osinc.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>,
        "H.J. Lu" <hjl.tools@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v4 03/36] arm64/gcs: Document the ABI for Guarded Control
 Stacks

On Fri, Aug 18, 2023 at 06:29:54PM +0100, Catalin Marinas wrote:

> A related question - it may have been discussed intensively on the x86
> thread (I may read it sometime) - why not have the libc map the shadow

Your assumption that this is a single thread feels optimistic there.

> stack and pass the pointer/size to clone3()? It saves us from having to
> guess what the right size we'd need. struct clone_args is extensible.

I can't recall or locate the specific reasoning there right now, perhaps
Rick or someone else can?  I'd guess there would be compat concerns for
things that don't go via libc which would complicate the story with
identifying and marking things as GCS/SS safe, it's going to be more
robust to just supply a GCS if the process is using it.  That said
having a default doesn't preclude us using the extensibility to allow
userspace directly to control the GCS size, I would certainly be in
favour of adding support for that.

> (I plan to get back next week to this series, I'll need to read a bit
> more on the spec)

I've been making changes, mostly in response to your feedback, so there
should be a new version on Monday even if not everything is addressed
yet.

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