linux-kernel - Re: [PATCH v7 21/39] arm64/gcs: Allocate a new GCS for threads with GCS enabled

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Message-ID: <87il5bhyhy.fsf@linaro.org>
Date:   Wed, 06 Dec 2023 17:22:49 -0300
From:   Thiago Jung Bauermann <thiago.bauermann@...aro.org>
To:     Mark Brown <broonie@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        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>,
        Florian Weimer <fweimer@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        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 v7 21/39] arm64/gcs: Allocate a new GCS for threads with
 GCS enabled


Mark Brown <broonie@...nel.org> writes:

> When a new thread is created by a thread with GCS enabled the GCS needs
> to be specified along with the regular stack.  clone3() has been
> extended to support this case, allowing userspace to explicitly request
> the size for the GCS to be created, but plain clone() is not extensible
> and existing clone3() users will not specify a size.
>
> For compatibility with these cases and also x86 (which did not initially
> implement clone3() support for shadow stacks) if no GCS is specified we
> will allocate one thread so when a thread is created which has GCS
                    ~~~~~~

This "thread" seems extraneous in the sentence. Remove it?

> enabled allocate one for it.  We follow the extensively discussed x86
> implementation and allocate min(RLIMIT_STACK, 4G).  Since the GCS only

Isn't it min(RLIMIT_STACK/2, 2G)?

> stores the call stack and not any variables this should be more than
> sufficient for most applications.
>
> GCSs allocated via this mechanism then it will be freed when the thread
> exits.

I'm not sure I parsed this sentence correctly. Is it missing an "If" at
the beginning?

-- 
Thiago