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References: <20231122-arm64-gcs-v7-0-201c483bd775@kernel.org>
 <20231122-arm64-gcs-v7-21-201c483bd775@kernel.org>
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From:   Thiago Jung Bauermann <thiago.bauermann@linaro.org>
To:     Mark Brown <broonie@kernel.org>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Arnd Bergmann <arnd@arndb.de>, Oleg Nesterov <oleg@redhat.com>,
        Eric Biederman <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Shuah Khan <shuah@kernel.org>,
        "Rick P. Edgecombe" <rick.p.edgecombe@intel.com>,
        Deepak Gupta <debug@rivosinc.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Szabolcs Nagy <Szabolcs.Nagy@arm.com>,
        "H.J. Lu" <hjl.tools@gmail.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
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        Albert Ou <aou@eecs.berkeley.edu>,
        Florian Weimer <fweimer@redhat.com>,
        Christian Brauner <brauner@kernel.org>,
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        kvmarm@lists.linux.dev, linux-fsdevel@vger.kernel.org,
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        linux-riscv@lists.infradead.org
Subject: Re: [PATCH v7 21/39] arm64/gcs: Allocate a new GCS for threads with
 GCS enabled
In-reply-to: <20231122-arm64-gcs-v7-21-201c483bd775@kernel.org>
Date:   Wed, 06 Dec 2023 17:22:49 -0300
Message-ID: <87il5bhyhy.fsf@linaro.org>
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Mark Brown <broonie@kernel.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