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References: <20231122-arm64-gcs-v7-0-201c483bd775@kernel.org> <20231122-arm64-gcs-v7-20-201c483bd775@kernel.org>
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From:   Deepak Gupta <debug@rivosinc.com>
Date:   Wed, 13 Dec 2023 11:59:45 -0800
Message-ID: <CAKC1njQVB71A8fQBLmBnx++agM12XDLhS=7iL7-A4NTXSqUM+A@mail.gmail.com>
Subject: Re: [PATCH v7 20/39] arm64/gcs: Context switch GCS state for EL0
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>,
        Ard Biesheuvel <ardb@kernel.org>,
        Szabolcs Nagy <Szabolcs.Nagy@arm.com>,
        "H.J. Lu" <hjl.tools@gmail.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Florian Weimer <fweimer@redhat.com>,
        Christian Brauner <brauner@kernel.org>,
        Thiago Jung Bauermann <thiago.bauermann@linaro.org>,
        linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
        kvmarm@lists.linux.dev, linux-fsdevel@vger.kernel.org,
        linux-arch@vger.kernel.org, linux-mm@kvack.org,
        linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-riscv@lists.infradead.org
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On Wed, Nov 22, 2023 at 1:45=E2=80=AFAM Mark Brown <broonie@kernel.org> wro=
te:
> +
> +/*
> + * Apply the GCS mode configured for the specified task to the
> + * hardware.
> + */
> +void gcs_set_el0_mode(struct task_struct *task)
> +{
> +       u64 gcscre0_el1 =3D GCSCRE0_EL1_nTR;
> +
> +       if (task->thread.gcs_el0_mode & PR_SHADOW_STACK_ENABLE)
> +               gcscre0_el1 |=3D GCSCRE0_EL1_RVCHKEN | GCSCRE0_EL1_PCRSEL=
;

If the intent is to disable, is the GCS stack freed or kept around?
I expect if libc is taking the decision to disable, kernel should free it u=
p.
Is it freed in some other flow?

> +
> +       if (task->thread.gcs_el0_mode & PR_SHADOW_STACK_WRITE)
> +               gcscre0_el1 |=3D GCSCRE0_EL1_STREn;
> +
> +       if (task->thread.gcs_el0_mode & PR_SHADOW_STACK_PUSH)
> +               gcscre0_el1 |=3D GCSCRE0_EL1_PUSHMEn;
> +
> +       write_sysreg_s(gcscre0_el1, SYS_GCSCRE0_EL1);
> +}