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Date:   Wed, 13 Dec 2023 20:02:24 +0000
From:   Mark Brown <broonie@kernel.org>
To:     Deepak Gupta <debug@rivosinc.com>
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>,
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        Palmer Dabbelt <palmer@dabbelt.com>,
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Subject: Re: [PATCH v7 20/39] arm64/gcs: Context switch GCS state for EL0
Message-ID: <b2a717a7-e6d3-4fa9-921e-156c17ad5424@sirena.org.uk>
References: <20231122-arm64-gcs-v7-0-201c483bd775@kernel.org>
 <20231122-arm64-gcs-v7-20-201c483bd775@kernel.org>
 <CAKC1njQVB71A8fQBLmBnx++agM12XDLhS=7iL7-A4NTXSqUM+A@mail.gmail.com>
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On Wed, Dec 13, 2023 at 11:59:45AM -0800, Deepak Gupta wrote:
> On Wed, Nov 22, 2023 at 1:45=E2=80=AFAM Mark Brown <broonie@kernel.org> w=
rote:

> > +       if (task->thread.gcs_el0_mode & PR_SHADOW_STACK_ENABLE)
> > +               gcscre0_el1 |=3D GCSCRE0_EL1_RVCHKEN | GCSCRE0_EL1_PCRS=
EL;

> 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=
 up.
> Is it freed in some other flow?

Kept around and freed on thread exit.  There is a potential race between
for example disabling in a signal handler and something trying to walk
the stack so we err on the side of caution.

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