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Date:   Tue, 18 Jul 2023 14:55:07 +0100
From:   Mark Brown <broonie@kernel.org>
To:     Szabolcs Nagy <Szabolcs.Nagy@arm.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>,
        Deepak Gupta <debug@rivosinc.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        "H.J. Lu" <hjl.tools@gmail.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
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        linux-riscv@lists.infradead.org
Subject: Re: [PATCH 22/35] arm64/mm: Implement map_shadow_stack()
Message-ID: <8ea03eda-e630-40c5-acc1-a63f5c5b3102@sirena.org.uk>
References: <20230716-arm64-gcs-v1-0-bf567f93bba6@kernel.org>
 <20230716-arm64-gcs-v1-22-bf567f93bba6@kernel.org>
 <ZLZW7Kvg2Rep8ySO@arm.com>
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On Tue, Jul 18, 2023 at 10:10:04AM +0100, Szabolcs Nagy wrote:

>   uint64_t *p = map_shadow_stack(0, N*8, 0);

> i'd expect p[N-1] to be the end token and p[N-2] to be the cap token,
> not p[PAGE_ALIGN(N*8)/8-2].

Yes, that probably would be more helpful.

> if we allow misalligned size here (and in munmap) then i think it's
> better to not page align.  size%8!=0 || size<16 can be an error.

Honestly I'd be a lot happier to just not allow misalignment but that
raises the issue with binaries randomly not working when moved to a
kernel with a different page size.  I'll have a think but possibly the
safest thing would be requiring a multiple of 4K then rounding up to our
actual page size.

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