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Date:   Mon, 7 Aug 2023 14:00:42 +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 v3 21/36] arm64/mm: Implement map_shadow_stack()
Message-ID: <3a077ff3-8627-4337-9b4a-6a8828eda0e7@sirena.org.uk>
References: <20230731-arm64-gcs-v3-0-cddf9f980d98@kernel.org>
 <20230731-arm64-gcs-v3-21-cddf9f980d98@kernel.org>
 <ZNDFioFIM34VcsuV@arm.com>
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On Mon, Aug 07, 2023 at 11:20:58AM +0100, Szabolcs Nagy wrote:
> The 07/31/2023 14:43, Mark Brown wrote:
> > +SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags)
> > +{
> > +	unsigned long alloc_size;
> > +	unsigned long __user *cap_ptr;
> > +	unsigned long cap_val;
> > +	int ret;
> > +
> > +	if (!system_supports_gcs())
> > +		return -EOPNOTSUPP;
> > +
> > +	if (flags)
> > +		return -EINVAL;
> > +
> > +	if (addr % 16)
> > +		return -EINVAL;

> mmap addr must be page aligned (and there is no align req on size).

> i'd expect similar api here.

That's not what the manual page or a quick check of the code suggest
that mmap() does, they say that the kernel just takes it as a hint and
chooses a nearby page boundary, though I didn't test.  I'm not sure why
I have that alignment check at all TBH, and to the extent it's needed I
could just be 8 - this level of code doesn't really care.

> > +	if (size == 16 || size % 16)
> > +		return -EINVAL;

> why %16 and not %8 ?

I don't think that's needed any more - there was some stuff in an
earlier version of the code but no longer.

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