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Date:   Tue, 8 Aug 2023 21:42:20 +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>,
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        Palmer Dabbelt <palmer@dabbelt.com>,
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Subject: Re: [PATCH v3 21/36] arm64/mm: Implement map_shadow_stack()
Message-ID: <0cf8f1fd-f4d4-45b3-ad49-5ec5ebc5a24f@sirena.org.uk>
References: <20230731-arm64-gcs-v3-0-cddf9f980d98@kernel.org>
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 <ZNDFioFIM34VcsuV@arm.com>
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On Tue, Aug 08, 2023 at 09:21:03AM +0100, Szabolcs Nagy wrote:
> The 08/07/2023 14:00, Mark Brown wrote:

> > 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

> i should have said that i expect MAP_FIXED_NOREPLACE semantics
> (since the x86 code seemed to use that) and then the mapped
> address must match exactly thus page aligned.

Ah, I see.  We do pass MAP_FIXED_NOREPLACE when allocating the stack if
an address was specified but currently leave it up to the VM subsystem
to figure out what to do with the address.  I don't immediately see
where mmap() enforces this requirement, but I have to admit I didn't
look overly hard.  I don't see a problem with enforcing a PAGE_SIZE
constraint here.

> > > > +	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.

> it's kind of important to know the exact logic so the cap token
> location can be computed in userspace for arbitrary size.

> (this is why i wanted to see the map_shadow_stack man page first
> but i was told that comes separately on linux..)

Right, I'd already changed it to % 8 in the version I posted yesterday.

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