linux-kernel - Re: [PATCH v4 03/36] arm64/gcs: Document the ABI for Guarded Control Stacks

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Date:   Thu, 10 Aug 2023 14:34:04 +0100
From:   Szabolcs Nagy <Szabolcs.Nagy@....com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Arnd Bergmann <arnd@...db.de>, Oleg Nesterov <oleg@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Shuah Khan <shuah@...nel.org>,
        "Rick P. Edgecombe" <rick.p.edgecombe@...el.com>,
        Deepak Gupta <debug@...osinc.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        "H.J. Lu" <hjl.tools@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v4 03/36] arm64/gcs: Document the ABI for Guarded Control
 Stacks

The 08/10/2023 12:41, Mark Brown wrote:
> On Thu, Aug 10, 2023 at 09:55:50AM +0100, Szabolcs Nagy wrote:
> > The 08/09/2023 16:34, Mark Brown wrote:
> 
> > > It's actually based on bitrot that I'd initially chosen a smaller value
> > > since it's likely that functions will push at least something as you
> > > suggest, the patches now just use RLIMIT_STACK.  I'll fix.
> 
> > the pcs requires 16byte aligned stack frames, with 8byte per gcs entry
> > there is no need for same gcs size as stack size in userspace.
> 
> I agree that it's going to be excessive for pretty much all
> applications, I adjusted it to match x86 as part of the general effort
> to avoid divergence and because I was a bit concerned about non-PCS
> cases (eg, JITed code) potentially running into trouble, especially with

is that even possible?

16byte alignment is not a convention but architectural:
access via unaligned sp traps (at least in userspace).

it is possible to use bl such that the stack is not involved
e.g. if there is no bl/ret pairing, but if we base the gcs
size on the stack size then i'd expect one stack frame per
bl/ret pair with 16byte alignment, or is there a programming
model possible that uses 8byte stack per bl?

> smaller stack limits.  It's not an issue I have super strong opinions on
> though, as you can see I had implemented it both ways at various times.