linux-kernel - Re: [PATCH v8 00/12] add support for Clang's Shadow Call Stack

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Message-ID: <CABCJKudAYATQSVLHbM4873Yr2EYufrBWQ7Pmv+L97uHhBQUe4w@mail.gmail.com>
Date:   Wed, 19 Feb 2020 12:12:21 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     James Morse <james.morse@....com>
Cc:     Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 00/12] add support for Clang's Shadow Call Stack

On Wed, Feb 19, 2020 at 10:38 AM James Morse <james.morse@....com> wrote:
> This looks like reserving x18 is causing Clang to not-inline the __kern_hyp_va() calls,
> losing the vitally important section information. (I can see why the compiler thinks this
> is fair)

Thanks for catching this. This doesn't appear to be caused by
reserving x18, it looks like SCS itself is causing clang to avoid
inlining these. If I add __noscs to __kern_hyp_va(), clang inlines the
function again. __always_inline also works, as you pointed out.

> Is this a known, er, thing, with clang-9?

I can reproduce this with ToT clang as well.

> I suspect repainting all KVM's 'inline' with __always_inline will fix it. (yuck!) I'll try
> tomorrow.

I think switching to __always_inline is the correct solution here.

Sami