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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Fri, 22 Jan 2021 18:54:44 +0100
Message-ID: <CAMj1kXF31FxCTbo4M8MX0aaegaq7AQXMUdCtsm6xrKUFSpkzjA@mail.gmail.com>
Subject: Re: [RFC PATCH 00/17] objtool: add base support for arm64
To:     Mark Brown <broonie@kernel.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Julien Thierry <jthierry@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Mark Rutland <mark.rutland@arm.com>,
        linux-efi <linux-efi@vger.kernel.org>,
        linux-hardening@vger.kernel.org, live-patching@vger.kernel.org
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On Fri, 22 Jan 2021 at 18:44, Mark Brown <broonie@kernel.org> wrote:
>
> On Thu, Jan 21, 2021 at 12:54:52PM -0600, Josh Poimboeuf wrote:
>
> > 2) The shadow stack idea sounds promising -- how hard would it be to
> >    make a prototype reliable unwinder?
>
> In theory it doesn't look too hard and I can't see a particular reason
> not to try doing this - there's going to be edge cases but hopefully for
> reliable stack trace they're all in areas where we would be happy to
> just decide the stack isn't reliable anyway, things like nesting which
> allocates separate shadow stacks for each nested level for example.
> I'll take a look.

This reminds me - a while ago, I had a stab at writing a rudimentary
GCC plugin that pushes/pops return addresses to a shadow call stack
pointed to by x18 [0]
I am by no means suggesting that we should rely on a GCC plugin for
this, only that it does seem rather straight-forward for the compiler
to manage a stack with return addresses like that (although the devil
is probably in the details, as usual)

[0] https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=arm64-scs-gcc