linux-kernel - Re: [RFC PATCH 12/17] gcc-plugins: objtool: Add plugin to detect switch table on arm64

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Message-ID: <20210203130453.GB4880@sirena.org.uk>
Date:   Wed, 3 Feb 2021 13:04:53 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Julien Thierry <jthierry@...hat.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Kees Cook <keescook@...omium.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Peter Zijlstra <peterz@...radead.org>, raphael.gault@....com,
        Will Deacon <will@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Bill Wendling <morbo@...gle.com>, swine@...gle.com,
        yonghyun@...gle.com
Subject: Re: [RFC PATCH 12/17] gcc-plugins: objtool: Add plugin to detect
 switch table on arm64

On Tue, Feb 02, 2021 at 06:14:14PM -0600, Josh Poimboeuf wrote:
> On Tue, Feb 02, 2021 at 03:01:22PM -0800, Nick Desaulniers wrote:

> > I wonder if PAC or BTI also make this slightly more complex?  PAC at
> > least has implications for unwinders, IIUC.

> What is PAC/BTI?

PAC and BTI are ARM architecture extensions.  PAC uses a tag in pointers
to sign and verify them, presenting a barrier to ROP, and when BTI is
active only specific instructions can be branched to.  Since PAC
modifies pointers when it is active the unwinder has to undo the tagging
to understand what's being pointed to, that's already there.

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