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

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Message-ID: <20210127232651.rj3mo7c2oqh4ytsr@treble>
Date:   Wed, 27 Jan 2021 17:26:51 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     jthierry@...hat.com, ardb@...nel.org, broonie@...nel.org,
        catalin.marinas@....com, keescook@...omium.org,
        linux-arm-kernel@...ts.infradead.org, linux-efi@...r.kernel.org,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, masahiroy@...nel.org,
        michal.lkml@...kovi.net, peterz@...radead.org,
        raphael.gault@....com, will@...nel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [RFC PATCH 12/17] gcc-plugins: objtool: Add plugin to detect
 switch table on arm64

On Wed, Jan 27, 2021 at 02:15:57PM -0800, Nick Desaulniers wrote:
> > From: Raphael Gault <raphael.gault@....com>
> > 
> > This plugins comes into play before the final 2 RTL passes of GCC and
> > detects switch-tables that are to be outputed in the ELF and writes
> > information in an ".discard.switch_table_info" section which will be
> > used by objtool.
> > 
> > Signed-off-by: Raphael Gault <raphael.gault@....com>
> > [J.T.: Change section name to store switch table information,
> >        Make plugin Kconfig be selected rather than opt-in by user,
> >        Add a relocation in the switch_table_info that points to
> >        the jump operation itself]
> > Signed-off-by: Julien Thierry <jthierry@...hat.com>
> 
> Rather than tightly couple this feature to a particular toolchain via
> plugin, it might be nice to consider what features could be spec'ed out
> for toolchains to implement (perhaps via a -f flag).

The problem is being able to detect switch statement jump table vectors.

For a given indirect branch (due to a switch statement), what are all
the corresponding jump targets?

We would need the compiler to annotate that information somehow.

> Distributions (like Android, CrOS) wont be able to use such a feature as
> is.

Would a Clang plugin be out of the question?

-- 
Josh