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

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Message-Id: <20210127221557.1119744-1-ndesaulniers@google.com>
Date:   Wed, 27 Jan 2021 14:15:57 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     jthierry@...hat.com
Cc:     ardb@...nel.org, broonie@...nel.org, catalin.marinas@....com,
        jpoimboe@...hat.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

> 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).

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