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Date:   Tue, 2 Mar 2021 17:26:49 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        linux-hardening@vger.kernel.org, linux-kbuild@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Justin Forbes <jforbes@redhat.com>,
        Ondrej Mosnacek <omosnace@redhat.com>,
        Frank Eigler <fche@redhat.com>,
        Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH RFC] gcc-plugins: Handle GCC version mismatch for OOT
 modules
Message-ID: <20210302232649.y2tutffhxsblwqlb@treble>
References: <efe6b039a544da8215d5e54aa7c4b6d1986fc2b0.1611607264.git.jpoimboe@redhat.com>
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On Mon, Jan 25, 2021 at 02:42:10PM -0600, Josh Poimboeuf wrote:
> When building out-of-tree kernel modules, the build system doesn't
> require the GCC version to match the version used to build the original
> kernel.  That's probably [1] fine.
> 
> In fact, for many distros, the version of GCC used to build the latest
> kernel doesn't necessarily match the latest released GCC, so a GCC
> mismatch turns out to be pretty common.  And with CONFIG_MODVERSIONS
> it's probably more common.
> 
> So a lot of users have come to rely on being able to use a different
> version of GCC when building OOT modules.
> 
> But with GCC plugins enabled, that's no longer allowed:
> 
>   cc1: error: incompatible gcc/plugin versions
>   cc1: error: failed to initialize plugin ./scripts/gcc-plugins/structleak_plugin.so
> 
> That error comes from the plugin's call to
> plugin_default_version_check(), which strictly enforces the GCC version.
> The strict check makes sense, because there's nothing to prevent the GCC
> plugin ABI from changing -- and it often does.
> 
> But failing the build isn't necessary.  For most plugins, OOT modules
> will otherwise work just fine without the plugin instrumentation.
> 
> When a GCC version mismatch is detected, print a warning and disable the
> plugin.  The only exception is the RANDSTRUCT plugin which needs all
> code to see the same struct layouts.  In that case print an error.

Hi Masahiro,

This problem is becoming more prevalent.  We will need to fix it one way
or another, if we want to support distro adoption of these GCC
plugin-based features.

Frank suggested a possibly better idea: always rebuild the plugins when
the GCC version changes.  What do you think?  Any suggestions on how to
implement that?  Otherwise I can try to hack something together.

-- 
Josh