linux-hardening - Re: [PATCH RFC] gcc-plugins: Handle GCC version mismatch for OOT modules

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Message-ID: <YBAyGU7H8E98xKng@hirez.programming.kicks-ass.net>
Date:   Tue, 26 Jan 2021 16:15:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Greg KH <greg@...ah.com>, Justin Forbes <jforbes@...hat.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-hardening@...r.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Ondrej Mosnacek <omosnace@...hat.com>
Subject: Re: [PATCH RFC] gcc-plugins: Handle GCC version mismatch for OOT
 modules

On Tue, Jan 26, 2021 at 08:51:55AM -0600, Josh Poimboeuf wrote:
> User space mixes compiler versions all the time.  The C ABI is stable.
> 
> What specifically is the harder issue you're referring to?

I don't think the C ABI captures nearly enough. Imagine trying to mix a
compiler with and without asm-goto support (ok, we fail to build without
by now, but just imagine).

No C ABI violated, but having that GCC extention vs not having it
radically changes the kernel ABI.

I think I'm with Greg here, just don't do it.