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

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Message-ID: <20210126154651.itfrnhwfistia3ss@treble>
Date:   Tue, 26 Jan 2021 09:46:51 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 04:15:37PM +0100, Peter Zijlstra wrote:
> 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.

Ok, thank you for an actual example.  asm goto is a good one.

But it's not a cut-and-dry issue.  Otherwise how could modversions
possibly work?

So yes, we should enforce GCC versions, but I still haven't seen a
reason it should be more than just "same compiler and *major* version".

-- 
Josh