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

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Date:   Tue, 26 Jan 2021 17:05:03 +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 09:46:51AM -0600, Josh Poimboeuf wrote:
> 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".

Why bother? rebuilding the kernel and all modules is a matter of 10
minutes at most on a decently beefy build box.

What actual problem are we trying to solve here?