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

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Date:   Tue, 26 Jan 2021 10:15:52 -0600
From:   Justin Forbes <jforbes@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, Greg KH <greg@...ah.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 10:05 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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?



This is true for those of us used to working with source and building
by hand. For users who want everything packaged, rebuilding a kernel
package for install is considerably longer, and for distros providing
builds for multiple arches, we are looking at a couple of hours at
best.  From a Fedora standpoint, I am perfectly fine with it failing
if someone tries to build a module on gcc10 when the kernel was built
with gcc11.  It's tedious when the kernel was built with gcc11
yesterday, and a new gcc11 build today means that kernel needs to be
rebuilt.