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

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Message-ID: <YBAeYaDReAc9VscA@kroah.com>
Date:   Tue, 26 Jan 2021 14:51:29 +0100
From:   Greg KH <greg@...ah.com>
To:     Justin Forbes <jforbes@...hat.com>
Cc:     Josh Poimboeuf <jpoimboe@...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>,
        Peter Zijlstra <peterz@...radead.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 06:44:44AM -0600, Justin Forbes wrote:
> On Tue, Jan 26, 2021 at 2:21 AM Greg KH <greg@...ah.com> wrote:
> >
> > On Mon, Jan 25, 2021 at 04:07:57PM -0600, Josh Poimboeuf wrote:
> > > On Tue, Jan 26, 2021 at 06:44:35AM +0900, Masahiro Yamada wrote:
> > > > > > If people use a different compiler, they must be
> > > > > > prepared for any possible problem.
> > > > > >
> > > > > > Using different compiler flags for in-tree and out-of-tree
> > > > > > is even more dangerous.
> > > > > >
> > > > > > For example, CONFIG_GCC_PLUGIN_RANDSTRUCT is enabled
> > > > > > for in-tree build, and then disabled for out-of-tree modules,
> > > > > > the struct layout will mismatch, won't it?
> > > > >
> > > > > If you read the patch you'll notice that it handles that case, when it's
> > > > > caused by GCC mismatch.
> > > > >
> > > > > However, as alluded to in the [1] footnote, it doesn't handle the case
> > > > > where the OOT build system doesn't have gcc-plugin-devel installed.
> > > > > Then CONFIG_GCC_PLUGIN_RANDSTRUCT gets silently disabled and the build
> > > > > succeeds!  That happens even without a GCC mismatch.
> > > >
> > > >
> > > > Ah, sorry.
> > > >
> > > > I responded too early before reading the patch fully.
> > > >
> > > > But, I do not like to make RANDSTRUCT a special case.
> > > >
> > > > I'd rather want to stop building for any plugin.
> > >
> > > Other than RANDSTRUCT there doesn't seem to be any problem with
> > > disabling them (and printing a warning) in the OOT build.  Why not give
> > > users that option?  It's harmless, and will make distro's (and their
> > > users') lives easier.
> > >
> > > Either GCC mismatch is ok, or it's not.  Let's not half-enforce it.
> >
> > As I said earlier, it's not ok, we can not support it at all.
> >
> 
> Support and enforce are 2 completely different things.  To shed a bit
> more light on this, the real issue that prompted this was breaking CI
> systems.  As we enabled gcc plugins in Fedora, and the toolchain folks
> went through 3 different snapshots of gcc 11 in a week. Any CI process
> that built an out of tree module failed. I don't think this is nearly
> as much of a concern for stable distros, as it is for CI in
> development cycles.

It's better to have an obvious break like this than to silently accept
things and then have a much harder issue to debug at runtime, right?

Don't allow things that we know we will not support, this sounds like an
issue with your CI systems, not with the kernel build system, why not
just fix that?  :)

thnaks,

greg k-h