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

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Date:   Tue, 26 Jan 2021 06:44:44 -0600
From:   Justin Forbes <jforbes@...hat.com>
To:     Greg KH <greg@...ah.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 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.

Justin