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

Masahiro Yamada
2021-01-25T21:47:42Z

On Tue, Jan 26, 2021 at 6:28 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Tue, Jan 26, 2021 at 06:16:01AM +0900, Masahiro Yamada wrote:
> > On Tue, Jan 26, 2021 at 5:42 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > >
> > > When building out-of-tree kernel modules, the build system doesn't
> > > require the GCC version to match the version used to build the original
> > > kernel. That's probably [1] fine.
> > >
> > > In fact, for many distros, the version of GCC used to build the latest
> > > kernel doesn't necessarily match the latest released GCC, so a GCC
> > > mismatch turns out to be pretty common. And with CONFIG_MODVERSIONS
> > > it's probably more common.
> > >
> > > So a lot of users have come to rely on being able to use a different
> > > version of GCC when building OOT modules.
> > >
> > > But with GCC plugins enabled, that's no longer allowed:
> > >
> > > cc1: error: incompatible gcc/plugin versions
> > > cc1: error: failed to initialize plugin ./scripts/gcc-plugins/structleak_plugin.so
> > >
> > > That error comes from the plugin's call to
> > > plugin_default_version_check(), which strictly enforces the GCC version.
> > > The strict check makes sense, because there's nothing to prevent the GCC
> > > plugin ABI from changing -- and it often does.
> > >
> > > But failing the build isn't necessary. For most plugins, OOT modules
> > > will otherwise work just fine without the plugin instrumentation.
> > >
> > > When a GCC version mismatch is detected, print a warning and disable the
> > > plugin. The only exception is the RANDSTRUCT plugin which needs all
> > > code to see the same struct layouts. In that case print an error.
> > >
> > > [1] Ignoring, for the moment, that the kernel now has
> > > toolchain-dependent kconfig options, which can silently disable
> > > features and cause havoc when compiler versions differ, or even when
> > > certain libraries are missing. This is a separate problem which
> > > also needs to be addressed.
> > >
> > > Reported-by: Ondrej Mosnacek <omosnace@redhat.com>
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> > > ---
> >
> >
> > We are based on the assumption that we use the same
> > compiler for in-tree and out-of-tree.
>
> Sorry, but that assumption isn't based in reality. And it's not
> enforced.
>
> > 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.

> > This patch is ugly, and not doing the right thing.
>
> Maybe, but I doubt the solution is to make assumptions.
>
> --
> Josh
>

--
Best Regards
Masahiro Yamada