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

Masahiro Yamada
2021-01-25T23:47:12Z

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.

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?

This patch is ugly, and not doing the right thing.

> scripts/Makefile.gcc-plugins | 19 +++++++++++++++++++
> scripts/Makefile.kcov | 11 +++++++++++
> 2 files changed, 30 insertions(+)
>
> diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
> index 952e46876329..7227692fba59 100644
> --- a/scripts/Makefile.gcc-plugins
> +++ b/scripts/Makefile.gcc-plugins
> @@ -51,6 +51,25 @@ export DISABLE_ARM_SSP_PER_TASK_PLUGIN
> GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y))
> # The sancov_plugin.so is included via CFLAGS_KCOV, so it is removed here.
> GCC_PLUGINS_CFLAGS := $(filter-out %/sancov_plugin.so, $(GCC_PLUGINS_CFLAGS))
> +
> +# Out-of-tree module check: If there's a GCC version mismatch, disable plugins
> +# and print a warning. Otherwise the OOT module build will fail due to
> +# plugin_default_version_check().
> +ifneq ($(GCC_PLUGINS_CFLAGS),)
> + ifneq ($(KBUILD_EXTMOD),)
> + ifneq ($(CONFIG_GCC_VERSION), $(shell $(srctree)/scripts/gcc-version.sh $(HOSTCXX)))
> +
> + ifdef CONFIG_GCC_PLUGIN_RANDSTRUCT
> + $(error error: CONFIG_GCC_PLUGIN_RANDSTRUCT requires out-of-tree modules to be built using the same GCC version as the kernel.)
> + endif
> +
> + $(warning warning: Disabling GCC plugins for out-of-tree modules due to GCC version mismatch.)
> + $(warning warning: The following plugins have been disabled: $(gcc-plugin-y))
> + GCC_PLUGINS_CFLAGS :=
> + endif
> + endif
> +endif
> +
> export GCC_PLUGINS_CFLAGS
>
> # Add the flags to the build!
> diff --git a/scripts/Makefile.kcov b/scripts/Makefile.kcov
> index 67e8cfe3474b..63a2bc2aabb2 100644
> --- a/scripts/Makefile.kcov
> +++ b/scripts/Makefile.kcov
> @@ -3,4 +3,15 @@ kcov-flags-$(CONFIG_CC_HAS_SANCOV_TRACE_PC) += -fsanitize-coverage=trace-pc
> kcov-flags-$(CONFIG_KCOV_ENABLE_COMPARISONS) += -fsanitize-coverage=trace-cmp
> kcov-flags-$(CONFIG_GCC_PLUGIN_SANCOV) += -fplugin=$(objtree)/scripts/gcc-plugins/sancov_plugin.so
>
> +# Out-of-tree module check for GCC version mismatch.
> +# See the similar check in scripts/Makefile.gcc-plugins
> +ifneq ($(CONFIG_GCC_PLUGIN_SANCOV),)
> + ifneq ($(KBUILD_EXTMOD),)
> + ifneq ($(CONFIG_GCC_VERSION), $(shell $(srctree)/scripts/gcc-version.sh $(HOSTCXX)))
> + $(warning warning: Disabling CONFIG_GCC_PLUGIN_SANCOV for out-of-tree modules due to GCC version mismatch.)
> + kcov-flags-y := $(filter-out %/sancov_plugin.so, $(kcov-flags-y))
> + endif
> + endif
> +endif
> +
> export CFLAGS_KCOV := $(kcov-flags-y)
> --
> 2.29.2
>

--
Best Regards
Masahiro Yamada