Date:   Tue, 26 Jan 2021 06:16:01 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     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>,
        Justin Forbes <jforbes@...hat.com>,
        Ondrej Mosnacek <omosnace@...hat.com>
Subject: Re: [PATCH RFC] gcc-plugins: Handle GCC version mismatch for OOT modules

On Tue, Jan 26, 2021 at 5:42 AM Josh Poimboeuf <jpoimboe@...hat.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@...hat.com>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.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

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