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

Josh Poimboeuf
2021-01-25T20:55:47Z

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>
---
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