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Subject[PATCH v4 27/31] kcov: test compiler capability in Kconfig and correct dependency
Date
As Documentation/kbuild/kconfig-language.txt notes, 'select' should be
be used with care - it forces a lower limit of another symbol, ignoring
the dependency. Currently, KCOV can select GCC_PLUGINS even if arch
does not select HAVE_GCC_PLUGINS. This could cause the unmet direct
dependency.

Now that Kconfig can test compiler capability, let's handle this in a
more sophisticated way.

There are two ways to enable KCOV; use the compiler that natively
supports -fsanitize-coverage=trace-pc, or build the SANCOV plugin if
the compiler has ability to build GCC plugins. Hence, the correct
dependency for KCOV is:

depends on CC_HAS_SANCOV_TRACE_PC || GCC_PLUGINS

You do not need to build the SANCOV plugin if the compiler already
supports -fsanitize-coverage=trace-pc. Hence, the select should be:

select GCC_PLUGIN_SANCOV if !CC_HAS_SANCOV_TRACE_PC

With this, GCC_PLUGIN_SANCOV is selected only when necessary, so
scripts/Makefile.gcc-plugins can be cleaner.

I also cleaned up Kconfig and scripts/Makefile.kcov as well.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
---

Changes in v4: None
Changes in v3:
- Replace the previous 'select -> imply' patch with
a new approach

Changes in v2:
- Drop depends on GCC_VERSION

Makefile | 2 +-
lib/Kconfig.debug | 11 +++++++----
scripts/Makefile.gcc-plugins | 6 +-----
scripts/Makefile.kcov | 10 ++++++----
4 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/Makefile b/Makefile
index 12c222d..b5c1301 100644
--- a/Makefile
+++ b/Makefile
@@ -623,7 +623,7 @@ all: vmlinux
KBUILD_CFLAGS += $(call cc-option,-fno-PIE)
KBUILD_AFLAGS += $(call cc-option,-fno-PIE)
CFLAGS_GCOV := -fprofile-arcs -ftest-coverage -fno-tree-loop-im $(call cc-disable-warning,maybe-uninitialized,)
-export CFLAGS_GCOV CFLAGS_KCOV
+export CFLAGS_GCOV

# The arch Makefile can set ARCH_{CPP,A,C}FLAGS to override the default
# values of the respective KBUILD_* variables
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index c40c7b7..bba2298 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -736,12 +736,15 @@ config ARCH_HAS_KCOV
only for x86_64. KCOV requires testing on other archs, and most likely
disabling of instrumentation for some early boot code.

+config CC_HAS_SANCOV_TRACE_PC
+ def_bool $(cc-option, -fsanitize-coverage=trace-pc)
+
config KCOV
bool "Code coverage for fuzzing"
depends on ARCH_HAS_KCOV
+ depends on CC_HAS_SANCOV_TRACE_PC || GCC_PLUGINS
select DEBUG_FS
- select GCC_PLUGINS if !COMPILE_TEST
- select GCC_PLUGIN_SANCOV if !COMPILE_TEST
+ select GCC_PLUGIN_SANCOV if !CC_HAS_SANCOV_TRACE_PC
help
KCOV exposes kernel code coverage information in a form suitable
for coverage-guided fuzzing (randomized testing).
@@ -755,7 +758,7 @@ config KCOV
config KCOV_ENABLE_COMPARISONS
bool "Enable comparison operands collection by KCOV"
depends on KCOV
- default n
+ depends on $(cc-option, -fsanitize-coverage=trace-cmp)
help
KCOV also exposes operands of every comparison in the instrumented
code along with operand sizes and PCs of the comparison instructions.
@@ -765,7 +768,7 @@ config KCOV_ENABLE_COMPARISONS
config KCOV_INSTRUMENT_ALL
bool "Instrument all code by default"
depends on KCOV
- default y if KCOV
+ default y
help
If you are doing generic system call fuzzing (like e.g. syzkaller),
then you will want to instrument the whole kernel and you should
diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
index 7f5c862..0ce3802 100644
--- a/scripts/Makefile.gcc-plugins
+++ b/scripts/Makefile.gcc-plugins
@@ -14,16 +14,12 @@ ifdef CONFIG_GCC_PLUGINS
endif

ifdef CONFIG_GCC_PLUGIN_SANCOV
- ifeq ($(strip $(CFLAGS_KCOV)),)
# It is needed because of the gcc-plugin.sh and gcc version checks.
gcc-plugin-$(CONFIG_GCC_PLUGIN_SANCOV) += sancov_plugin.so

- ifneq ($(PLUGINCC),)
- CFLAGS_KCOV := $(SANCOV_PLUGIN)
- else
+ ifeq ($(PLUGINCC),)
$(warning warning: cannot use CONFIG_KCOV: -fsanitize-coverage=trace-pc is not supported by compiler)
endif
- endif
endif

gcc-plugin-$(CONFIG_GCC_PLUGIN_STRUCTLEAK) += structleak_plugin.so
diff --git a/scripts/Makefile.kcov b/scripts/Makefile.kcov
index 5cc7203..d71ba73 100644
--- a/scripts/Makefile.kcov
+++ b/scripts/Makefile.kcov
@@ -1,7 +1,9 @@
ifdef CONFIG_KCOV
-CFLAGS_KCOV := $(call cc-option,-fsanitize-coverage=trace-pc,)
-ifeq ($(CONFIG_KCOV_ENABLE_COMPARISONS),y)
-CFLAGS_KCOV += $(call cc-option,-fsanitize-coverage=trace-cmp,)
-endif
+
+kcov-flags-$(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_SANKOV) += -fplugin=$(objtree)/scripts/gcc-plugins/sancov_plugin.so
+
+export CFLAGS_KCOV := $(kcov-flags-y)

endif
--
2.7.4
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