[PATCH v4 27/31] kcov: test compiler capability in Kconfig and correct dependency

[Masahiro Yamada]

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@xxxxxxxxxxxxx>
---

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