BACKPORT: kcov: test compiler capability in Kconfig and correct dependency

Work around missing cc-option support in Kconfig by checking required
compiler flags in Makefile.

(Upstream commit 5aadfdeb8de001ca04d500586e3b033404c28617.)

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>
Reviewed-by: Kees Cook <keescook@chromium.org>
Change-Id: Iad9110eb7b6ecef6dfcec38cf483699c1b85af01
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Bug: 147413187

lib/Kconfig.debug

@@ -740,12 +740,17 @@ 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.
 
+# Upstream uses $(cc-option, -fsanitize-coverage=trace-pc), which requires
+# cc-option support. Here we instead check CC in scripts/Makefile.kcov.
+config CC_HAS_SANCOV_TRACE_PC
+	def_bool ARCH_HAS_KCOV
+
 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 !CC_HAS_SANCOV_TRACE_PC
-	select GCC_PLUGIN_SANCOV if !COMPILE_TEST
 	help
 	  KCOV exposes kernel code coverage information in a form suitable
 	  for coverage-guided fuzzing (randomized testing).
@@ -756,10 +761,11 @@ config KCOV
 
 	  For more details, see Documentation/dev-tools/kcov.rst.
 
+# Upstream uses $(cc-option, -fsanitize-coverage=trace-cmp), which requires
+# cc-option support. Here we instead check CC in scripts/Makefile.kcov.
 config KCOV_ENABLE_COMPARISONS
 	bool "Enable comparison operands collection by KCOV"
 	depends on KCOV
-	default n
 	help
 	  KCOV also exposes operands of every comparison in the instrumented
 	  code along with operand sizes and PCs of the comparison instructions.
@@ -769,7 +775,7 @@ 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

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 ($(PLUGINCC),)
+      ifeq ($(PLUGINCC),)
-        CFLAGS_KCOV := $(SANCOV_PLUGIN)
-      else
         $(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
@@ -38,7 +34,7 @@ ifdef CONFIG_GCC_PLUGINS
   GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y))
 
   export PLUGINCC GCC_PLUGINS_CFLAGS GCC_PLUGIN GCC_PLUGIN_SUBDIR
-  export SANCOV_PLUGIN DISABLE_LATENT_ENTROPY_PLUGIN
+  export DISABLE_LATENT_ENTROPY_PLUGIN
 
   ifneq ($(PLUGINCC),)
     # SANCOV_PLUGIN can be only in CFLAGS_KCOV because avoid duplication.

scripts/Makefile.kcov

@@ -1,7 +1,25 @@
 ifdef CONFIG_KCOV
-CFLAGS_KCOV	:= $(call cc-option,-fsanitize-coverage=trace-pc,)
+
-ifeq ($(CONFIG_KCOV_ENABLE_COMPARISONS),y)
+ifeq ($(call cc-option, -fsanitize-coverage=trace-pc -Werror),)
-CFLAGS_KCOV += $(call cc-option,-fsanitize-coverage=trace-cmp,)
+    ifneq ($(CONFIG_COMPILE_TEST),y)
+        $(error Cannot use CONFIG_KCOV: \
+          -fsanitize-coverage=trace-pc is not supported by compiler)
+    endif
 endif
 
+ifdef CONFIG_KCOV_ENABLE_COMPARISONS
+    ifeq ($(call cc-option, -fsanitize-coverage=trace-cmp -Werror),)
+        ifneq ($(CONFIG_COMPILE_TEST),y)
+            $(error Cannot use CONFIG_KCOV_ENABLE_COMPARISONS: \
+              -fsanitize-coverage=trace-cmp is not supported by compiler)
+        endif
+    endif
+endif
+
+kcov-flags-$(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_SANCOV)		+= -fplugin=$(objtree)/scripts/gcc-plugins/sancov_plugin.so
+
+export CFLAGS_KCOV := $(kcov-flags-y)
+
 endif

scripts/gcc-plugins/Makefile

@@ -13,10 +13,6 @@ else
   export HOST_EXTRACXXFLAGS
 endif
 
-ifneq ($(CFLAGS_KCOV), $(SANCOV_PLUGIN))
-  GCC_PLUGIN := $(filter-out $(SANCOV_PLUGIN), $(GCC_PLUGIN))
-endif
-
 export HOSTLIBS
 
 $(obj)/randomize_layout_plugin.o: $(objtree)/$(obj)/randomize_layout_seed.h