Makefile: support flag -fsanitizer-coverage=trace-cmp

The flag enables Clang instrumentation of comparison operations
(currently not supported by GCC).  This instrumentation is needed by the
new KCOV device to collect comparison operands.

Link: http://lkml.kernel.org/r/20171011095459.70721-2-glider@google.com
Signed-off-by: Victor Chibotaru <tchibo@google.com>
Signed-off-by: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Popov <alex.popov@linux.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: <syzkaller@googlegroups.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 18 insertions, 2 deletions

diff --git a/Makefile b/Makefile
index 763ab35df12a..ccb7d5b2fbf5 100644
--- a/Makefile
+++ b/Makefile
@@ -375,8 +375,6 @@ CFLAGS_KERNEL	=
 AFLAGS_KERNEL   =
 LDFLAGS_vmlinux =
 CFLAGS_GCOV     := -fprofile-arcs -ftest-coverage -fno-tree-loop-im $(call cc-disable-warning,maybe-uninitialized,)
-CFLAGS_KCOV     := $(call cc-option,-fsanitize-coverage=trace-pc,)
-
 
 # Use USERINCLUDE when you must reference the UAPI directories only.
 USERINCLUDE    := \
@@ -659,6 +657,7 @@ ifeq ($(shell $(CONFIG_SHELL) $(srctree)/scripts/gcc-goto.sh $(CC) $(KBUILD_CFLA
         KBUILD_AFLAGS += -DCC_HAVE_ASM_GOTO
 endif
 
+include scripts/Makefile.kcov
 include scripts/Makefile.gcc-plugins
 
 ifdef CONFIG_READABLE_ASM
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index ce813731269f..947d3e2ed5c2 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -756,6 +756,16 @@ config KCOV
 
           For more details, see Documentation/dev-tools/kcov.rst.
 
+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.
+          These operands can be used by fuzzing engines to improve the quality
+          of fuzzing coverage.
+
 config KCOV_INSTRUMENT_ALL
         bool "Instrument all code by default"
         depends on KCOV
diff --git a/scripts/Makefile.kcov b/scripts/Makefile.kcov
new file mode 100644
index 000000000000..5cc72037e423
--- /dev/null
+++ b/scripts/Makefile.kcov
@@ -0,0 +1,7 @@
+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
+
+endif