[PATCH v3 2/3] Makefile: support flag -fsanitizer-coverage=trace-cmp

[Alexander Potapenko]

From: Victor Chibotaru <tchibo@xxxxxxxxxx>

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.

Signed-off-by: Victor Chibotaru <tchibo@xxxxxxxxxx>
Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Alexander Popov <alex.popov@xxxxxxxxx>
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
Cc: Quentin Casasnovas <quentin.casasnovas@xxxxxxxxxx>
Cc: syzkaller@xxxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
Clang instrumentation:
https://clang.llvm.org/docs/SanitizerCoverage.html#tracing-data-flow

v3: - Andrey Ryabinin's comments: reinstated scripts/Makefile.kcov
      and moved CFLAGS_KCOV there, dropped CFLAGS_KCOV_COMPS
    - allow building with GCC
v2: - updated KCOV_ENABLE_COMPARISONS description
---
 Makefile              |  3 +--
 lib/Kconfig.debug     | 10 ++++++++++
 scripts/Makefile.kcov |  7 +++++++
 scripts/Makefile.lib  |  6 ++++++
 4 files changed, 24 insertions(+), 2 deletions(-)
 create mode 100644 scripts/Makefile.kcov

diff --git a/Makefile b/Makefile
index 2835863bdd5a..43f642167d68 100644
--- a/Makefile
+++ b/Makefile
@@ -373,8 +373,6 @@ 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    := \
@@ -657,6 +655,7 @@ 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 2689b7c50c52..a10eb4e34719 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -759,6 +759,16 @@ 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
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 5e975fee0f5b..7ddd5932c832 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -142,6 +142,12 @@ _c_flags += $(if $(patsubst n%,, \
 	$(CFLAGS_KCOV))
 endif
 
+ifeq ($(CONFIG_KCOV_ENABLE_COMPARISONS),y)
+_c_flags += $(if $(patsubst n%,, \
+	$(KCOV_INSTRUMENT_$(basetarget).o)$(KCOV_INSTRUMENT)$(CONFIG_KCOV_INSTRUMENT_ALL)), \
+	$(CFLAGS_KCOV_COMPS))
+endif
+
 # If building the kernel in a separate objtree expand all occurrences
 # of -Idir to -I$(srctree)/dir except for absolute paths (starting with '/').
 
-- 
2.14.2.920.gcf0c67979c-goog