kcov: allow more fine-grained coverage instrumentation

For more targeted fuzzing, it's better to disable kernel-wide
instrumentation and instead enable it on a per-subsystem basis.  This
follows the pattern of UBSAN and allows you to compile in the kcov
driver without instrumenting the whole kernel.

To instrument a part of the kernel, you can use either

    # for a single file in the current directory
    KCOV_INSTRUMENT_filename.o := y

or

    # for all the files in the current directory (excluding subdirectories)
    KCOV_INSTRUMENT := y

or

    # (same as above)
    ccflags-y += $(CFLAGS_KCOV)

or

    # for all the files in the current directory (including subdirectories)
    subdir-ccflags-y += $(CFLAGS_KCOV)

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Vegard Nossum <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Quentin Casasnovas <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

lib/Kconfig.debug

@@ -719,6 +719,17 @@ config KCOV
 
 	  For more details, see Documentation/kcov.txt.
 
+config KCOV_INSTRUMENT_ALL
+	bool "Instrument all code by default"
+	depends on KCOV
+	default y if KCOV
+	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
+	  say y here. If you are doing more targeted fuzzing (like e.g.
+	  filesystem fuzzing with AFL) then you will want to enable coverage
+	  for more specific subsets of files, and should say n here.
+
 config DEBUG_SHIRQ
 	bool "Debug shared IRQ handlers"
 	depends on DEBUG_KERNEL

scripts/Makefile.lib

@@ -138,7 +138,7 @@ endif
 
 ifeq ($(CONFIG_KCOV),y)
 _c_flags += $(if $(patsubst n%,, \
-	$(KCOV_INSTRUMENT_$(basetarget).o)$(KCOV_INSTRUMENT)y), \
+	$(KCOV_INSTRUMENT_$(basetarget).o)$(KCOV_INSTRUMENT)$(CONFIG_KCOV_INSTRUMENT_ALL)), \
 	$(CFLAGS_KCOV))
 endif