build, kcsan: Add KCSAN build exceptions

This blacklists several compilation units from KCSAN. See the respective
inline comments for the reasoning.

Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

drivers/firmware/efi/libstub/Makefile

@@ -31,7 +31,9 @@ KBUILD_CFLAGS			:= $(cflags-y) -DDISABLE_BRANCH_PROFILING \
 				   -D__DISABLE_EXPORTS
 
 GCOV_PROFILE			:= n
+# Sanitizer runtimes are unavailable and cannot be linked here.
 KASAN_SANITIZE			:= n
+KCSAN_SANITIZE			:= n
 UBSAN_SANITIZE			:= n
 OBJECT_FILES_NON_STANDARD	:= y
 

kernel/Makefile

@@ -23,6 +23,9 @@ endif
 # Prevents flicker of uninteresting __do_softirq()/__local_bh_disable_ip()
 # in coverage traces.
 KCOV_INSTRUMENT_softirq.o := n
+# Avoid KCSAN instrumentation in softirq ("No shared variables, all the data
+# are CPU local" => assume no data races), to reduce overhead in interrupts.
+KCSAN_SANITIZE_softirq.o = n
 # These are called from save_stack_trace() on slub debug path,
 # and produce insane amounts of uninteresting coverage.
 KCOV_INSTRUMENT_module.o := n
@@ -30,6 +33,7 @@ KCOV_INSTRUMENT_extable.o := n
 # Don't self-instrument.
 KCOV_INSTRUMENT_kcov.o := n
 KASAN_SANITIZE_kcov.o := n
+KCSAN_SANITIZE_kcov.o := n
 CFLAGS_kcov.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector)
 
 # cond_syscall is currently not LTO compatible
@@ -118,6 +122,7 @@ obj-$(CONFIG_RSEQ) += rseq.o
 
 obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += stackleak.o
 KASAN_SANITIZE_stackleak.o := n
+KCSAN_SANITIZE_stackleak.o := n
 KCOV_INSTRUMENT_stackleak.o := n
 
 $(obj)/configs.o: $(obj)/config_data.gz

kernel/sched/Makefile

@@ -7,6 +7,12 @@ endif
 # that is not a function of syscall inputs. E.g. involuntary context switches.
 KCOV_INSTRUMENT := n
 
+# There are numerous races here, however, most of them due to plain accesses.
+# This would make it even harder for syzbot to find reproducers, because these
+# bugs trigger without specific input. Disable by default, but should re-enable
+# eventually.
+KCSAN_SANITIZE := n
+
 ifneq ($(CONFIG_SCHED_OMIT_FRAME_POINTER),y)
 # According to Alan Modra <alan@linuxcare.com.au>, the -fno-omit-frame-pointer is
 # needed for x86 only.  Why this used to be enabled for all architectures is beyond

mm/Makefile

@@ -7,6 +7,14 @@ KASAN_SANITIZE_slab_common.o := n
 KASAN_SANITIZE_slab.o := n
 KASAN_SANITIZE_slub.o := n
 
+# These produce frequent data race reports: most of them are due to races on
+# the same word but accesses to different bits of that word. Re-enable KCSAN
+# for these when we have more consensus on what to do about them.
+KCSAN_SANITIZE_slab_common.o := n
+KCSAN_SANITIZE_slab.o := n
+KCSAN_SANITIZE_slub.o := n
+KCSAN_SANITIZE_page_alloc.o := n
+
 # These files are disabled because they produce non-interesting and/or
 # flaky coverage that is not a function of syscall inputs. E.g. slab is out of
 # free pages, or a task is migrated between nodes.