kcsan: Pass option tsan-instrument-read-before-write to Clang

Clang (unlike GCC) removes reads before writes with matching addresses
in the same basic block. This is an optimization for TSAN, since writes
will always cause conflict if the preceding read would have.

However, for KCSAN we cannot rely on this option, because we apply
several special rules to writes, in particular when the
KCSAN_ASSUME_PLAIN_WRITES_ATOMIC option is selected. To avoid missing
potential data races, pass the -tsan-instrument-read-before-write option
to Clang if it is available [1].

[1] llvm/llvm-project@151ed6a

Signed-off-by: Marco Elver <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Will Deacon <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: melver

scripts/Makefile.kcsan

@@ -13,6 +13,7 @@ endif
 # of some options does not break KCSAN nor causes false positive reports.
 CFLAGS_KCSAN := -fsanitize=thread \
 	$(call cc-option,$(call cc-param,tsan-instrument-func-entry-exit=0) -fno-optimize-sibling-calls) \
+	$(call cc-option,$(call cc-param,tsan-instrument-read-before-write=1)) \
 	$(call cc-param,tsan-distinguish-volatile=1)
 
 endif # CONFIG_KCSAN