kcsan: Support distinguishing volatile accesses

In the kernel, the "volatile" keyword is used in various concurrent
contexts, whether in low-level synchronization primitives or for
legacy reasons. If supported by the compiler, it will be assumed
that aligned volatile accesses up to sizeof(long long) (matching
compiletime_assert_rwonce_type()) are atomic.

Recent versions of Clang [1] (GCC tentative [2]) can instrument
volatile accesses differently. Add the option (required) to enable the
instrumentation, and provide the necessary runtime functions. None of
the updated compilers are widely available yet (Clang 11 will be the
first release to support the feature).

[1] https://github.com/llvm/llvm-project/commit/5a2c31116f412c3b6888be361137efd705e05814
[2] https://gcc.gnu.org/pipermail/gcc-patches/2020-April/544452.html

This change allows removing of any explicit checks in primitives such as
READ_ONCE() and WRITE_ONCE().

 [ bp: Massage commit message a bit. ]
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200521142047.169334-4-elver@google.com

kernel/kcsan/core.c

@@ -789,6 +789,49 @@ void __tsan_write_range(void *ptr, size_t size)
 }
 EXPORT_SYMBOL(__tsan_write_range);
 
+/*
+ * Use of explicit volatile is generally disallowed [1], however, volatile is
+ * still used in various concurrent context, whether in low-level
+ * synchronization primitives or for legacy reasons.
+ * [1] https://lwn.net/Articles/233479/
+ *
+ * We only consider volatile accesses atomic if they are aligned and would pass
+ * the size-check of compiletime_assert_rwonce_type().
+ */
+#define DEFINE_TSAN_VOLATILE_READ_WRITE(size)                                  \
+	void __tsan_volatile_read##size(void *ptr)                             \
+	{                                                                      \
+		const bool is_atomic = size <= sizeof(long long) &&            \
+				       IS_ALIGNED((unsigned long)ptr, size);   \
+		if (IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS) && is_atomic)      \
+			return;                                                \
+		check_access(ptr, size, is_atomic ? KCSAN_ACCESS_ATOMIC : 0);  \
+	}                                                                      \
+	EXPORT_SYMBOL(__tsan_volatile_read##size);                             \
+	void __tsan_unaligned_volatile_read##size(void *ptr)                   \
+		__alias(__tsan_volatile_read##size);                           \
+	EXPORT_SYMBOL(__tsan_unaligned_volatile_read##size);                   \
+	void __tsan_volatile_write##size(void *ptr)                            \
+	{                                                                      \
+		const bool is_atomic = size <= sizeof(long long) &&            \
+				       IS_ALIGNED((unsigned long)ptr, size);   \
+		if (IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS) && is_atomic)      \
+			return;                                                \
+		check_access(ptr, size,                                        \
+			     KCSAN_ACCESS_WRITE |                              \
+				     (is_atomic ? KCSAN_ACCESS_ATOMIC : 0));   \
+	}                                                                      \
+	EXPORT_SYMBOL(__tsan_volatile_write##size);                            \
+	void __tsan_unaligned_volatile_write##size(void *ptr)                  \
+		__alias(__tsan_volatile_write##size);                          \
+	EXPORT_SYMBOL(__tsan_unaligned_volatile_write##size)
+
+DEFINE_TSAN_VOLATILE_READ_WRITE(1);
+DEFINE_TSAN_VOLATILE_READ_WRITE(2);
+DEFINE_TSAN_VOLATILE_READ_WRITE(4);
+DEFINE_TSAN_VOLATILE_READ_WRITE(8);
+DEFINE_TSAN_VOLATILE_READ_WRITE(16);
+
 /*
  * The below are not required by KCSAN, but can still be emitted by the
  * compiler.

scripts/Makefile.kcsan

@@ -9,7 +9,10 @@ else
 cc-param = --param -$(1)
 endif
 
+# Keep most options here optional, to allow enabling more compilers if absence
+# 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-func-entry-exit=0) -fno-optimize-sibling-calls) \
+	$(call cc-param,tsan-distinguish-volatile=1)
 
 endif # CONFIG_KCSAN