[PATCH -tip 03/10] kcsan: Support distinguishing volatile accesses

[Marco Elver]

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

Recent versions 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 patch allows removing any explicit checks in primitives such as
READ_ONCE() and WRITE_ONCE().

Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
 kernel/kcsan/core.c    | 43 ++++++++++++++++++++++++++++++++++++++++++
 scripts/Makefile.kcsan |  5 ++++-
 2 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index a73a66cf79df..15f67949d11e 100644
--- a/kernel/kcsan/core.c
+++ b/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.
diff --git a/scripts/Makefile.kcsan b/scripts/Makefile.kcsan
index 20337a7ecf54..c02662b30a7c 100644
--- a/scripts/Makefile.kcsan
+++ b/scripts/Makefile.kcsan
@@ -9,7 +9,10 @@ else
 cc-param = --param -$(1)
 endif
 
+# Most options here should be kept optional, to allow enabling more compilers
+# if the absence of some options still allows us to use KCSAN in most cases.
 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
-- 
2.26.2.761.g0e0b3e54be-goog