kcsan: Add core support for a subset of weak memory modeling

Add support for modeling a subset of weak memory, which will enable
detection of a subset of data races due to missing memory barriers.

KCSAN's approach to detecting missing memory barriers is based on
modeling access reordering, and enabled if `CONFIG_KCSAN_WEAK_MEMORY=y`,
which depends on `CONFIG_KCSAN_STRICT=y`. The feature can be enabled or
disabled at boot and runtime via the `kcsan.weak_memory` boot parameter.

Each memory access for which a watchpoint is set up, is also selected
for simulated reordering within the scope of its function (at most 1
in-flight access).

We are limited to modeling the effects of "buffering" (delaying the
access), since the runtime cannot "prefetch" accesses (therefore no
acquire modeling). Once an access has been selected for reordering, it
is checked along every other access until the end of the function scope.
If an appropriate memory barrier is encountered, the access will no
longer be considered for reordering.

When the result of a memory operation should be ordered by a barrier,
KCSAN can then detect data races where the conflict only occurs as a
result of a missing barrier due to reordering accesses.

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

1 files changed, 9 insertions, 1 deletions

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index 5f5965246877..a1c6a89fde71 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -99,7 +99,15 @@ void kcsan_set_access_mask(unsigned long mask);
 
 /* Scoped access information. */
 struct kcsan_scoped_access {
-	struct list_head list;
+	union {
+		struct list_head list; /* scoped_accesses list */
+		/*
+		 * Not an entry in scoped_accesses list; stack depth from where
+		 * the access was initialized.
+		 */
+		int stack_depth;
+	};
+
 	/* Access information. */
 	const volatile void *ptr;
 	size_t size;