kcsan: Fix 0-sized checks

Instrumentation of arbitrary memory-copy functions, such as user-copies,
may be called with size of 0, which could lead to false positives.

To avoid this, add a comparison in check_access() for size==0, which
will be optimized out for constant sized instrumentation
(__tsan_{read,write}N), and therefore not affect the common-case
fast-path.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/kcsan/core.c

@@ -455,6 +455,13 @@ static __always_inline void check_access(const volatile void *ptr, size_t size,
 	atomic_long_t *watchpoint;
 	long encoded_watchpoint;
 
+	/*
+	 * Do nothing for 0 sized check; this comparison will be optimized out
+	 * for constant sized instrumentation (__tsan_{read,write}N).
+	 */
+	if (unlikely(size == 0))
+		return;
+
 	/*
 	 * Avoid user_access_save in fast-path: find_watchpoint is safe without
 	 * user_access_save, as the address that ptr points to is only used to

kernel/kcsan/test.c

@@ -92,6 +92,16 @@ static bool test_matching_access(void)
 		return false;
 	if (WARN_ON(matching_access(9, 1, 10, 1)))
 		return false;
+
+	/*
+	 * An access of size 0 could match another access, as demonstrated here.
+	 * Rather than add more comparisons to 'matching_access()', which would
+	 * end up in the fast-path for *all* checks, check_access() simply
+	 * returns for all accesses of size 0.
+	 */
+	if (WARN_ON(!matching_access(8, 8, 12, 0)))
+		return false;
+
 	return true;
 }