kcsan: Introduce kcsan_value_change type

Introduces kcsan_value_change type, which explicitly points out if we
either observed a value-change (TRUE), or we could not observe one but
cannot rule out a value-change happened (MAYBE). The MAYBE state can
either be reported or not, depending on configuration preferences.

A follow-up patch introduces the FALSE state, which should never be
reported.

No functional change intended.

Acked-by: John Hubbard <jhubbard@nvidia.com>
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>

1 files changed, 22 insertions, 16 deletions

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 498b1eb3c1cd..3f89801161d3 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -341,7 +341,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 		u32 _4;
 		u64 _8;
 	} expect_value;
-	bool value_change = false;
+	enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
 	unsigned long ua_flags = user_access_save();
 	unsigned long irq_flags;
 
@@ -398,6 +398,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	 * Read the current value, to later check and infer a race if the data
 	 * was modified via a non-instrumented access, e.g. from a device.
 	 */
+	expect_value._8 = 0;
 	switch (size) {
 	case 1:
 		expect_value._1 = READ_ONCE(*(const u8 *)ptr);
@@ -436,24 +437,37 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 	 */
 	switch (size) {
 	case 1:
-		value_change = expect_value._1 != READ_ONCE(*(const u8 *)ptr);
+		expect_value._1 ^= READ_ONCE(*(const u8 *)ptr);
 		break;
 	case 2:
-		value_change = expect_value._2 != READ_ONCE(*(const u16 *)ptr);
+		expect_value._2 ^= READ_ONCE(*(const u16 *)ptr);
 		break;
 	case 4:
-		value_change = expect_value._4 != READ_ONCE(*(const u32 *)ptr);
+		expect_value._4 ^= READ_ONCE(*(const u32 *)ptr);
 		break;
 	case 8:
-		value_change = expect_value._8 != READ_ONCE(*(const u64 *)ptr);
+		expect_value._8 ^= READ_ONCE(*(const u64 *)ptr);
 		break;
 	default:
 		break; /* ignore; we do not diff the values */
 	}
 
+	/* Were we able to observe a value-change? */
+	if (expect_value._8 != 0)
+		value_change = KCSAN_VALUE_CHANGE_TRUE;
+
 	/* Check if this access raced with another. */
 	if (!remove_watchpoint(watchpoint)) {
 		/*
+		 * Depending on the access type, map a value_change of MAYBE to
+		 * TRUE (require reporting).
+		 */
+		if (value_change == KCSAN_VALUE_CHANGE_MAYBE && (size > 8 || is_assert)) {
+			/* Always assume a value-change. */
+			value_change = KCSAN_VALUE_CHANGE_TRUE;
+		}
+
+		/*
 		 * No need to increment 'data_races' counter, as the racing
 		 * thread already did.
 		 *
@@ -461,20 +475,12 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 		 * therefore both this thread and the racing thread may
 		 * increment this counter.
 		 */
-		if (is_assert)
+		if (is_assert && value_change == KCSAN_VALUE_CHANGE_TRUE)
 			kcsan_counter_inc(KCSAN_COUNTER_ASSERT_FAILURES);
 
-		/*
-		 * - If we were not able to observe a value change due to size
-		 *   constraints, always assume a value change.
-		 * - If the access type is an assertion, we also always assume a
-		 *   value change to always report the race.
-		 */
-		value_change = value_change || size > 8 || is_assert;
-
 		kcsan_report(ptr, size, type, value_change, smp_processor_id(),
 			     KCSAN_REPORT_RACE_SIGNAL);
-	} else if (value_change) {
+	} else if (value_change == KCSAN_VALUE_CHANGE_TRUE) {
 		/* Inferring a race, since the value should not have changed. */
 
 		kcsan_counter_inc(KCSAN_COUNTER_RACES_UNKNOWN_ORIGIN);
@@ -482,7 +488,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 			kcsan_counter_inc(KCSAN_COUNTER_ASSERT_FAILURES);
 
 		if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN) || is_assert)
-			kcsan_report(ptr, size, type, true,
+			kcsan_report(ptr, size, type, KCSAN_VALUE_CHANGE_TRUE,
 				     smp_processor_id(),
 				     KCSAN_REPORT_RACE_UNKNOWN_ORIGIN);
 	}