kcsan: Address missing case with KCSAN_REPORT_VALUE_CHANGE_ONLY

Even with KCSAN_REPORT_VALUE_CHANGE_ONLY, KCSAN still reports data
races between reads and watchpointed writes, even if the writes wrote
values already present.  This commit causes KCSAN to unconditionally
skip reporting in this case.
Signed-off-by: NMarco Elver <elver@google.com>
Signed-off-by: NPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: NIngo Molnar <mingo@kernel.org>

kernel/kcsan/report.c

@@ -130,12 +130,25 @@ static bool rate_limit_report(unsigned long frame1, unsigned long frame2)
  * Special rules to skip reporting.
  */
 static bool
-skip_report(int access_type, bool value_change, unsigned long top_frame)
+skip_report(bool value_change, unsigned long top_frame)
 {
-	const bool is_write = (access_type & KCSAN_ACCESS_WRITE) != 0;
-
-	if (IS_ENABLED(CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY) && is_write &&
-	    !value_change) {
+	/*
+	 * The first call to skip_report always has value_change==true, since we
+	 * cannot know the value written of an instrumented access. For the 2nd
+	 * call there are 6 cases with CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY:
+	 *
+	 * 1. read watchpoint, conflicting write (value_change==true): report;
+	 * 2. read watchpoint, conflicting write (value_change==false): skip;
+	 * 3. write watchpoint, conflicting write (value_change==true): report;
+	 * 4. write watchpoint, conflicting write (value_change==false): skip;
+	 * 5. write watchpoint, conflicting read (value_change==false): skip;
+	 * 6. write watchpoint, conflicting read (value_change==true): impossible;
+	 *
+	 * Cases 1-4 are intuitive and expected; case 5 ensures we do not report
+	 * data races where the write may have rewritten the same value; and
+	 * case 6 is simply impossible.
+	 */
+	if (IS_ENABLED(CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY) && !value_change) {
 		/*
 		 * The access is a write, but the data value did not change.
 		 *
@@ -228,7 +241,7 @@ static bool print_report(const volatile void *ptr, size_t size, int access_type,
 	/*
 	 * Must check report filter rules before starting to print.
 	 */
-	if (skip_report(access_type, true, stack_entries[skipnr]))
+	if (skip_report(true, stack_entries[skipnr]))
 		return false;
 
 	if (type == KCSAN_REPORT_RACE_SIGNAL) {
@@ -237,7 +250,7 @@ static bool print_report(const volatile void *ptr, size_t size, int access_type,
 		other_frame = other_info.stack_entries[other_skipnr];
 
 		/* @value_change is only known for the other thread */
-		if (skip_report(other_info.access_type, value_change, other_frame))
+		if (skip_report(value_change, other_frame))
 			return false;
 	}