kcsan: Fix function matching in report

Pass string length as returned by scnprintf() to strnstr(), since
strnstr() searches exactly len bytes in haystack, even if it contains a
NUL-terminator before haystack+len.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/kcsan/report.c

@@ -192,11 +192,11 @@ skip_report(enum kcsan_value_change value_change, unsigned long top_frame)
 		 * maintainers.
 		 */
 		char buf[64];
+		int len = scnprintf(buf, sizeof(buf), "%ps", (void *)top_frame);
 
-		snprintf(buf, sizeof(buf), "%ps", (void *)top_frame);
-		if (!strnstr(buf, "rcu_", sizeof(buf)) &&
-		    !strnstr(buf, "_rcu", sizeof(buf)) &&
-		    !strnstr(buf, "_srcu", sizeof(buf)))
+		if (!strnstr(buf, "rcu_", len) &&
+		    !strnstr(buf, "_rcu", len) &&
+		    !strnstr(buf, "_srcu", len))
 			return true;
 	}
 
@@ -262,15 +262,15 @@ static const char *get_thread_desc(int task_id)
 static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries)
 {
 	char buf[64];
+	int len;
 	int skip = 0;
 
 	for (; skip < num_entries; ++skip) {
-		snprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skip]);
-		if (!strnstr(buf, "csan_", sizeof(buf)) &&
-		    !strnstr(buf, "tsan_", sizeof(buf)) &&
-		    !strnstr(buf, "_once_size", sizeof(buf))) {
+		len = scnprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skip]);
+		if (!strnstr(buf, "csan_", len) &&
+		    !strnstr(buf, "tsan_", len) &&
+		    !strnstr(buf, "_once_size", len))
 			break;
-		}
 	}
 	return skip;
 }