diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index f917b4c4c943..44627d4010f2 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -1317,7 +1317,7 @@ static thread_state thread_state_get(pid_t tid)
 		p = strstr(tmp, header);
 		if (p) {
 			/* For example, "State:\tD (disk sleep)" */
-			c = *(p + sizeof(header) - 1);
+			c = *(p + strlen(header));
 			return c == 'D' ?
 			    THR_STATE_UNINTERRUPTIBLE : THR_STATE_UNKNOWN;
 		}