alpine: sigaltstack returns EINVAL if SS_ONSTACK is set

Actually this flag is ignored by kernel.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm/static/sigaltstack.c

@@ -64,7 +64,7 @@ static void *thread_func(void *arg)
 	sas_state[SAS_THRD_OLD] = (stack_t) {
 		.ss_size	= sizeof(stack_thread) - 8,
 		.ss_sp		= stack_thread,
-		.ss_flags	= SS_ONSTACK,
+		.ss_flags	= 0,
 	};
 
 	struct sigaction sa = {
@@ -106,7 +106,7 @@ int main(int argc, char *argv[])
 	sas_state[SAS_MAIN_OLD] = (stack_t) {
 		.ss_size	= sizeof(stack_main) - 8,
 		.ss_sp		= stack_main,
-		.ss_flags	= SS_ONSTACK,
+		.ss_flags	= 0,
 	};
 
 	struct sigaction sa = {
@@ -151,6 +151,9 @@ int main(int argc, char *argv[])
 	}
 	task_waiter_fini(&t);
 
+	sas_state[SAS_THRD_OLD].ss_flags = SS_ONSTACK;
+	sas_state[SAS_MAIN_OLD].ss_flags = SS_ONSTACK;
+
 	show_ss("main old", &sas_state[SAS_MAIN_OLD]);
 	show_ss("main new", &sas_state[SAS_MAIN_NEW]);
 	show_ss("thrd old", &sas_state[SAS_THRD_OLD]);