selftests/bpf: Add testcase for async callback return value failure

A previous commit updated the verifier to print an accurate failure
message for when someone specifies a nonzero return value from an async
callback. This adds a testcase for validating that the verifier emits
the correct message in such a case.

Signed-off-by: David Vernet <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/prog_tests/timer.c

···
 /* Copyright (c) 2021 Facebook */
 #include <test_progs.h>
 #include "timer.skel.h"
+#include "timer_failure.skel.h"
 
 static int timer(struct timer *timer_skel)
 {
···
 
 	timer_skel = timer__open_and_load();
 	if (!ASSERT_OK_PTR(timer_skel, "timer_skel_load"))
-		goto cleanup;
+		return;
 
 	err = timer(timer_skel);
 	ASSERT_OK(err, "timer");
-cleanup:
 	timer__destroy(timer_skel);
+
+	RUN_TESTS(timer_failure);
 }

tools/testing/selftests/bpf/progs/timer_failure.c

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
+
+#include <linux/bpf.h>
+#include <time.h>
+#include <errno.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+#include "bpf_tcp_helpers.h"
+
+char _license[] SEC("license") = "GPL";
+
+struct elem {
+	struct bpf_timer t;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, struct elem);
+} timer_map SEC(".maps");
+
+static int timer_cb_ret1(void *map, int *key, struct bpf_timer *timer)
+{
+	if (bpf_get_smp_processor_id() % 2)
+		return 1;
+	else
+		return 0;
+}
+
+SEC("fentry/bpf_fentry_test1")
+__failure __msg("should have been in (0x0; 0x0)")
+int BPF_PROG2(test_ret_1, int, a)
+{
+	int key = 0;
+	struct bpf_timer *timer;
+
+	timer = bpf_map_lookup_elem(&timer_map, &key);
+	if (timer) {
+		bpf_timer_init(timer, &timer_map, CLOCK_BOOTTIME);
+		bpf_timer_set_callback(timer, timer_cb_ret1);
+		bpf_timer_start(timer, 1000, 0);
+	}
+
+	return 0;
+}