selftests/hid: add subprog call test

I got a weird verifier error with a subprog once, so let's have a test
for it.

Link: https://lore.kernel.org/r/20240608-hid_bpf_struct_ops-v3-9-6ac6ade58329@kernel.orgSigned-off-by: Benjamin Tissoires <bentiss@kernel.org>

tools/testing/selftests/hid/hid_bpf.c

@@ -638,6 +638,47 @@ TEST_F(hid_bpf, raw_event)
 	ASSERT_EQ(buf[2], 52);
 }
 
+/*
+ * Attach hid_first_event to the given uhid device,
+ * retrieve and open the matching hidraw node,
+ * inject one event in the uhid device,
+ * check that the program sees it and can change the data
+ */
+TEST_F(hid_bpf, subprog_raw_event)
+{
+	const struct test_program progs[] = {
+		{ .name = "hid_subprog_first_event" },
+	};
+	__u8 buf[10] = {0};
+	int err;
+
+	LOAD_PROGRAMS(progs);
+
+	/* inject one event */
+	buf[0] = 1;
+	buf[1] = 42;
+	uhid_send_event(_metadata, self->uhid_fd, buf, 6);
+
+	/* read the data from hidraw */
+	memset(buf, 0, sizeof(buf));
+	err = read(self->hidraw_fd, buf, sizeof(buf));
+	ASSERT_EQ(err, 6) TH_LOG("read_hidraw");
+	ASSERT_EQ(buf[0], 1);
+	ASSERT_EQ(buf[2], 47);
+
+	/* inject another event */
+	memset(buf, 0, sizeof(buf));
+	buf[0] = 1;
+	buf[1] = 47;
+	uhid_send_event(_metadata, self->uhid_fd, buf, 6);
+
+	/* read the data from hidraw */
+	memset(buf, 0, sizeof(buf));
+	err = read(self->hidraw_fd, buf, sizeof(buf));
+	ASSERT_EQ(err, 6) TH_LOG("read_hidraw");
+	ASSERT_EQ(buf[2], 52);
+}
+
 /*
  * Ensures that we can attach/detach programs
  */

tools/testing/selftests/hid/progs/hid.c

@@ -35,6 +35,30 @@ struct hid_bpf_ops first_event = {
 	.hid_id = 2,
 };
 
+int __hid_subprog_first_event(struct hid_bpf_ctx *hid_ctx, enum hid_report_type type)
+{
+	__u8 *rw_data = hid_bpf_get_data(hid_ctx, 0 /* offset */, 3 /* size */);
+
+	if (!rw_data)
+		return 0; /* EPERM check */
+
+	rw_data[2] = rw_data[1] + 5;
+
+	return hid_ctx->size;
+}
+
+SEC("?struct_ops/hid_device_event")
+int BPF_PROG(hid_subprog_first_event, struct hid_bpf_ctx *hid_ctx, enum hid_report_type type)
+{
+	return __hid_subprog_first_event(hid_ctx, type);
+}
+
+SEC(".struct_ops.link")
+struct hid_bpf_ops subprog_first_event = {
+	.hid_device_event = (void *)hid_subprog_first_event,
+	.hid_id = 2,
+};
+
 SEC("?struct_ops/hid_device_event")
 int BPF_PROG(hid_second_event, struct hid_bpf_ctx *hid_ctx, enum hid_report_type type)
 {