selftests/pidfd: add second supported_mask test

Verify that supported_mask is returned even when other fields are
requested.

Link: https://patch.msgid.link/[e-mail]
Reviewed-by: Alexander Mikhalitsyn <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

tools/testing/selftests/pidfd/pidfd_info_test.c

···
 	close(pidfd);
 }
 
+/*
+ * Test: PIDFD_INFO_SUPPORTED_MASK always available
+ *
+ * Verify that supported_mask is returned even when other fields are requested.
+ */
+TEST(supported_mask_with_other_fields)
+{
+	struct pidfd_info info = {
+		.mask = PIDFD_INFO_CGROUPID | PIDFD_INFO_SUPPORTED_MASK,
+	};
+	int pidfd;
+	pid_t pid;
+
+	pid = create_child(&pidfd, 0);
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0)
+		pause();
+
+	ASSERT_EQ(ioctl(pidfd, PIDFD_GET_INFO, &info), 0);
+
+	/* Both fields should be present */
+	ASSERT_TRUE(!!(info.mask & PIDFD_INFO_CGROUPID));
+	ASSERT_TRUE(!!(info.mask & PIDFD_INFO_SUPPORTED_MASK));
+	ASSERT_NE(info.supported_mask, 0);
+
+	/* Clean up */
+	sys_pidfd_send_signal(pidfd, SIGKILL, NULL, 0);
+	sys_waitid(P_PIDFD, pidfd, NULL, WEXITED);
+	close(pidfd);
+}
+
 TEST_HARNESS_MAIN