selftests: add listmount() iteration tests

Add a forward and backward iteration test for listmount().

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

tools/testing/selftests/filesystems/statmount/Makefile

···
 # SPDX-License-Identifier: GPL-2.0-or-later
 
 CFLAGS += -Wall -O2 -g $(KHDR_INCLUDES)
-TEST_GEN_PROGS := statmount_test statmount_test_ns
+TEST_GEN_PROGS := statmount_test statmount_test_ns listmount_test
 
 include ../../lib.mk

tools/testing/selftests/filesystems/statmount/listmount_test.c

···
+// SPDX-License-Identifier: GPL-2.0-or-later
+// Copyright (c) 2024 Christian Brauner <[e-mail]>
+
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <sched.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/mount.h>
+#include <unistd.h>
+
+#include "statmount.h"
+#include "../../kselftest_harness.h"
+
+#ifndef LISTMOUNT_REVERSE
+#define LISTMOUNT_REVERSE    (1 << 0) /* List later mounts first */
+#endif
+
+#define LISTMNT_BUFFER 10
+
+/* Check that all mount ids are in increasing order. */
+TEST(listmount_forward)
+{
+	uint64_t list[LISTMNT_BUFFER], last_mnt_id = 0;
+
+	for (;;) {
+		ssize_t nr_mounts;
+
+		nr_mounts = listmount(LSMT_ROOT, 0, last_mnt_id,
+				      list, LISTMNT_BUFFER, 0);
+		ASSERT_GE(nr_mounts, 0);
+		if (nr_mounts == 0)
+			break;
+
+		for (size_t cur = 0; cur < nr_mounts; cur++) {
+			if (cur < nr_mounts - 1)
+				ASSERT_LT(list[cur], list[cur + 1]);
+			last_mnt_id = list[cur];
+		}
+	}
+}
+
+/* Check that all mount ids are in decreasing order. */
+TEST(listmount_backward)
+{
+	uint64_t list[LISTMNT_BUFFER], last_mnt_id = 0;
+
+	for (;;) {
+		ssize_t nr_mounts;
+
+		nr_mounts = listmount(LSMT_ROOT, 0, last_mnt_id,
+				      list, LISTMNT_BUFFER, LISTMOUNT_REVERSE);
+		ASSERT_GE(nr_mounts, 0);
+		if (nr_mounts == 0)
+			break;
+
+		for (size_t cur = 0; cur < nr_mounts; cur++) {
+			if (cur < nr_mounts - 1)
+				ASSERT_GT(list[cur], list[cur + 1]);
+			last_mnt_id = list[cur];
+		}
+	}
+}
+
+TEST_HARNESS_MAIN