selftests: add FSMOUNT_NAMESPACE tests

Add selftests for FSMOUNT_NAMESPACE which creates a new mount namespace
with the newly created filesystem mounted onto a copy of the real
rootfs.

Link: https://patch.msgid.link/20260122-work-fsmount-namespace-v1-6-5ef0a886e646@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

tools/testing/selftests/filesystems/fsmount_ns/.gitignore

@@ -0,0 +1 @@
+fsmount_ns_test

tools/testing/selftests/filesystems/fsmount_ns/Makefile

@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: GPL-2.0
+TEST_GEN_PROGS := fsmount_ns_test
+
+CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
+LDLIBS := -lcap
+
+include ../../lib.mk
+
+$(OUTPUT)/fsmount_ns_test: fsmount_ns_test.c ../utils.c
+	$(CC) $(CFLAGS) -o $@ $^ $(LDLIBS)

tools/testing/selftests/filesystems/open_tree_ns/Makefile

@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 TEST_GEN_PROGS := open_tree_ns_test
 
-CFLAGS := -Wall -Werror -g $(KHDR_INCLUDES)
+CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
 LDLIBS := -lcap
 
 include ../../lib.mk

tools/testing/selftests/filesystems/open_tree_ns/open_tree_ns_test.c

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 /*
+ * Copyright (c) 2026 Christian Brauner <brauner@kernel.org>
+ *
  * Test for OPEN_TREE_NAMESPACE flag.
  *
  * Test that open_tree() with OPEN_TREE_NAMESPACE creates a new mount
@@ -50,31 +52,6 @@ static int get_mnt_ns_id_from_path(const char *path, uint64_t *mnt_ns_id)
 	return ret;
 }
 
-#define STATMOUNT_BUFSIZE (1 << 15)
-
-static struct statmount *statmount_alloc(uint64_t mnt_id, uint64_t mnt_ns_id, uint64_t mask)
-{
-	struct statmount *buf;
-	size_t bufsize = STATMOUNT_BUFSIZE;
-	int ret;
-
-	for (;;) {
-		buf = malloc(bufsize);
-		if (!buf)
-			return NULL;
-
-		ret = statmount(mnt_id, mnt_ns_id, mask, buf, bufsize, 0);
-		if (ret == 0)
-			return buf;
-
-		free(buf);
-		if (errno != EOVERFLOW)
-			return NULL;
-
-		bufsize <<= 1;
-	}
-}
-
 static void log_mount(struct __test_metadata *_metadata, struct statmount *sm)
 {
 	const char *fs_type = "";
@@ -221,7 +198,7 @@ FIXTURE_SETUP(open_tree_ns)
 		SKIP(return, "open_tree() syscall not supported");
 
 	/* Check if statmount/listmount are supported */
-	ret = statmount(0, 0, 0, NULL, 0, 0);
+	ret = statmount(0, 0, 0, 0, NULL, 0, 0);
 	if (ret == -1 && errno == ENOSYS)
 		SKIP(return, "statmount() syscall not supported");
 
@@ -340,7 +317,7 @@ TEST_F(open_tree_ns, verify_mount_properties)
 	ASSERT_GE(nr_mounts, 1);
 
 	/* Get info about the root mount (the bind mount, rootfs is hidden) */
-	ret = statmount(list[0], new_ns_id, STATMOUNT_MNT_BASIC, &sm, sizeof(sm), 0);
+	ret = statmount(list[0], new_ns_id, 0, STATMOUNT_MNT_BASIC, &sm, sizeof(sm), 0);
 	ASSERT_EQ(ret, 0);
 
 	ASSERT_NE(sm.mnt_id, sm.mnt_parent_id);
@@ -452,7 +429,7 @@ FIXTURE_SETUP(open_tree_ns_userns)
 		SKIP(return, "open_tree() syscall not supported");
 
 	/* Check if statmount/listmount are supported */
-	ret = statmount(0, 0, 0, NULL, 0, 0);
+	ret = statmount(0, 0, 0, 0, NULL, 0, 0);
 	if (ret == -1 && errno == ENOSYS)
 		SKIP(return, "statmount() syscall not supported");
 }
@@ -904,7 +881,7 @@ TEST_F(open_tree_ns_userns, umount_succeeds)
 		ASSERT_FALSE(true) TH_LOG("setns into new namespace failed");
 		break;
 	case 7:
-		ASSERT_FALSE(true) TH_LOG("umount succeeded but should have failed with EINVAL");
+		ASSERT_FALSE(true) TH_LOG("umount failed but should have succeeded");
 		break;
 	case 9:
 		ASSERT_FALSE(true) TH_LOG("listmount failed");