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From:   Alexander Larsson <alexl@redhat.com>
To:     linux-fsdevel@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, gscrivan@redhat.com,
        david@fromorbit.com, brauner@kernel.org, viro@zeniv.linux.org.uk,
        Alexander Larsson <alexl@redhat.com>
Subject: [PATCH v3 4/6] composefs: Add filesystem implementation
Date:   Fri, 20 Jan 2023 16:23:32 +0100
Message-Id: <9b799ec7e403ba814e7bc097b1e8bd5f7662d596.1674227308.git.alexl@redhat.com>
In-Reply-To: <cover.1674227308.git.alexl@redhat.com>
References: <cover.1674227308.git.alexl@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

This is the basic inode and filesystem implementation.

We open a private mount for the specified base directories at the time
of mount, and all backing data files are looked up with this mount as
root, to protect against changes in the mount tables, or links pointing
outside the base directories.

Access to the backing data is done with the callers credentials, to
the backing files need to be readable by all users of the
filesystem. We also use open_with_fake_path() to ensure the backing
filename is not visible in /proc/.

Signed-off-by: Alexander Larsson <alexl@redhat.com>
Co-developed-by: Giuseppe Scrivano <gscrivan@redhat.com>
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
---
 fs/composefs/cfs.c | 750 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 750 insertions(+)
 create mode 100644 fs/composefs/cfs.c

diff --git a/fs/composefs/cfs.c b/fs/composefs/cfs.c
new file mode 100644
index 000000000000..cd54d3751186
--- /dev/null
+++ b/fs/composefs/cfs.c
@@ -0,0 +1,750 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * composefs
+ *
+ * Copyright (C) 2000 Linus Torvalds.
+ *               2000 Transmeta Corp.
+ * Copyright (C) 2021 Giuseppe Scrivano
+ * Copyright (C) 2022 Alexander Larsson
+ *
+ * This file is released under the GPL.
+ */
+
+#include <linux/exportfs.h>
+#include <linux/fsverity.h>
+#include <linux/fs_parser.h>
+#include <linux/module.h>
+#include <linux/namei.h>
+#include <linux/seq_file.h>
+#include <linux/version.h>
+#include <linux/xattr.h>
+#include <linux/statfs.h>
+
+#include "cfs-internals.h"
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Giuseppe Scrivano <gscrivan@redhat.com>");
+
+#define CFS_MAX_STACK 500
+
+/* Backing file fs-verity check policy, ordered in strictness */
+enum cfs_verity_policy {
+	CFS_VERITY_CHECK_NONE = 0, /* Never verify digest */
+	CFS_VERITY_CHECK_IF_SPECIFIED = 1, /* Verify if specified in image */
+	CFS_VERITY_CHECK_REQUIRED = 2, /* Always verify, fail if not specified in image */
+};
+
+#define CFS_VERITY_CHECK_MAX_POLICY 2
+
+struct cfs_info {
+	struct cfs_context cfs_ctx;
+
+	char *base_path;
+
+	size_t n_bases;
+	struct vfsmount **bases;
+
+	enum cfs_verity_policy verity_check;
+	bool has_digest;
+	u8 digest[SHA256_DIGEST_SIZE]; /* fs-verity digest */
+};
+
+struct cfs_inode {
+	struct inode vfs_inode;
+	struct cfs_inode_extra_data inode_data;
+};
+
+static inline struct cfs_inode *CFS_I(struct inode *inode)
+{
+	return container_of(inode, struct cfs_inode, vfs_inode);
+}
+
+static struct file empty_file;
+
+static const struct file_operations cfs_file_operations;
+
+static const struct super_operations cfs_ops;
+static const struct file_operations cfs_dir_operations;
+static const struct inode_operations cfs_dir_inode_operations;
+static const struct inode_operations cfs_file_inode_operations;
+static const struct inode_operations cfs_link_inode_operations;
+
+static const struct xattr_handler *cfs_xattr_handlers[];
+
+static const struct address_space_operations cfs_aops = {
+	.direct_IO = noop_direct_IO,
+};
+
+static ssize_t cfs_listxattr(struct dentry *dentry, char *names, size_t size);
+
+/* split array of basedirs at ':', copied from overlayfs.  */
+static unsigned int cfs_split_basedirs(char *str)
+{
+	unsigned int ctr = 1;
+	char *s, *d;
+
+	for (s = d = str;; s++, d++) {
+		if (*s == '\\') {
+			s++;
+		} else if (*s == ':') {
+			*d = '\0';
+			ctr++;
+			continue;
+		}
+		*d = *s;
+		if (!*s)
+			break;
+	}
+	return ctr;
+}
+
+static struct inode *cfs_make_inode(struct cfs_context *ctx, struct super_block *sb,
+				    ino_t ino_num, const struct inode *dir)
+{
+	struct inode *inode;
+	struct cfs_inode *cino;
+	int ret;
+
+	inode = new_inode(sb);
+	if (!inode)
+		return ERR_PTR(-ENOMEM);
+
+	cino = CFS_I(inode);
+
+	ret = cfs_init_inode(ctx, ino_num, inode, &cino->inode_data);
+	if (ret < 0)
+		goto fail;
+
+	inode_init_owner(&init_user_ns, inode, dir, inode->i_mode);
+	inode->i_mapping->a_ops = &cfs_aops;
+
+	switch (inode->i_mode & S_IFMT) {
+	case S_IFREG:
+		inode->i_op = &cfs_file_inode_operations;
+		inode->i_fop = &cfs_file_operations;
+		break;
+	case S_IFLNK:
+		inode->i_link = cino->inode_data.path_payload;
+		inode->i_op = &cfs_link_inode_operations;
+		inode->i_fop = &cfs_file_operations;
+		break;
+	case S_IFDIR:
+		inode->i_op = &cfs_dir_inode_operations;
+		inode->i_fop = &cfs_dir_operations;
+		break;
+	case S_IFCHR:
+	case S_IFBLK:
+		if (current_user_ns() != &init_user_ns) {
+			ret = -EPERM;
+			goto fail;
+		}
+		fallthrough;
+	default:
+		inode->i_op = &cfs_file_inode_operations;
+		init_special_inode(inode, inode->i_mode, inode->i_rdev);
+		break;
+	}
+
+	return inode;
+
+fail:
+	iput(inode);
+	return ERR_PTR(ret);
+}
+
+static bool cfs_iterate_cb(void *private, const char *name, int name_len,
+			   u64 ino, unsigned int dtype)
+{
+	struct dir_context *ctx = private;
+
+	if (!dir_emit(ctx, name, name_len, ino, dtype))
+		return 0;
+
+	ctx->pos++;
+	return 1;
+}
+
+static int cfs_iterate(struct file *file, struct dir_context *ctx)
+{
+	struct inode *inode = file->f_inode;
+	struct cfs_info *fsi = inode->i_sb->s_fs_info;
+	struct cfs_inode *cino = CFS_I(inode);
+
+	if (!dir_emit_dots(file, ctx))
+		return 0;
+
+	return cfs_dir_iterate(&fsi->cfs_ctx, inode->i_ino, &cino->inode_data,
+			       ctx->pos - 2, cfs_iterate_cb, ctx);
+}
+
+static struct dentry *cfs_lookup(struct inode *dir, struct dentry *dentry,
+				 unsigned int flags)
+{
+	struct cfs_info *fsi = dir->i_sb->s_fs_info;
+	struct cfs_inode *cino = CFS_I(dir);
+	struct inode *inode = NULL;
+	u64 index;
+	int ret;
+
+	if (dentry->d_name.len > NAME_MAX)
+		return ERR_PTR(-ENAMETOOLONG);
+
+	ret = cfs_dir_lookup(&fsi->cfs_ctx, dir->i_ino, &cino->inode_data,
+			     dentry->d_name.name, dentry->d_name.len, &index);
+	if (ret) {
+		if (ret < 0)
+			return ERR_PTR(ret);
+		inode = cfs_make_inode(&fsi->cfs_ctx, dir->i_sb, index, dir);
+	}
+
+	return d_splice_alias(inode, dentry);
+}
+
+static const struct file_operations cfs_dir_operations = {
+	.llseek = generic_file_llseek,
+	.read = generic_read_dir,
+	.iterate_shared = cfs_iterate,
+};
+
+static const struct inode_operations cfs_dir_inode_operations = {
+	.lookup = cfs_lookup,
+	.listxattr = cfs_listxattr,
+};
+
+static const struct inode_operations cfs_link_inode_operations = {
+	.get_link = simple_get_link,
+	.listxattr = cfs_listxattr,
+};
+
+static int digest_from_string(const char *digest_str, u8 *digest)
+{
+	int res;
+
+	res = hex2bin(digest, digest_str, SHA256_DIGEST_SIZE);
+	if (res < 0)
+		return res;
+
+	if (digest_str[2 * SHA256_DIGEST_SIZE] != 0)
+		return -EINVAL; /* Too long string */
+
+	return 0;
+}
+
+/*
+ * Display the mount options in /proc/mounts.
+ */
+static int cfs_show_options(struct seq_file *m, struct dentry *root)
+{
+	struct cfs_info *fsi = root->d_sb->s_fs_info;
+
+	if (fsi->base_path)
+		seq_show_option(m, "basedir", fsi->base_path);
+	if (fsi->has_digest)
+		seq_printf(m, ",digest=%*phN", SHA256_DIGEST_SIZE, fsi->digest);
+	if (fsi->verity_check != 0)
+		seq_printf(m, ",verity_check=%u", fsi->verity_check);
+
+	return 0;
+}
+
+static struct kmem_cache *cfs_inode_cachep;
+
+static struct inode *cfs_alloc_inode(struct super_block *sb)
+{
+	struct cfs_inode *cino = alloc_inode_sb(sb, cfs_inode_cachep, GFP_KERNEL);
+
+	if (!cino)
+		return NULL;
+
+	memset(&cino->inode_data, 0, sizeof(cino->inode_data));
+
+	return &cino->vfs_inode;
+}
+
+static void cfs_free_inode(struct inode *inode)
+{
+	struct cfs_inode *cino = CFS_I(inode);
+
+	kfree(cino->inode_data.path_payload);
+	kmem_cache_free(cfs_inode_cachep, cino);
+}
+
+static void cfs_put_super(struct super_block *sb)
+{
+	struct cfs_info *fsi = sb->s_fs_info;
+
+	cfs_ctx_put(&fsi->cfs_ctx);
+	if (fsi->bases) {
+		kern_unmount_array(fsi->bases, fsi->n_bases);
+		kfree(fsi->bases);
+	}
+	kfree(fsi->base_path);
+
+	kfree(fsi);
+}
+
+static int cfs_statfs(struct dentry *dentry, struct kstatfs *buf)
+{
+	struct cfs_info *fsi = dentry->d_sb->s_fs_info;
+	int err = 0;
+
+	/* We return the free space, etc from the first base dir. */
+	if (fsi->n_bases > 0) {
+		struct path root = { .mnt = fsi->bases[0],
+				     .dentry = fsi->bases[0]->mnt_root };
+		err = vfs_statfs(&root, buf);
+	}
+
+	if (!err) {
+		buf->f_namelen = NAME_MAX;
+		buf->f_type = dentry->d_sb->s_magic;
+	}
+
+	return err;
+}
+
+static const struct super_operations cfs_ops = {
+	.statfs = cfs_statfs,
+	.drop_inode = generic_delete_inode,
+	.show_options = cfs_show_options,
+	.put_super = cfs_put_super,
+	.alloc_inode = cfs_alloc_inode,
+	.free_inode = cfs_free_inode,
+};
+
+enum cfs_param {
+	Opt_base_path,
+	Opt_digest,
+	Opt_verity_check,
+};
+
+const struct fs_parameter_spec cfs_parameters[] = {
+	fsparam_string("basedir", Opt_base_path),
+	fsparam_string("digest", Opt_digest),
+	fsparam_u32("verity_check", Opt_verity_check),
+	{}
+};
+
+static int cfs_parse_param(struct fs_context *fc, struct fs_parameter *param)
+{
+	struct cfs_info *fsi = fc->s_fs_info;
+	struct fs_parse_result result;
+	int opt, r;
+
+	opt = fs_parse(fc, cfs_parameters, param, &result);
+	if (opt == -ENOPARAM)
+		return vfs_parse_fs_param_source(fc, param);
+	if (opt < 0)
+		return opt;
+
+	switch (opt) {
+	case Opt_base_path:
+		kfree(fsi->base_path);
+		/* Take ownership.  */
+		fsi->base_path = param->string;
+		param->string = NULL;
+		break;
+	case Opt_digest:
+		r = digest_from_string(param->string, fsi->digest);
+		if (r < 0)
+			return r;
+		fsi->has_digest = true;
+		fsi->verity_check = CFS_VERITY_CHECK_REQUIRED; /* Default to full verity check */
+		break;
+	case Opt_verity_check:
+		if (result.uint_32 > CFS_VERITY_CHECK_MAX_POLICY)
+			return invalfc(fc, "Invalid verity_check mode");
+		fsi->verity_check = result.uint_32;
+		break;
+	}
+
+	return 0;
+}
+
+static struct vfsmount *resolve_basedir(const char *name)
+{
+	struct path path = {};
+	struct vfsmount *mnt;
+	int err = -EINVAL;
+
+	if (!*name) {
+		pr_err("empty basedir\n");
+		return ERR_PTR(-EINVAL);
+	}
+	err = kern_path(name, LOOKUP_FOLLOW | LOOKUP_DIRECTORY, &path);
+	if (err) {
+		pr_err("failed to resolve '%s': %i\n", name, err);
+		return ERR_PTR(-EINVAL);
+	}
+
+	mnt = clone_private_mount(&path);
+	path_put(&path);
+	if (!IS_ERR(mnt)) {
+		/* Don't inherit atime flags */
+		mnt->mnt_flags &= ~(MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME);
+	}
+
+	return mnt;
+}
+
+static int cfs_fill_super(struct super_block *sb, struct fs_context *fc)
+{
+	struct cfs_info *fsi = sb->s_fs_info;
+	struct vfsmount **bases = NULL;
+	size_t numbasedirs = 0;
+	struct inode *inode;
+	struct vfsmount *mnt;
+	int ret;
+
+	/* Set up the inode allocator early */
+	sb->s_op = &cfs_ops;
+	sb->s_flags |= SB_RDONLY;
+	sb->s_magic = CFS_MAGIC;
+	sb->s_xattr = cfs_xattr_handlers;
+
+	if (fsi->base_path) {
+		char *lower, *splitlower = NULL;
+
+		ret = -ENOMEM;
+		splitlower = kstrdup(fsi->base_path, GFP_KERNEL);
+		if (!splitlower)
+			goto fail;
+
+		ret = -EINVAL;
+		numbasedirs = cfs_split_basedirs(splitlower);
+		if (numbasedirs > CFS_MAX_STACK) {
+			pr_err("too many lower directories, limit is %d\n",
+			       CFS_MAX_STACK);
+			kfree(splitlower);
+			goto fail;
+		}
+
+		ret = -ENOMEM;
+		bases = kcalloc(numbasedirs, sizeof(struct vfsmount *), GFP_KERNEL);
+		if (!bases) {
+			kfree(splitlower);
+			goto fail;
+		}
+
+		lower = splitlower;
+		for (size_t i = 0; i < numbasedirs; i++) {
+			mnt = resolve_basedir(lower);
+			if (IS_ERR(mnt)) {
+				ret = PTR_ERR(mnt);
+				kfree(splitlower);
+				goto fail;
+			}
+			bases[i] = mnt;
+
+			lower = strchr(lower, '\0') + 1;
+		}
+		kfree(splitlower);
+	}
+
+	/* Must be inited before calling cfs_get_inode.  */
+	ret = cfs_init_ctx(fc->source, fsi->has_digest ? fsi->digest : NULL,
+			   &fsi->cfs_ctx);
+	if (ret < 0)
+		goto fail;
+
+	inode = cfs_make_inode(&fsi->cfs_ctx, sb, CFS_ROOT_INO, NULL);
+	if (IS_ERR(inode)) {
+		ret = PTR_ERR(inode);
+		goto fail;
+	}
+	sb->s_root = d_make_root(inode);
+
+	ret = -ENOMEM;
+	if (!sb->s_root)
+		goto fail;
+
+	sb->s_maxbytes = MAX_LFS_FILESIZE;
+	sb->s_blocksize = PAGE_SIZE;
+	sb->s_blocksize_bits = PAGE_SHIFT;
+
+	fsi->bases = bases;
+	fsi->n_bases = numbasedirs;
+	return 0;
+fail:
+	if (bases) {
+		for (size_t i = 0; i < numbasedirs; i++) {
+			if (bases[i])
+				kern_unmount(bases[i]);
+		}
+		kfree(bases);
+	}
+	cfs_ctx_put(&fsi->cfs_ctx);
+	return ret;
+}
+
+static int cfs_get_tree(struct fs_context *fc)
+{
+	return get_tree_nodev(fc, cfs_fill_super);
+}
+
+static const struct fs_context_operations cfs_context_ops = {
+	.parse_param = cfs_parse_param,
+	.get_tree = cfs_get_tree,
+};
+
+static struct file *open_base_file(struct cfs_info *fsi, struct inode *inode,
+				   struct file *file)
+{
+	struct cfs_inode *cino = CFS_I(inode);
+	struct file *real_file;
+	char *real_path = cino->inode_data.path_payload;
+
+	for (size_t i = 0; i < fsi->n_bases; i++) {
+		real_file = file_open_root_mnt(fsi->bases[i], real_path,
+					       file->f_flags, 0);
+		if (real_file != ERR_PTR(-ENOENT))
+			return real_file;
+	}
+
+	return ERR_PTR(-ENOENT);
+}
+
+static int cfs_open_file(struct inode *inode, struct file *file)
+{
+	struct cfs_info *fsi = inode->i_sb->s_fs_info;
+	struct cfs_inode *cino = CFS_I(inode);
+	char *real_path = cino->inode_data.path_payload;
+	struct file *faked_file;
+	struct file *real_file;
+
+	if (file->f_flags & (O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_TRUNC))
+		return -EROFS;
+
+	if (!real_path) {
+		file->private_data = &empty_file;
+		return 0;
+	}
+
+	if (fsi->verity_check >= CFS_VERITY_CHECK_REQUIRED &&
+	    !cino->inode_data.has_digest) {
+		pr_warn("WARNING: composefs image file '%pD' specified no fs-verity digest\n",
+			file);
+		return -EIO;
+	}
+
+	real_file = open_base_file(fsi, inode, file);
+
+	if (IS_ERR(real_file))
+		return PTR_ERR(real_file);
+
+	/* If metadata records a digest for the file, ensure it is there
+	 * and correct before using the contents.
+	 */
+	if (cino->inode_data.has_digest &&
+	    fsi->verity_check >= CFS_VERITY_CHECK_IF_SPECIFIED) {
+		u8 verity_digest[FS_VERITY_MAX_DIGEST_SIZE];
+		enum hash_algo verity_algo;
+		int res;
+
+		res = fsverity_get_digest(d_inode(real_file->f_path.dentry),
+					  verity_digest, &verity_algo);
+		if (res < 0) {
+			pr_warn("WARNING: composefs backing file '%pD' has no fs-verity digest\n",
+				real_file);
+			fput(real_file);
+			return -EIO;
+		}
+		if (verity_algo != HASH_ALGO_SHA256 ||
+		    memcmp(cino->inode_data.digest, verity_digest,
+			   SHA256_DIGEST_SIZE) != 0) {
+			pr_warn("WARNING: composefs backing file '%pD' has the wrong fs-verity digest\n",
+				real_file);
+			fput(real_file);
+			return -EIO;
+		}
+	}
+
+	faked_file = open_with_fake_path(&file->f_path, file->f_flags,
+					 real_file->f_inode, current_cred());
+	fput(real_file);
+
+	if (IS_ERR(faked_file))
+		return PTR_ERR(faked_file);
+
+	file->private_data = faked_file;
+	return 0;
+}
+
+#ifdef CONFIG_MMU
+static unsigned long cfs_mmu_get_unmapped_area(struct file *file, unsigned long addr,
+					       unsigned long len, unsigned long pgoff,
+					       unsigned long flags)
+{
+	struct file *realfile = file->private_data;
+
+	if (realfile == &empty_file)
+		return 0;
+
+	return current->mm->get_unmapped_area(file, addr, len, pgoff, flags);
+}
+#endif
+
+static int cfs_release_file(struct inode *inode, struct file *file)
+{
+	struct file *realfile = file->private_data;
+
+	if (WARN_ON(!realfile))
+		return -EIO;
+
+	if (realfile == &empty_file)
+		return 0;
+
+	fput(realfile);
+
+	return 0;
+}
+
+static int cfs_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct file *realfile = file->private_data;
+	int ret;
+
+	if (realfile == &empty_file)
+		return 0;
+
+	if (!realfile->f_op->mmap)
+		return -ENODEV;
+
+	if (WARN_ON(file != vma->vm_file))
+		return -EIO;
+
+	vma_set_file(vma, realfile);
+
+	ret = call_mmap(vma->vm_file, vma);
+
+	return ret;
+}
+
+static ssize_t cfs_read_iter(struct kiocb *iocb, struct iov_iter *iter)
+{
+	struct file *file = iocb->ki_filp;
+	struct file *realfile = file->private_data;
+	int ret;
+
+	if (realfile == &empty_file)
+		return 0;
+
+	if (!realfile->f_op->read_iter)
+		return -ENODEV;
+
+	iocb->ki_filp = realfile;
+	ret = call_read_iter(realfile, iocb, iter);
+	iocb->ki_filp = file;
+
+	return ret;
+}
+
+static int cfs_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
+{
+	struct file *realfile = file->private_data;
+
+	if (realfile == &empty_file)
+		return 0;
+
+	return vfs_fadvise(realfile, offset, len, advice);
+}
+
+static int cfs_getxattr(const struct xattr_handler *handler,
+			struct dentry *unused2, struct inode *inode,
+			const char *name, void *value, size_t size)
+{
+	struct cfs_info *fsi = inode->i_sb->s_fs_info;
+	struct cfs_inode *cino = CFS_I(inode);
+
+	return cfs_get_xattr(&fsi->cfs_ctx, &cino->inode_data, name, value, size);
+}
+
+static ssize_t cfs_listxattr(struct dentry *dentry, char *names, size_t size)
+{
+	struct inode *inode = d_inode(dentry);
+	struct cfs_info *fsi = inode->i_sb->s_fs_info;
+	struct cfs_inode *cino = CFS_I(inode);
+
+	return cfs_list_xattrs(&fsi->cfs_ctx, &cino->inode_data, names, size);
+}
+
+static const struct file_operations cfs_file_operations = {
+	.read_iter = cfs_read_iter,
+	.mmap = cfs_mmap,
+	.fadvise = cfs_fadvise,
+	.fsync = noop_fsync,
+	.splice_read = generic_file_splice_read,
+	.llseek = generic_file_llseek,
+#ifdef CONFIG_MMU
+	.get_unmapped_area = cfs_mmu_get_unmapped_area,
+#endif
+	.release = cfs_release_file,
+	.open = cfs_open_file,
+};
+
+static const struct xattr_handler cfs_xattr_handler = {
+	.prefix = "", /* catch all */
+	.get = cfs_getxattr,
+};
+
+static const struct xattr_handler *cfs_xattr_handlers[] = {
+	&cfs_xattr_handler,
+	NULL,
+};
+
+static const struct inode_operations cfs_file_inode_operations = {
+	.listxattr = cfs_listxattr,
+};
+
+static int cfs_init_fs_context(struct fs_context *fc)
+{
+	struct cfs_info *fsi;
+
+	fsi = kzalloc(sizeof(*fsi), GFP_KERNEL);
+	if (!fsi)
+		return -ENOMEM;
+
+	fc->s_fs_info = fsi;
+	fc->ops = &cfs_context_ops;
+	return 0;
+}
+
+static struct file_system_type cfs_type = {
+	.owner = THIS_MODULE,
+	.name = "composefs",
+	.init_fs_context = cfs_init_fs_context,
+	.parameters = cfs_parameters,
+	.kill_sb = kill_anon_super,
+};
+
+static void cfs_inode_init_once(void *foo)
+{
+	struct cfs_inode *cino = foo;
+
+	inode_init_once(&cino->vfs_inode);
+}
+
+static int __init init_cfs(void)
+{
+	cfs_inode_cachep = kmem_cache_create(
+		"cfs_inode", sizeof(struct cfs_inode), 0,
+		(SLAB_RECLAIM_ACCOUNT | SLAB_MEM_SPREAD | SLAB_ACCOUNT),
+		cfs_inode_init_once);
+	if (!cfs_inode_cachep)
+		return -ENOMEM;
+
+	return register_filesystem(&cfs_type);
+}
+
+static void __exit exit_cfs(void)
+{
+	unregister_filesystem(&cfs_type);
+
+	/* Ensure all RCU free inodes are safe to be destroyed. */
+	rcu_barrier();
+
+	kmem_cache_destroy(cfs_inode_cachep);
+}
+
+module_init(init_cfs);
+module_exit(exit_cfs);
-- 
2.39.0