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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, alexl@redhat.com
Subject: [PATCH 4/6] composefs: Add filesystem implementation
Date:   Mon, 28 Nov 2022 12:17:12 +0100
Message-Id: <1f0bd3e3a0c68ee19dd96ee0d573bb113428f1b6.1669631086.git.alexl@redhat.com>
In-Reply-To: <cover.1669631086.git.alexl@redhat.com>
References: <cover.1669631086.git.alexl@redhat.com>
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This is the basic inode and filesystem implementation.

Signed-off-by: Alexander Larsson <alexl@redhat.com>
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
---
 fs/composefs/cfs.c | 941 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 941 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..4ed355ab079d
--- /dev/null
+++ b/fs/composefs/cfs.c
@@ -0,0 +1,941 @@
+// 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
+
+struct cfs_info {
+	struct cfs_context_s cfs_ctx;
+
+	char *base_path;
+
+	size_t n_bases;
+	struct vfsmount **bases;
+
+	u32 verity_check; /* 0 == none, 1 == if specified in image, 2 == require in image */
+	bool has_digest;
+	u8 digest[SHA256_DIGEST_SIZE]; /* fs-verity digest */
+};
+
+struct cfs_inode {
+	/* must be first for clear in cfs_alloc_inode to work */
+	struct inode vfs_inode;
+
+	struct cfs_inode_data_s 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 vm_operations_struct generic_file_vm_ops;
+
+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 export_operations cfs_export_operations;
+
+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);
+
+/* 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_s *ctx,
+				    struct super_block *sb, ino_t ino_num,
+				    struct cfs_inode_s *ino,
+				    const struct inode *dir)
+{
+	struct cfs_xattr_header_s *xattrs = NULL;
+	struct cfs_inode *cino;
+	struct inode *inode = NULL;
+	struct cfs_inode_data_s inode_data = { 0 };
+	int ret, res;
+
+	res = cfs_init_inode_data(ctx, ino, ino_num, &inode_data);
+	if (res < 0) {
+		ret = res;
+		goto fail;
+	}
+
+	inode = new_inode(sb);
+	if (inode) {
+		inode_init_owner(&init_user_ns, inode, dir, ino->st_mode);
+		inode->i_mapping->a_ops = &cfs_aops;
+
+		cino = CFS_I(inode);
+		cino->inode_data = inode_data;
+
+		inode->i_ino = ino_num;
+		set_nlink(inode, ino->st_nlink);
+		inode->i_rdev = ino->st_rdev;
+		inode->i_uid = make_kuid(current_user_ns(), ino->st_uid);
+		inode->i_gid = make_kgid(current_user_ns(), ino->st_gid);
+		inode->i_mode = ino->st_mode;
+		inode->i_atime = ino->st_mtim;
+		inode->i_mtime = ino->st_mtim;
+		inode->i_ctime = ino->st_ctim;
+
+		switch (ino->st_mode & S_IFMT) {
+		case S_IFREG:
+			inode->i_op = &cfs_file_inode_operations;
+			inode->i_fop = &cfs_file_operations;
+			inode->i_size = ino->st_size;
+			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;
+			inode->i_size = 4096;
+			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, ino->st_mode, ino->st_rdev);
+			break;
+		}
+	}
+	return inode;
+
+fail:
+	if (inode)
+		iput(inode);
+	kfree(xattrs);
+	cfs_inode_data_put(&inode_data);
+	return ERR_PTR(ret);
+}
+
+static struct inode *cfs_get_root_inode(struct super_block *sb)
+{
+	struct cfs_info *fsi = sb->s_fs_info;
+	struct cfs_inode_s ino_buf;
+	struct cfs_inode_s *ino;
+	u64 index;
+
+	ino = cfs_get_root_ino(&fsi->cfs_ctx, &ino_buf, &index);
+	if (IS_ERR(ino))
+		return ERR_CAST(ino);
+
+	return cfs_make_inode(&fsi->cfs_ctx, sb, index, ino, NULL);
+}
+
+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 cfs_inode_s ino_buf;
+	struct inode *inode;
+	struct cfs_inode_s *ino_s;
+	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 < 0)
+		return ERR_PTR(ret);
+	if (ret == 0)
+		goto return_negative;
+
+	ino_s = cfs_get_ino_index(&fsi->cfs_ctx, index, &ino_buf);
+	if (IS_ERR(ino_s))
+		return ERR_CAST(ino_s);
+
+	inode = cfs_make_inode(&fsi->cfs_ctx, dir->i_sb, index, ino_s, dir);
+	if (IS_ERR(inode))
+		return ERR_CAST(inode);
+
+	return d_splice_alias(inode, dentry);
+
+return_negative:
+	d_add(dentry, NULL);
+	return NULL;
+}
+
+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 void digest_to_string(const u8 *digest, char *buf)
+{
+	static const char hexchars[] = "0123456789abcdef";
+	u32 i, j;
+
+	for (i = 0, j = 0; i < SHA256_DIGEST_SIZE; i++, j += 2) {
+		u8 byte = digest[i];
+
+		buf[j] = hexchars[byte >> 4];
+		buf[j + 1] = hexchars[byte & 0xF];
+	}
+	buf[j] = '\0';
+}
+
+static int digest_from_string(const char *digest_str, u8 *digest)
+{
+	size_t i, j;
+
+	for (i = 0, j = 0; i < SHA256_DIGEST_SIZE; i += 1, j += 2) {
+		int big, little;
+
+		if (digest_str[j] == 0 || digest_str[j + 1] == 0)
+			return -EINVAL; /* Too short string */
+
+		big = cfs_xdigit_value(digest_str[j]);
+		little = cfs_xdigit_value(digest_str[j + 1]);
+
+		if (big == -1 || little == -1)
+			return -EINVAL; /* Not hex digit */
+
+		digest[i] = (big << 4) | little;
+	}
+
+	if (digest_str[j] != 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) {
+		char buf[SHA256_DIGEST_SIZE * 2 + 1];
+
+		digest_to_string(fsi->digest, buf);
+		seq_show_option(m, "digest", buf);
+	}
+	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((u8 *)cino + sizeof(struct inode), 0,
+	       sizeof(struct cfs_inode) - sizeof(struct inode));
+
+	return &cino->vfs_inode;
+}
+
+static void cfs_destroy_inode(struct inode *inode)
+{
+	struct cfs_inode *cino = CFS_I(inode);
+
+	cfs_inode_data_put(&cino->inode_data);
+}
+
+static void cfs_free_inode(struct inode *inode)
+{
+	struct cfs_inode *cino = CFS_I(inode);
+
+	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,
+	.destroy_inode = cfs_destroy_inode,
+	.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 fs_parse_result result;
+	struct cfs_info *fsi = fc->s_fs_info;
+	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 = 2; /* Default to full verity check */
+		break;
+	case Opt_verity_check:
+		if (result.uint_32 > 2)
+			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");
+		goto out;
+	}
+	err = kern_path(name, LOOKUP_FOLLOW, &path);
+	if (err) {
+		pr_err("failed to resolve '%s': %i\n", name, err);
+		goto out;
+	}
+
+	mnt = clone_private_mount(&path);
+	err = PTR_ERR(mnt);
+	if (IS_ERR(mnt)) {
+		pr_err("failed to clone basedir\n");
+		goto out_put;
+	}
+
+	path_put(&path);
+
+	/* Don't inherit atime flags */
+	mnt->mnt_flags &= ~(MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME);
+
+	return mnt;
+
+out_put:
+	path_put(&path);
+out:
+	return ERR_PTR(err);
+}
+
+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;
+
+	if (sb->s_root)
+		return -EINVAL;
+
+	/* 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;
+	sb->s_export_op = &cfs_export_operations;
+
+	if (fsi->base_path) {
+		char *lower, *splitlower = NULL;
+		size_t i;
+
+		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 (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_get_root_inode(sb);
+	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;
+
+	sb->s_time_gran = 1;
+
+	fsi->bases = bases;
+	fsi->n_bases = numbasedirs;
+	return 0;
+fail:
+	if (bases) {
+		size_t i;
+
+		for (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;
+	size_t i;
+
+	for (i = 0; i < fsi->n_bases; i++) {
+		real_file = file_open_root_mnt(fsi->bases[i], real_path,
+					       file->f_flags, 0);
+		if (!IS_ERR(real_file) || PTR_ERR(real_file) != -ENOENT)
+			return real_file;
+	}
+
+	return ERR_PTR(-ENOENT);
+}
+
+static int cfs_open_file(struct inode *inode, struct file *file)
+{
+	struct cfs_inode *cino = CFS_I(inode);
+	struct cfs_info *fsi = inode->i_sb->s_fs_info;
+	struct file *real_file;
+	struct file *faked_file;
+	char *real_path = cino->inode_data.path_payload;
+
+	if (WARN_ON(!file))
+		return -EIO;
+
+	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 >= 2 && !cino->inode_data.has_digest) {
+		pr_warn("WARNING: composefs image file '%pd' specified no fs-verity digest\n",
+			file->f_path.dentry);
+		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 >= 1) {
+		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->f_path.dentry);
+			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->f_path.dentry);
+			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;
+}
+
+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);
+}
+
+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(file->private_data);
+
+	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_encode_fh(struct inode *inode, u32 *fh, int *max_len,
+			 struct inode *parent)
+{
+	int len = 3;
+	u64 nodeid;
+	u32 generation;
+
+	if (*max_len < len) {
+		*max_len = len;
+		return FILEID_INVALID;
+	}
+
+	nodeid = inode->i_ino;
+	generation = inode->i_generation;
+
+	fh[0] = (u32)(nodeid >> 32);
+	fh[1] = (u32)(nodeid & 0xffffffff);
+	fh[2] = generation;
+
+	*max_len = len;
+
+	return 0x91;
+}
+
+static struct dentry *cfs_fh_to_dentry(struct super_block *sb, struct fid *fid,
+				       int fh_len, int fh_type)
+{
+	struct cfs_info *fsi = sb->s_fs_info;
+	struct inode *ino;
+	u64 inode_index;
+	u32 generation;
+
+	if (fh_type != 0x91 || fh_len < 3)
+		return NULL;
+
+	inode_index = (u64)(fid->raw[0]) << 32;
+	inode_index |= fid->raw[1];
+	generation = fid->raw[2];
+
+	ino = ilookup(sb, inode_index);
+	if (!ino) {
+		struct cfs_inode_s inode_buf;
+		struct cfs_inode_s *inode;
+
+		inode = cfs_get_ino_index(&fsi->cfs_ctx, inode_index,
+					  &inode_buf);
+		if (IS_ERR(inode))
+			return ERR_CAST(inode);
+
+		ino = cfs_make_inode(&fsi->cfs_ctx, sb, inode_index, inode,
+				     NULL);
+		if (IS_ERR(ino))
+			return ERR_CAST(ino);
+	}
+	if (ino->i_generation != generation) {
+		iput(ino);
+		return ERR_PTR(-ESTALE);
+	}
+	return d_obtain_alias(ino);
+}
+
+static struct dentry *cfs_fh_to_parent(struct super_block *sb, struct fid *fid,
+				       int fh_len, int fh_type)
+{
+	return ERR_PTR(-EACCES);
+}
+
+static int cfs_get_name(struct dentry *parent, char *name, struct dentry *child)
+{
+	WARN_ON_ONCE(1);
+	return -EIO;
+}
+
+static struct dentry *cfs_get_parent(struct dentry *dentry)
+{
+	WARN_ON_ONCE(1);
+	return ERR_PTR(-EIO);
+}
+
+static const struct export_operations cfs_export_operations = {
+	.fh_to_dentry = cfs_fh_to_dentry,
+	.fh_to_parent = cfs_fh_to_parent,
+	.encode_fh = cfs_encode_fh,
+	.get_parent = cfs_get_parent,
+	.get_name = cfs_get_name,
+};
+
+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,
+	.get_unmapped_area = cfs_mmu_get_unmapped_area,
+	.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 = {
+	.setattr = simple_setattr,
+	.getattr = simple_getattr,
+
+	.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);
-- 
2.38.1