// SPDX-License-Identifier: GPL-2.0-only
/*
 *      fs/libfs.c
 *      Library for filesystems writers.
 */

#include <linux/blkdev.h>
#include <linux/export.h>
#include <linux/pagemap.h>
#include <linux/slab.h>
#include <linux/cred.h>
#include <linux/mount.h>
#include <linux/vfs.h>
#include <linux/quotaops.h>
#include <linux/mutex.h>
#include <linux/namei.h>
#include <linux/exportfs.h>
#include <linux/writeback.h>
#include <linux/buffer_head.h> /* sync_mapping_buffers */
#include <linux/fs_context.h>
#include <linux/pseudo_fs.h>

#include <linux/uaccess.h>

#include "internal.h"

int simple_getattr(const struct path *path, struct kstat *stat,
                   u32 request_mask, unsigned int query_flags)
{
        struct inode *inode = d_inode(path->dentry);
        generic_fillattr(inode, stat);
        stat->blocks = inode->i_mapping->nrpages << (PAGE_SHIFT - 9);
        return 0;
}
EXPORT_SYMBOL(simple_getattr);

int simple_statfs(struct dentry *dentry, struct kstatfs *buf)
{
        buf->f_type = dentry->d_sb->s_magic;
        buf->f_bsize = PAGE_SIZE;
        buf->f_namelen = NAME_MAX;
        return 0;
}
EXPORT_SYMBOL(simple_statfs);

/*
 * Retaining negative dentries for an in-memory filesystem just wastes
 * memory and lookup time: arrange for them to be deleted immediately.
 */
int always_delete_dentry(const struct dentry *dentry)
{
        return 1;
}
EXPORT_SYMBOL(always_delete_dentry);

const struct dentry_operations simple_dentry_operations = {
        .d_delete = always_delete_dentry,
};
EXPORT_SYMBOL(simple_dentry_operations);

/*
 * Lookup the data. This is trivial - if the dentry didn't already
 * exist, we know it is negative.  Set d_op to delete negative dentries.
 */
struct dentry *simple_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
{
        if (dentry->d_name.len > NAME_MAX)
                return ERR_PTR(-ENAMETOOLONG);
        if (!dentry->d_sb->s_d_op)
                d_set_d_op(dentry, &simple_dentry_operations);
        d_add(dentry, NULL);
        return NULL;
}
EXPORT_SYMBOL(simple_lookup);

int dcache_dir_open(struct inode *inode, struct file *file)
{
        file->private_data = d_alloc_cursor(file->f_path.dentry);

        return file->private_data ? 0 : -ENOMEM;
}
EXPORT_SYMBOL(dcache_dir_open);

int dcache_dir_close(struct inode *inode, struct file *file)
{
        dput(file->private_data);
        return 0;
}
EXPORT_SYMBOL(dcache_dir_close);

/* parent is locked at least shared */
static struct dentry *next_positive(struct dentry *parent,
                                    struct list_head *from,
                                    int count)
{
        unsigned *seq = &parent->d_inode->i_dir_seq, n;
        struct dentry *res;
        struct list_head *p;
        bool skipped;
        int i;

retry:
        i = count;
        skipped = false;
        n = smp_load_acquire(seq) & ~1;
        res = NULL;
        rcu_read_lock();
        for (p = from->next; p != &parent->d_subdirs; p = p->next) {
                struct dentry *d = list_entry(p, struct dentry, d_child);
                if (!simple_positive(d)) {
                        skipped = true;
                } else if (!--i) {
                        res = d;
                        break;
                }
        }
        rcu_read_unlock();
        if (skipped) {
                smp_rmb();
                if (unlikely(*seq != n))
                        goto retry;
        }
        return res;
}

static void move_cursor(struct dentry *cursor, struct list_head *after)
{
        struct dentry *parent = cursor->d_parent;
        unsigned n, *seq = &parent->d_inode->i_dir_seq;
        spin_lock(&parent->d_lock);
        for (;;) {
                n = *seq;
                if (!(n & 1) && cmpxchg(seq, n, n + 1) == n)
                        break;
                cpu_relax();
        }
        __list_del(cursor->d_child.prev, cursor->d_child.next);
        if (after)
                list_add(&cursor->d_child, after);
        else
                list_add_tail(&cursor->d_child, &parent->d_subdirs);
        smp_store_release(seq, n + 2);
        spin_unlock(&parent->d_lock);
}

loff_t dcache_dir_lseek(struct file *file, loff_t offset, int whence)
{
        struct dentry *dentry = file->f_path.dentry;
        switch (whence) {
                case 1:
                        offset += file->f_pos;
                        /* fall through */
                case 0:
                        if (offset >= 0)
                                break;
                        /* fall through */
                default:
                        return -EINVAL;
        }
        if (offset != file->f_pos) {
                file->f_pos = offset;
                if (file->f_pos >= 2) {
                        struct dentry *cursor = file->private_data;
                        struct dentry *to;
                        loff_t n = file->f_pos - 2;

                        inode_lock_shared(dentry->d_inode);
                        to = next_positive(dentry, &dentry->d_subdirs, n);
                        move_cursor(cursor, to ? &to->d_child : NULL);
                        inode_unlock_shared(dentry->d_inode);
                }
        }
        return offset;
}
EXPORT_SYMBOL(dcache_dir_lseek);

/* Relationship between i_mode and the DT_xxx types */
static inline unsigned char dt_type(struct inode *inode)
{
        return (inode->i_mode >> 12) & 15;
}

/*
 * Directory is locked and all positive dentries in it are safe, since
 * for ramfs-type trees they can't go away without unlink() or rmdir(),
 * both impossible due to the lock on directory.
 */

int dcache_readdir(struct file *file, struct dir_context *ctx)
{
        struct dentry *dentry = file->f_path.dentry;
        struct dentry *cursor = file->private_data;
        struct list_head *p = &cursor->d_child;
        struct dentry *next;
        bool moved = false;

        if (!dir_emit_dots(file, ctx))
                return 0;

        if (ctx->pos == 2)
                p = &dentry->d_subdirs;
        while ((next = next_positive(dentry, p, 1)) != NULL) {
                if (!dir_emit(ctx, next->d_name.name, next->d_name.len,
                              d_inode(next)->i_ino, dt_type(d_inode(next))))
                        break;
                moved = true;
                p = &next->d_child;
                ctx->pos++;
        }
        if (moved)
                move_cursor(cursor, p);
        return 0;
}
EXPORT_SYMBOL(dcache_readdir);

ssize_t generic_read_dir(struct file *filp, char __user *buf, size_t siz, loff_t *ppos)
{
        return -EISDIR;
}
EXPORT_SYMBOL(generic_read_dir);

const struct file_operations simple_dir_operations = {
        .open           = dcache_dir_open,
        .release        = dcache_dir_close,
        .llseek         = dcache_dir_lseek,
        .read           = generic_read_dir,
        .iterate_shared = dcache_readdir,
        .fsync          = noop_fsync,
};
EXPORT_SYMBOL(simple_dir_operations);

const struct inode_operations simple_dir_inode_operations = {
        .lookup         = simple_lookup,
};
EXPORT_SYMBOL(simple_dir_inode_operations);

static const struct super_operations simple_super_operations = {
        .statfs         = simple_statfs,
};

static int pseudo_fs_fill_super(struct super_block *s, struct fs_context *fc)
{
        struct pseudo_fs_context *ctx = fc->fs_private;
        struct inode *root;

        s->s_maxbytes = MAX_LFS_FILESIZE;
        s->s_blocksize = PAGE_SIZE;
        s->s_blocksize_bits = PAGE_SHIFT;
        s->s_magic = ctx->magic;
        s->s_op = ctx->ops ?: &simple_super_operations;
        s->s_xattr = ctx->xattr;
        s->s_time_gran = 1;
        root = new_inode(s);
        if (!root)
                return -ENOMEM;

        /*
         * since this is the first inode, make it number 1. New inodes created
         * after this must take care not to collide with it (by passing
         * max_reserved of 1 to iunique).
         */
        root->i_ino = 1;
        root->i_mode = S_IFDIR | S_IRUSR | S_IWUSR;
        root->i_atime = root->i_mtime = root->i_ctime = current_time(root);
        s->s_root = d_make_root(root);
        if (!s->s_root)
                return -ENOMEM;
        s->s_d_op = ctx->dops;
        return 0;
}

static int pseudo_fs_get_tree(struct fs_context *fc)
{
        return get_tree_nodev(fc, pseudo_fs_fill_super);
}

static void pseudo_fs_free(struct fs_context *fc)
{
        kfree(fc->fs_private);
}

static const struct fs_context_operations pseudo_fs_context_ops = {
        .free           = pseudo_fs_free,
        .get_tree       = pseudo_fs_get_tree,
};

/*
 * Common helper for pseudo-filesystems (sockfs, pipefs, bdev - stuff that
 * will never be mountable)
 */
struct pseudo_fs_context *init_pseudo(struct fs_context *fc,
                                        unsigned long magic)
{
        struct pseudo_fs_context *ctx;

        ctx = kzalloc(sizeof(struct pseudo_fs_context), GFP_KERNEL);
        if (likely(ctx)) {
                ctx->magic = magic;
                fc->fs_private = ctx;
                fc->ops = &pseudo_fs_context_ops;
                fc->sb_flags |= SB_NOUSER;
                fc->global = true;
        }
        return ctx;
}
EXPORT_SYMBOL(init_pseudo);

int simple_open(struct inode *inode, struct file *file)
{
        if (inode->i_private)
                file->private_data = inode->i_private;
        return 0;
}
EXPORT_SYMBOL(simple_open);

int simple_link(struct dentry *old_dentry, struct inode *dir, struct dentry *dentry)
{
        struct inode *inode = d_inode(old_dentry);

        inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
        inc_nlink(inode);
        ihold(inode);
        dget(dentry);
        d_instantiate(dentry, inode);
        return 0;
}
EXPORT_SYMBOL(simple_link);

int simple_empty(struct dentry *dentry)
{
        struct dentry *child;
        int ret = 0;

        spin_lock(&dentry->d_lock);
        list_for_each_entry(child, &dentry->d_subdirs, d_child) {
                spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
                if (simple_positive(child)) {
                        spin_unlock(&child->d_lock);
                        goto out;
                }
                spin_unlock(&child->d_lock);
        }
        ret = 1;
out:
        spin_unlock(&dentry->d_lock);
        return ret;
}
EXPORT_SYMBOL(simple_empty);

int simple_unlink(struct inode *dir, struct dentry *dentry)
{
        struct inode *inode = d_inode(dentry);

        inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
        drop_nlink(inode);
        dput(dentry);
        return 0;
}
EXPORT_SYMBOL(simple_unlink);

int simple_rmdir(struct inode *dir, struct dentry *dentry)
{
        if (!simple_empty(dentry))
                return -ENOTEMPTY;

        drop_nlink(d_inode(dentry));
        simple_unlink(dir, dentry);
        drop_nlink(dir);
        return 0;
}
EXPORT_SYMBOL(simple_rmdir);

int simple_rename(struct inode *old_dir, struct dentry *old_dentry,
                  struct inode *new_dir, struct dentry *new_dentry,
                  unsigned int flags)
{
        struct inode *inode = d_inode(old_dentry);
        int they_are_dirs = d_is_dir(old_dentry);

        if (flags & ~RENAME_NOREPLACE)
                return -EINVAL;

        if (!simple_empty(new_dentry))
                return -ENOTEMPTY;

        if (d_really_is_positive(new_dentry)) {
                simple_unlink(new_dir, new_dentry);
                if (they_are_dirs) {
                        drop_nlink(d_inode(new_dentry));
                        drop_nlink(old_dir);
                }
        } else if (they_are_dirs) {
                drop_nlink(old_dir);
                inc_nlink(new_dir);
        }

        old_dir->i_ctime = old_dir->i_mtime = new_dir->i_ctime =
                new_dir->i_mtime = inode->i_ctime = current_time(old_dir);

        return 0;
}
EXPORT_SYMBOL(simple_rename);

/**
 * simple_setattr - setattr for simple filesystem
 * @dentry: dentry
 * @iattr: iattr structure
 *
 * Returns 0 on success, -error on failure.
 *
 * simple_setattr is a simple ->setattr implementation without a proper
 * implementation of size changes.
 *
 * It can either be used for in-memory filesystems or special files
 * on simple regular filesystems.  Anything that needs to change on-disk
 * or wire state on size changes needs its own setattr method.
 */
int simple_setattr(struct dentry *dentry, struct iattr *iattr)
{
        struct inode *inode = d_inode(dentry);
        int error;

        error = setattr_prepare(dentry, iattr);
        if (error)
                return error;

        if (iattr->ia_valid & ATTR_SIZE)
                truncate_setsize(inode, iattr->ia_size);
        setattr_copy(inode, iattr);
        mark_inode_dirty(inode);
        return 0;
}
EXPORT_SYMBOL(simple_setattr);

int simple_readpage(struct file *file, struct page *page)
{
        clear_highpage(page);
        flush_dcache_page(page);
        SetPageUptodate(page);
        unlock_page(page);
        return 0;
}
EXPORT_SYMBOL(simple_readpage);

int simple_write_begin(struct file *file, struct address_space *mapping,
                        loff_t pos, unsigned len, unsigned flags,
                        struct page **pagep, void **fsdata)
{
        struct page *page;
        pgoff_t index;

        index = pos >> PAGE_SHIFT;

        page = grab_cache_page_write_begin(mapping, index, flags);
        if (!page)
                return -ENOMEM;

        *pagep = page;

        if (!PageUptodate(page) && (len != PAGE_SIZE)) {
                unsigned from = pos & (PAGE_SIZE - 1);

                zero_user_segments(page, 0, from, from + len, PAGE_SIZE);
        }
        return 0;
}
EXPORT_SYMBOL(simple_write_begin);

/**
 * simple_write_end - .write_end helper for non-block-device FSes
 * @available: See .write_end of address_space_operations
 * @file:               "
 * @mapping:            "
 * @pos:                "
 * @len:                "
 * @copied:             "
 * @page:               "
 * @fsdata:             "
 *
 * simple_write_end does the minimum needed for updating a page after writing is
 * done. It has the same API signature as the .write_end of
 * address_space_operations vector. So it can just be set onto .write_end for
 * FSes that don't need any other processing. i_mutex is assumed to be held.
 * Block based filesystems should use generic_write_end().
 * NOTE: Even though i_size might get updated by this function, mark_inode_dirty
 * is not called, so a filesystem that actually does store data in .write_inode
 * should extend on what's done here with a call to mark_inode_dirty() in the
 * case that i_size has changed.
 *
 * Use *ONLY* with simple_readpage()
 */
int simple_write_end(struct file *file, struct address_space *mapping,
                        loff_t pos, unsigned len, unsigned copied,
                        struct page *page, void *fsdata)
{
        struct inode *inode = page->mapping->host;
        loff_t last_pos = pos + copied;

        /* zero the stale part of the page if we did a short copy */
        if (!PageUptodate(page)) {
                if (copied < len) {
                        unsigned from = pos & (PAGE_SIZE - 1);

                        zero_user(page, from + copied, len - copied);
                }
                SetPageUptodate(page);
        }
        /*
         * No need to use i_size_read() here, the i_size
         * cannot change under us because we hold the i_mutex.
         */
        if (last_pos > inode->i_size)
                i_size_write(inode, last_pos);

        set_page_dirty(page);
        unlock_page(page);
        put_page(page);

        return copied;
}
EXPORT_SYMBOL(simple_write_end);

/*
 * the inodes created here are not hashed. If you use iunique to generate
 * unique inode values later for this filesystem, then you must take care
 * to pass it an appropriate max_reserved value to avoid collisions.
 */
int simple_fill_super(struct super_block *s, unsigned long magic,
                      const struct tree_descr *files)
{
        struct inode *inode;
        struct dentry *root;
        struct dentry *dentry;
        int i;

        s->s_blocksize = PAGE_SIZE;
        s->s_blocksize_bits = PAGE_SHIFT;
        s->s_magic = magic;
        s->s_op = &simple_super_operations;
        s->s_time_gran = 1;

        inode = new_inode(s);
        if (!inode)
                return -ENOMEM;
        /*
         * because the root inode is 1, the files array must not contain an
         * entry at index 1
         */
        inode->i_ino = 1;
        inode->i_mode = S_IFDIR | 0755;
        inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
        inode->i_op = &simple_dir_inode_operations;
        inode->i_fop = &simple_dir_operations;
        set_nlink(inode, 2);
        root = d_make_root(inode);
        if (!root)
                return -ENOMEM;
        for (i = 0; !files->name || files->name[0]; i++, files++) {
                if (!files->name)
                        continue;

                /* warn if it tries to conflict with the root inode */
                if (unlikely(i == 1))
                        printk(KERN_WARNING "%s: %s passed in a files array"
                                "with an index of 1!\n", __func__,
                                s->s_type->name);

                dentry = d_alloc_name(root, files->name);
                if (!dentry)
                        goto out;
                inode = new_inode(s);
                if (!inode) {
                        dput(dentry);
                        goto out;
                }
                inode->i_mode = S_IFREG | files->mode;
                inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
                inode->i_fop = files->ops;
                inode->i_ino = i;
                d_add(dentry, inode);
        }
        s->s_root = root;
        return 0;
out:
        d_genocide(root);
        shrink_dcache_parent(root);
        dput(root);
        return -ENOMEM;
}
EXPORT_SYMBOL(simple_fill_super);

static DEFINE_SPINLOCK(pin_fs_lock);

int simple_pin_fs(struct file_system_type *type, struct vfsmount **mount, int *count)
{
        struct vfsmount *mnt = NULL;
        spin_lock(&pin_fs_lock);
        if (unlikely(!*mount)) {
                spin_unlock(&pin_fs_lock);
                mnt = vfs_kern_mount(type, SB_KERNMOUNT, type->name, NULL);
                if (IS_ERR(mnt))
                        return PTR_ERR(mnt);
                spin_lock(&pin_fs_lock);
                if (!*mount)
                        *mount = mnt;
        }
        mntget(*mount);
        ++*count;
        spin_unlock(&pin_fs_lock);
        mntput(mnt);
        return 0;
}
EXPORT_SYMBOL(simple_pin_fs);

void simple_release_fs(struct vfsmount **mount, int *count)
{
        struct vfsmount *mnt;
        spin_lock(&pin_fs_lock);
        mnt = *mount;
        if (!--*count)
                *mount = NULL;
        spin_unlock(&pin_fs_lock);
        mntput(mnt);
}
EXPORT_SYMBOL(simple_release_fs);

/**
 * simple_read_from_buffer - copy data from the buffer to user space
 * @to: the user space buffer to read to
 * @count: the maximum number of bytes to read
 * @ppos: the current position in the buffer
 * @from: the buffer to read from
 * @available: the size of the buffer
 *
 * The simple_read_from_buffer() function reads up to @count bytes from the
 * buffer @from at offset @ppos into the user space address starting at @to.
 *
 * On success, the number of bytes read is returned and the offset @ppos is
 * advanced by this number, or negative value is returned on error.
 **/
ssize_t simple_read_from_buffer(void __user *to, size_t count, loff_t *ppos,
                                const void *from, size_t available)
{
        loff_t pos = *ppos;
        size_t ret;

        if (pos < 0)
                return -EINVAL;
        if (pos >= available || !count)
                return 0;
        if (count > available - pos)
                count = available - pos;
        ret = copy_to_user(to, from + pos, count);
        if (ret == count)
                return -EFAULT;
        count -= ret;
        *ppos = pos + count;
        return count;
}
EXPORT_SYMBOL(simple_read_from_buffer);

/**
 * simple_write_to_buffer - copy data from user space to the buffer
 * @to: the buffer to write to
 * @available: the size of the buffer
 * @ppos: the current position in the buffer
 * @from: the user space buffer to read from
 * @count: the maximum number of bytes to read
 *
 * The simple_write_to_buffer() function reads up to @count bytes from the user
 * space address starting at @from into the buffer @to at offset @ppos.
 *
 * On success, the number of bytes written is returned and the offset @ppos is
 * advanced by this number, or negative value is returned on error.
 **/
ssize_t simple_write_to_buffer(void *to, size_t available, loff_t *ppos,
                const void __user *from, size_t count)
{
        loff_t pos = *ppos;
        size_t res;

        if (pos < 0)
                return -EINVAL;
        if (pos >= available || !count)
                return 0;
        if (count > available - pos)
                count = available - pos;
        res = copy_from_user(to + pos, from, count);
        if (res == count)
                return -EFAULT;
        count -= res;
        *ppos = pos + count;
        return count;
}
EXPORT_SYMBOL(simple_write_to_buffer);

/**
 * memory_read_from_buffer - copy data from the buffer
 * @to: the kernel space buffer to read to
 * @count: the maximum number of bytes to read
 * @ppos: the current position in the buffer
 * @from: the buffer to read from
 * @available: the size of the buffer
 *
 * The memory_read_from_buffer() function reads up to @count bytes from the
 * buffer @from at offset @ppos into the kernel space address starting at @to.
 *
 * On success, the number of bytes read is returned and the offset @ppos is
 * advanced by this number, or negative value is returned on error.
 **/
ssize_t memory_read_from_buffer(void *to, size_t count, loff_t *ppos,
                                const void *from, size_t available)
{
        loff_t pos = *ppos;

        if (pos < 0)
                return -EINVAL;
        if (pos >= available)
                return 0;
        if (count > available - pos)
                count = available - pos;
        memcpy(to, from + pos, count);
        *ppos = pos + count;

        return count;
}
EXPORT_SYMBOL(memory_read_from_buffer);

/*
 * Transaction based IO.
 * The file expects a single write which triggers the transaction, and then
 * possibly a read which collects the result - which is stored in a
 * file-local buffer.
 */

void simple_transaction_set(struct file *file, size_t n)
{
        struct simple_transaction_argresp *ar = file->private_data;

        BUG_ON(n > SIMPLE_TRANSACTION_LIMIT);

        /*
         * The barrier ensures that ar->size will really remain zero until
         * ar->data is ready for reading.
         */
        smp_mb();
        ar->size = n;
}
EXPORT_SYMBOL(simple_transaction_set);

char *simple_transaction_get(struct file *file, const char __user *buf, size_t size)
{
        struct simple_transaction_argresp *ar;
        static DEFINE_SPINLOCK(simple_transaction_lock);

        if (size > SIMPLE_TRANSACTION_LIMIT - 1)
                return ERR_PTR(-EFBIG);

        ar = (struct simple_transaction_argresp *)get_zeroed_page(GFP_KERNEL);
        if (!ar)
                return ERR_PTR(-ENOMEM);

        spin_lock(&simple_transaction_lock);

        /* only one write allowed per open */
        if (file->private_data) {
                spin_unlock(&simple_transaction_lock);
                free_page((unsigned long)ar);
                return ERR_PTR(-EBUSY);
        }

        file->private_data = ar;

        spin_unlock(&simple_transaction_lock);

        if (copy_from_user(ar->data, buf, size))
                return ERR_PTR(-EFAULT);

        return ar->data;
}
EXPORT_SYMBOL(simple_transaction_get);

ssize_t simple_transaction_read(struct file *file, char __user *buf, size_t size, loff_t *pos)
{
        struct simple_transaction_argresp *ar = file->private_data;

        if (!ar)
                return 0;
        return simple_read_from_buffer(buf, size, pos, ar->data, ar->size);
}
EXPORT_SYMBOL(simple_transaction_read);

int simple_transaction_release(struct inode *inode, struct file *file)
{
        free_page((unsigned long)file->private_data);
        return 0;
}
EXPORT_SYMBOL(simple_transaction_release);

/* Simple attribute files */

struct simple_attr {
        int (*get)(void *, u64 *);
        int (*set)(void *, u64);
        char get_buf[24];       /* enough to store a u64 and "\n\0" */
        char set_buf[24];
        void *data;
        const char *fmt;        /* format for read operation */
        struct mutex mutex;     /* protects access to these buffers */
};

/* simple_attr_open is called by an actual attribute open file operation
 * to set the attribute specific access operations. */
int simple_attr_open(struct inode *inode, struct file *file,
                     int (*get)(void *, u64 *), int (*set)(void *, u64),
                     const char *fmt)
{
        struct simple_attr *attr;

        attr = kmalloc(sizeof(*attr), GFP_KERNEL);
        if (!attr)
                return -ENOMEM;

        attr->get = get;
        attr->set = set;
        attr->data = inode->i_private;
        attr->fmt = fmt;
        mutex_init(&attr->mutex);

        file->private_data = attr;

        return nonseekable_open(inode, file);
}
EXPORT_SYMBOL_GPL(simple_attr_open);

int simple_attr_release(struct inode *inode, struct file *file)
{
        kfree(file->private_data);
        return 0;
}
EXPORT_SYMBOL_GPL(simple_attr_release); /* GPL-only?  This?  Really? */

/* read from the buffer that is filled with the get function */
ssize_t simple_attr_read(struct file *file, char __user *buf,
                         size_t len, loff_t *ppos)
{
        struct simple_attr *attr;
        size_t size;
        ssize_t ret;

        attr = file->private_data;

        if (!attr->get)
                return -EACCES;

        ret = mutex_lock_interruptible(&attr->mutex);
        if (ret)
                return ret;

        if (*ppos) {            /* continued read */
                size = strlen(attr->get_buf);
        } else {                /* first read */
                u64 val;
                ret = attr->get(attr->data, &val);
                if (ret)
                        goto out;

                size = scnprintf(attr->get_buf, sizeof(attr->get_buf),
                                 attr->fmt, (unsigned long long)val);
        }

        ret = simple_read_from_buffer(buf, len, ppos, attr->get_buf, size);
out:
        mutex_unlock(&attr->mutex);
        return ret;
}
EXPORT_SYMBOL_GPL(simple_attr_read);

/* interpret the buffer as a number to call the set function with */
ssize_t simple_attr_write(struct file *file, const char __user *buf,
                          size_t len, loff_t *ppos)
{
        struct simple_attr *attr;
        u64 val;
        size_t size;
        ssize_t ret;

        attr = file->private_data;
        if (!attr->set)
                return -EACCES;

        ret = mutex_lock_interruptible(&attr->mutex);
        if (ret)
                return ret;

        ret = -EFAULT;
        size = min(sizeof(attr->set_buf) - 1, len);
        if (copy_from_user(attr->set_buf, buf, size))
                goto out;

        attr->set_buf[size] = '\0';
        val = simple_strtoll(attr->set_buf, NULL, 0);
        ret = attr->set(attr->data, val);
        if (ret == 0)
                ret = len; /* on success, claim we got the whole input */
out:
        mutex_unlock(&attr->mutex);
        return ret;
}
EXPORT_SYMBOL_GPL(simple_attr_write);

/**
 * generic_fh_to_dentry - generic helper for the fh_to_dentry export operation
 * @sb:         filesystem to do the file handle conversion on
 * @fid:        file handle to convert
 * @fh_len:     length of the file handle in bytes
 * @fh_type:    type of file handle
 * @get_inode:  filesystem callback to retrieve inode
 *
 * This function decodes @fid as long as it has one of the well-known
 * Linux filehandle types and calls @get_inode on it to retrieve the
 * inode for the object specified in the file handle.
 */
struct dentry *generic_fh_to_dentry(struct super_block *sb, struct fid *fid,
                int fh_len, int fh_type, struct inode *(*get_inode)
                        (struct super_block *sb, u64 ino, u32 gen))
{
        struct inode *inode = NULL;

        if (fh_len < 2)
                return NULL;

        switch (fh_type) {
        case FILEID_INO32_GEN:
        case FILEID_INO32_GEN_PARENT:
                inode = get_inode(sb, fid->i32.ino, fid->i32.gen);
                break;
        }

        return d_obtain_alias(inode);
}
EXPORT_SYMBOL_GPL(generic_fh_to_dentry);

/**
 * generic_fh_to_parent - generic helper for the fh_to_parent export operation
 * @sb:         filesystem to do the file handle conversion on
 * @fid:        file handle to convert
 * @fh_len:     length of the file handle in bytes
 * @fh_type:    type of file handle
 * @get_inode:  filesystem callback to retrieve inode
 *
 * This function decodes @fid as long as it has one of the well-known
 * Linux filehandle types and calls @get_inode on it to retrieve the
 * inode for the _parent_ object specified in the file handle if it
 * is specified in the file handle, or NULL otherwise.
 */
struct dentry *generic_fh_to_parent(struct super_block *sb, struct fid *fid,
                int fh_len, int fh_type, struct inode *(*get_inode)
                        (struct super_block *sb, u64 ino, u32 gen))
{
        struct inode *inode = NULL;

        if (fh_len <= 2)
                return NULL;

        switch (fh_type) {
        case FILEID_INO32_GEN_PARENT:
                inode = get_inode(sb, fid->i32.parent_ino,
                                  (fh_len > 3 ? fid->i32.parent_gen : 0));
                break;
        }

        return d_obtain_alias(inode);
}
EXPORT_SYMBOL_GPL(generic_fh_to_parent);

/**
 * __generic_file_fsync - generic fsync implementation for simple filesystems
 *
 * @file:       file to synchronize
 * @start:      start offset in bytes
 * @end:        end offset in bytes (inclusive)
 * @datasync:   only synchronize essential metadata if true
 *
 * This is a generic implementation of the fsync method for simple
 * filesystems which track all non-inode metadata in the buffers list
 * hanging off the address_space structure.
 */
int __generic_file_fsync(struct file *file, loff_t start, loff_t end,
                                 int datasync)
{
        struct inode *inode = file->f_mapping->host;
        int err;
        int ret;

        err = file_write_and_wait_range(file, start, end);
        if (err)
                return err;

        inode_lock(inode);

        ret = sync_mapping_buffers(inode->i_mapping);
        if (!(inode->i_state & I_DIRTY_ALL))
                goto out;
        if (datasync && !(inode->i_state & I_DIRTY_DATASYNC))
                goto out;

        err = sync_inode_metadata(inode, 1);
        if (ret == 0)
                ret = err;

out:
        inode_unlock(inode);
        /* check and advance again to catch errors after syncing out buffers */
        err = file_check_and_advance_wb_err(file);
        if (ret == 0)
                ret = err;
        return ret;
}
EXPORT_SYMBOL(__generic_file_fsync);

/**
 * generic_file_fsync - generic fsync implementation for simple filesystems
 *                      with flush
 * @file:       file to synchronize
 * @start:      start offset in bytes
 * @end:        end offset in bytes (inclusive)
 * @datasync:   only synchronize essential metadata if true
 *
 */

int generic_file_fsync(struct file *file, loff_t start, loff_t end,
                       int datasync)
{
        struct inode *inode = file->f_mapping->host;
        int err;

        err = __generic_file_fsync(file, start, end, datasync);
        if (err)
                return err;
        return blkdev_issue_flush(inode->i_sb->s_bdev, GFP_KERNEL, NULL);
}
EXPORT_SYMBOL(generic_file_fsync);

/**
 * generic_check_addressable - Check addressability of file system
 * @blocksize_bits:     log of file system block size
 * @num_blocks:         number of blocks in file system
 *
 * Determine whether a file system with @num_blocks blocks (and a
 * block size of 2**@blocksize_bits) is addressable by the sector_t
 * and page cache of the system.  Return 0 if so and -EFBIG otherwise.
 */
int generic_check_addressable(unsigned blocksize_bits, u64 num_blocks)
{
        u64 last_fs_block = num_blocks - 1;
        u64 last_fs_page =
                last_fs_block >> (PAGE_SHIFT - blocksize_bits);

        if (unlikely(num_blocks == 0))
                return 0;

        if ((blocksize_bits < 9) || (blocksize_bits > PAGE_SHIFT))
                return -EINVAL;

        if ((last_fs_block > (sector_t)(~0ULL) >> (blocksize_bits - 9)) ||
            (last_fs_page > (pgoff_t)(~0ULL))) {
                return -EFBIG;
        }
        return 0;
}
EXPORT_SYMBOL(generic_check_addressable);

/*
 * No-op implementation of ->fsync for in-memory filesystems.
 */
int noop_fsync(struct file *file, loff_t start, loff_t end, int datasync)
{
        return 0;
}
EXPORT_SYMBOL(noop_fsync);

int noop_set_page_dirty(struct page *page)
{
        /*
         * Unlike __set_page_dirty_no_writeback that handles dirty page
         * tracking in the page object, dax does all dirty tracking in
         * the inode address_space in response to mkwrite faults. In the
         * dax case we only need to worry about potentially dirty CPU
         * caches, not dirty page cache pages to write back.
         *
         * This callback is defined to prevent fallback to
         * __set_page_dirty_buffers() in set_page_dirty().
         */
        return 0;
}
EXPORT_SYMBOL_GPL(noop_set_page_dirty);

void noop_invalidatepage(struct page *page, unsigned int offset,
                unsigned int length)
{
        /*
         * There is no page cache to invalidate in the dax case, however
         * we need this callback defined to prevent falling back to
         * block_invalidatepage() in do_invalidatepage().
         */
}
EXPORT_SYMBOL_GPL(noop_invalidatepage);

ssize_t noop_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
{
        /*
         * iomap based filesystems support direct I/O without need for
         * this callback. However, it still needs to be set in
         * inode->a_ops so that open/fcntl know that direct I/O is
         * generally supported.
         */
        return -EINVAL;
}
EXPORT_SYMBOL_GPL(noop_direct_IO);

/* Because kfree isn't assignment-compatible with void(void*) ;-/ */
void kfree_link(void *p)
{
        kfree(p);
}
EXPORT_SYMBOL(kfree_link);

/*
 * nop .set_page_dirty method so that people can use .page_mkwrite on
 * anon inodes.
 */
static int anon_set_page_dirty(struct page *page)
{
        return 0;
};

/*
 * A single inode exists for all anon_inode files. Contrary to pipes,
 * anon_inode inodes have no associated per-instance data, so we need
 * only allocate one of them.
 */
struct inode *alloc_anon_inode(struct super_block *s)
{
        static const struct address_space_operations anon_aops = {
                .set_page_dirty = anon_set_page_dirty,
        };
        struct inode *inode = new_inode_pseudo(s);

        if (!inode)
                return ERR_PTR(-ENOMEM);

        inode->i_ino = get_next_ino();
        inode->i_mapping->a_ops = &anon_aops;

        /*
         * Mark the inode dirty from the very beginning,
         * that way it will never be moved to the dirty
         * list because mark_inode_dirty() will think
         * that it already _is_ on the dirty list.
         */
        inode->i_state = I_DIRTY;
        inode->i_mode = S_IRUSR | S_IWUSR;
        inode->i_uid = current_fsuid();
        inode->i_gid = current_fsgid();
        inode->i_flags |= S_PRIVATE;
        inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
        return inode;
}
EXPORT_SYMBOL(alloc_anon_inode);

/**
 * simple_nosetlease - generic helper for prohibiting leases
 * @filp: file pointer
 * @arg: type of lease to obtain
 * @flp: new lease supplied for insertion
 * @priv: private data for lm_setup operation
 *
 * Generic helper for filesystems that do not wish to allow leases to be set.
 * All arguments are ignored and it just returns -EINVAL.
 */
int
simple_nosetlease(struct file *filp, long arg, struct file_lock **flp,
                  void **priv)
{
        return -EINVAL;
}
EXPORT_SYMBOL(simple_nosetlease);

/**
 * simple_get_link - generic helper to get the target of "fast" symlinks
 * @dentry: not used here
 * @inode: the symlink inode
 * @done: not used here
 *
 * Generic helper for filesystems to use for symlink inodes where a pointer to
 * the symlink target is stored in ->i_link.  NOTE: this isn't normally called,
 * since as an optimization the path lookup code uses any non-NULL ->i_link
 * directly, without calling ->get_link().  But ->get_link() still must be set,
 * to mark the inode_operations as being for a symlink.
 *
 * Return: the symlink target
 */
const char *simple_get_link(struct dentry *dentry, struct inode *inode,
                            struct delayed_call *done)
{
        return inode->i_link;
}
EXPORT_SYMBOL(simple_get_link);

const struct inode_operations simple_symlink_inode_operations = {
        .get_link = simple_get_link,
};
EXPORT_SYMBOL(simple_symlink_inode_operations);

/*
 * Operations for a permanently empty directory.
 */
static struct dentry *empty_dir_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
{
        return ERR_PTR(-ENOENT);
}

static int empty_dir_getattr(const struct path *path, struct kstat *stat,
                             u32 request_mask, unsigned int query_flags)
{
        struct inode *inode = d_inode(path->dentry);
        generic_fillattr(inode, stat);
        return 0;
}

static int empty_dir_setattr(struct dentry *dentry, struct iattr *attr)
{
        return -EPERM;
}

static ssize_t empty_dir_listxattr(struct dentry *dentry, char *list, size_t size)
{
        return -EOPNOTSUPP;
}

static const struct inode_operations empty_dir_inode_operations = {
        .lookup         = empty_dir_lookup,
        .permission     = generic_permission,
        .setattr        = empty_dir_setattr,
        .getattr        = empty_dir_getattr,
        .listxattr      = empty_dir_listxattr,
};

static loff_t empty_dir_llseek(struct file *file, loff_t offset, int whence)
{
        /* An empty directory has two entries . and .. at offsets 0 and 1 */
        return generic_file_llseek_size(file, offset, whence, 2, 2);
}

static int empty_dir_readdir(struct file *file, struct dir_context *ctx)
{
        dir_emit_dots(file, ctx);
        return 0;
}

static const struct file_operations empty_dir_operations = {
        .llseek         = empty_dir_llseek,
        .read           = generic_read_dir,
        .iterate_shared = empty_dir_readdir,
        .fsync          = noop_fsync,
};


void make_empty_dir_inode(struct inode *inode)
{
        set_nlink(inode, 2);
        inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO;
        inode->i_uid = GLOBAL_ROOT_UID;
        inode->i_gid = GLOBAL_ROOT_GID;
        inode->i_rdev = 0;
        inode->i_size = 0;
        inode->i_blkbits = PAGE_SHIFT;
        inode->i_blocks = 0;

        inode->i_op = &empty_dir_inode_operations;
        inode->i_opflags &= ~IOP_XATTR;
        inode->i_fop = &empty_dir_operations;
}

bool is_empty_dir_inode(struct inode *inode)
{
        return (inode->i_fop == &empty_dir_operations) &&
                (inode->i_op == &empty_dir_inode_operations);
}