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* fs/anon_inodes.c
* Copyright (C) 2007 Davide Libenzi <>
* Thanks to Arnd Bergmann for code review and suggestions.
* More changes for Thomas Gleixner suggestions.
#include <linux/file.h>
#include <linux/poll.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/mount.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/magic.h>
#include <linux/anon_inodes.h>
#include <asm/uaccess.h>
static struct vfsmount *anon_inode_mnt __read_mostly;
static struct inode *anon_inode_inode;
static const struct file_operations anon_inode_fops;
static int anon_inodefs_get_sb(struct file_system_type *fs_type, int flags,
const char *dev_name, void *data,
struct vfsmount *mnt)
return get_sb_pseudo(fs_type, "anon_inode:", NULL, ANON_INODE_FS_MAGIC,
static int anon_inodefs_delete_dentry(struct dentry *dentry)
* We faked vfs to believe the dentry was hashed when we created it.
* Now we restore the flag so that dput() will work correctly.
dentry->d_flags |= DCACHE_UNHASHED;
return 1;
static struct file_system_type anon_inode_fs_type = {
.name = "anon_inodefs",
.get_sb = anon_inodefs_get_sb,
.kill_sb = kill_anon_super,
static const struct dentry_operations anon_inodefs_dentry_operations = {
.d_delete = anon_inodefs_delete_dentry,
* 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;
static const struct address_space_operations anon_aops = {
.set_page_dirty = anon_set_page_dirty,
* anon_inode_getfd - creates a new file instance by hooking it up to an
* anonymous inode, and a dentry that describe the "class"
* of the file
* @name: [in] name of the "class" of the new file
* @fops: [in] file operations for the new file
* @priv: [in] private data for the new file (will be file's private_data)
* @flags: [in] flags
* Creates a new file by hooking it on a single inode. This is useful for files
* that do not need to have a full-fledged inode in order to operate correctly.
* All the files created with anon_inode_getfd() will share a single inode,
* hence saving memory and avoiding code duplication for the file/inode/dentry
* setup. Returns new descriptor or -error.
int anon_inode_getfd(const char *name, const struct file_operations *fops,
void *priv, int flags)
struct qstr this;
struct dentry *dentry;
struct file *file;
int error, fd;
if (IS_ERR(anon_inode_inode))
return -ENODEV;
if (fops->owner && !try_module_get(fops->owner))
return -ENOENT;
error = get_unused_fd_flags(flags);
if (error < 0)
goto err_module;
fd = error;
* Link the inode to a directory entry by creating a unique name
* using the inode sequence number.
error = -ENOMEM; = name;
this.len = strlen(name);
this.hash = 0;
dentry = d_alloc(anon_inode_mnt->mnt_sb->s_root, &this);
if (!dentry)
goto err_put_unused_fd;
* We know the anon_inode inode count is always greater than zero,
* so we can avoid doing an igrab() and we can use an open-coded
* atomic_inc().
dentry->d_op = &anon_inodefs_dentry_operations;
/* Do not publish this dentry inside the global dentry hash table */
dentry->d_flags &= ~DCACHE_UNHASHED;
d_instantiate(dentry, anon_inode_inode);
error = -ENFILE;
file = alloc_file(anon_inode_mnt, dentry,
if (!file)
goto err_dput;
file->f_mapping = anon_inode_inode->i_mapping;
file->f_pos = 0;
file->f_flags = O_RDWR | (flags & O_NONBLOCK);
file->f_version = 0;
file->private_data = priv;
fd_install(fd, file);
return fd;
return error;
* 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.
static struct inode *anon_inode_mkinode(void)
struct inode *inode = new_inode(anon_inode_mnt->mnt_sb);
if (!inode)
return ERR_PTR(-ENOMEM);
inode->i_fop = &anon_inode_fops;
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_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
return inode;
static int __init anon_inode_init(void)
int error;
error = register_filesystem(&anon_inode_fs_type);
if (error)
goto err_exit;
anon_inode_mnt = kern_mount(&anon_inode_fs_type);
if (IS_ERR(anon_inode_mnt)) {
error = PTR_ERR(anon_inode_mnt);
goto err_unregister_filesystem;
anon_inode_inode = anon_inode_mkinode();
if (IS_ERR(anon_inode_inode)) {
error = PTR_ERR(anon_inode_inode);
goto err_mntput;
return 0;
panic(KERN_ERR "anon_inode_init() failed (%d)\n", error);