linux/fs/anon_inodes.c
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   1/*
   2 *  fs/anon_inodes.c
   3 *
   4 *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
   5 *
   6 *  Thanks to Arnd Bergmann for code review and suggestions.
   7 *  More changes for Thomas Gleixner suggestions.
   8 *
   9 */
  10
  11#include <linux/cred.h>
  12#include <linux/file.h>
  13#include <linux/poll.h>
  14#include <linux/sched.h>
  15#include <linux/init.h>
  16#include <linux/fs.h>
  17#include <linux/mount.h>
  18#include <linux/module.h>
  19#include <linux/kernel.h>
  20#include <linux/magic.h>
  21#include <linux/anon_inodes.h>
  22
  23#include <asm/uaccess.h>
  24
  25static struct vfsmount *anon_inode_mnt __read_mostly;
  26static struct inode *anon_inode_inode;
  27static const struct file_operations anon_inode_fops;
  28
  29/*
  30 * anon_inodefs_dname() is called from d_path().
  31 */
  32static char *anon_inodefs_dname(struct dentry *dentry, char *buffer, int buflen)
  33{
  34        return dynamic_dname(dentry, buffer, buflen, "anon_inode:%s",
  35                                dentry->d_name.name);
  36}
  37
  38static const struct dentry_operations anon_inodefs_dentry_operations = {
  39        .d_dname        = anon_inodefs_dname,
  40};
  41
  42static struct dentry *anon_inodefs_mount(struct file_system_type *fs_type,
  43                                int flags, const char *dev_name, void *data)
  44{
  45        return mount_pseudo(fs_type, "anon_inode:", NULL,
  46                        &anon_inodefs_dentry_operations, ANON_INODE_FS_MAGIC);
  47}
  48
  49static struct file_system_type anon_inode_fs_type = {
  50        .name           = "anon_inodefs",
  51        .mount          = anon_inodefs_mount,
  52        .kill_sb        = kill_anon_super,
  53};
  54
  55/*
  56 * nop .set_page_dirty method so that people can use .page_mkwrite on
  57 * anon inodes.
  58 */
  59static int anon_set_page_dirty(struct page *page)
  60{
  61        return 0;
  62};
  63
  64static const struct address_space_operations anon_aops = {
  65        .set_page_dirty = anon_set_page_dirty,
  66};
  67
  68/**
  69 * anon_inode_getfile - creates a new file instance by hooking it up to an
  70 *                      anonymous inode, and a dentry that describe the "class"
  71 *                      of the file
  72 *
  73 * @name:    [in]    name of the "class" of the new file
  74 * @fops:    [in]    file operations for the new file
  75 * @priv:    [in]    private data for the new file (will be file's private_data)
  76 * @flags:   [in]    flags
  77 *
  78 * Creates a new file by hooking it on a single inode. This is useful for files
  79 * that do not need to have a full-fledged inode in order to operate correctly.
  80 * All the files created with anon_inode_getfile() will share a single inode,
  81 * hence saving memory and avoiding code duplication for the file/inode/dentry
  82 * setup.  Returns the newly created file* or an error pointer.
  83 */
  84struct file *anon_inode_getfile(const char *name,
  85                                const struct file_operations *fops,
  86                                void *priv, int flags)
  87{
  88        struct qstr this;
  89        struct path path;
  90        struct file *file;
  91        int error;
  92
  93        if (IS_ERR(anon_inode_inode))
  94                return ERR_PTR(-ENODEV);
  95
  96        if (fops->owner && !try_module_get(fops->owner))
  97                return ERR_PTR(-ENOENT);
  98
  99        /*
 100         * Link the inode to a directory entry by creating a unique name
 101         * using the inode sequence number.
 102         */
 103        error = -ENOMEM;
 104        this.name = name;
 105        this.len = strlen(name);
 106        this.hash = 0;
 107        path.dentry = d_alloc_pseudo(anon_inode_mnt->mnt_sb, &this);
 108        if (!path.dentry)
 109                goto err_module;
 110
 111        path.mnt = mntget(anon_inode_mnt);
 112        /*
 113         * We know the anon_inode inode count is always greater than zero,
 114         * so ihold() is safe.
 115         */
 116        ihold(anon_inode_inode);
 117
 118        d_instantiate(path.dentry, anon_inode_inode);
 119
 120        error = -ENFILE;
 121        file = alloc_file(&path, OPEN_FMODE(flags), fops);
 122        if (!file)
 123                goto err_dput;
 124        file->f_mapping = anon_inode_inode->i_mapping;
 125
 126        file->f_pos = 0;
 127        file->f_flags = flags & (O_ACCMODE | O_NONBLOCK);
 128        file->f_version = 0;
 129        file->private_data = priv;
 130
 131        return file;
 132
 133err_dput:
 134        path_put(&path);
 135err_module:
 136        module_put(fops->owner);
 137        return ERR_PTR(error);
 138}
 139EXPORT_SYMBOL_GPL(anon_inode_getfile);
 140
 141/**
 142 * anon_inode_getfd - creates a new file instance by hooking it up to an
 143 *                    anonymous inode, and a dentry that describe the "class"
 144 *                    of the file
 145 *
 146 * @name:    [in]    name of the "class" of the new file
 147 * @fops:    [in]    file operations for the new file
 148 * @priv:    [in]    private data for the new file (will be file's private_data)
 149 * @flags:   [in]    flags
 150 *
 151 * Creates a new file by hooking it on a single inode. This is useful for files
 152 * that do not need to have a full-fledged inode in order to operate correctly.
 153 * All the files created with anon_inode_getfd() will share a single inode,
 154 * hence saving memory and avoiding code duplication for the file/inode/dentry
 155 * setup.  Returns new descriptor or an error code.
 156 */
 157int anon_inode_getfd(const char *name, const struct file_operations *fops,
 158                     void *priv, int flags)
 159{
 160        int error, fd;
 161        struct file *file;
 162
 163        error = get_unused_fd_flags(flags);
 164        if (error < 0)
 165                return error;
 166        fd = error;
 167
 168        file = anon_inode_getfile(name, fops, priv, flags);
 169        if (IS_ERR(file)) {
 170                error = PTR_ERR(file);
 171                goto err_put_unused_fd;
 172        }
 173        fd_install(fd, file);
 174
 175        return fd;
 176
 177err_put_unused_fd:
 178        put_unused_fd(fd);
 179        return error;
 180}
 181EXPORT_SYMBOL_GPL(anon_inode_getfd);
 182
 183/*
 184 * A single inode exists for all anon_inode files. Contrary to pipes,
 185 * anon_inode inodes have no associated per-instance data, so we need
 186 * only allocate one of them.
 187 */
 188static struct inode *anon_inode_mkinode(void)
 189{
 190        struct inode *inode = new_inode_pseudo(anon_inode_mnt->mnt_sb);
 191
 192        if (!inode)
 193                return ERR_PTR(-ENOMEM);
 194
 195        inode->i_ino = get_next_ino();
 196        inode->i_fop = &anon_inode_fops;
 197
 198        inode->i_mapping->a_ops = &anon_aops;
 199
 200        /*
 201         * Mark the inode dirty from the very beginning,
 202         * that way it will never be moved to the dirty
 203         * list because mark_inode_dirty() will think
 204         * that it already _is_ on the dirty list.
 205         */
 206        inode->i_state = I_DIRTY;
 207        inode->i_mode = S_IRUSR | S_IWUSR;
 208        inode->i_uid = current_fsuid();
 209        inode->i_gid = current_fsgid();
 210        inode->i_flags |= S_PRIVATE;
 211        inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
 212        return inode;
 213}
 214
 215static int __init anon_inode_init(void)
 216{
 217        int error;
 218
 219        error = register_filesystem(&anon_inode_fs_type);
 220        if (error)
 221                goto err_exit;
 222        anon_inode_mnt = kern_mount(&anon_inode_fs_type);
 223        if (IS_ERR(anon_inode_mnt)) {
 224                error = PTR_ERR(anon_inode_mnt);
 225                goto err_unregister_filesystem;
 226        }
 227        anon_inode_inode = anon_inode_mkinode();
 228        if (IS_ERR(anon_inode_inode)) {
 229                error = PTR_ERR(anon_inode_inode);
 230                goto err_mntput;
 231        }
 232
 233        return 0;
 234
 235err_mntput:
 236        kern_unmount(anon_inode_mnt);
 237err_unregister_filesystem:
 238        unregister_filesystem(&anon_inode_fs_type);
 239err_exit:
 240        panic(KERN_ERR "anon_inode_init() failed (%d)\n", error);
 241}
 242
 243fs_initcall(anon_inode_init);
 244
 245
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