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