fs/anon_inodes.c

   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/slab.h>
  16 #include <linux/init.h>
  17 #include <linux/fs.h>
  18 #include <linux/mount.h>
  19 #include <linux/module.h>
  20 #include <linux/kernel.h>
  21 #include <linux/magic.h>
  22 #include <linux/anon_inodes.h>
  23
  24 #include <asm/uaccess.h>
  25
  26 static struct vfsmount *anon_inode_mnt __read_mostly;
  27 static struct inode *anon_inode_inode;
  28 static const struct file_operations anon_inode_fops;
  29
  30 static int anon_inodefs_get_sb(struct file_system_type *fs_type, int flags,
  31                                const char *dev_name, void *data,
  32                                struct vfsmount *mnt)
  33 {
  34         return get_sb_pseudo(fs_type, "anon_inode:", NULL, ANON_INODE_FS_MAGIC,
  35                              mnt);
  36 }
  37
  38 /*
  39  * anon_inodefs_dname() is called from d_path().
  40  */
  41 static char *anon_inodefs_dname(struct dentry *dentry, char *buffer, int buflen)
  42 {
  43         return dynamic_dname(dentry, buffer, buflen, "anon_inode:%s",
  44                                 dentry->d_name.name);
  45 }
  46
  47 static struct file_system_type anon_inode_fs_type = {
  48         .name           = "anon_inodefs",
  49         .get_sb         = anon_inodefs_get_sb,
  50         .kill_sb        = kill_anon_super,
  51 };
  52 static const struct dentry_operations anon_inodefs_dentry_operations = {
  53         .d_dname        = anon_inodefs_dname,
  54 };
  55
  56 /*
  57  * nop .set_page_dirty method so that people can use .page_mkwrite on
  58  * anon inodes.
  59  */
  60 static int anon_set_page_dirty(struct page *page)
  61 {
  62         return 0;
  63 };
  64
  65 static const struct address_space_operations anon_aops = {
  66         .set_page_dirty = anon_set_page_dirty,
  67 };
  68
  69 /**
  70  * anon_inode_getfd - creates a new file instance by hooking it up to an
  71  *                    anonymous inode, and a dentry that describe the "class"
  72  *                    of the file
  73  *
  74  * @name:    [in]    name of the "class" of the new file
  75  * @fops:    [in]    file operations for the new file
  76  * @priv:    [in]    private data for the new file (will be file's private_data)
  77  * @flags:   [in]    flags
  78  *
  79  * Creates a new file by hooking it on a single inode. This is useful for files
  80  * that do not need to have a full-fledged inode in order to operate correctly.
  81  * All the files created with anon_inode_getfile() will share a single inode,
  82  * hence saving memory and avoiding code duplication for the file/inode/dentry
  83  * setup.  Returns the newly created file* or an error pointer.
  84  */
  85 struct file *anon_inode_getfile(const char *name,
  86                                 const struct file_operations *fops,
  87                                 void *priv, int flags)
  88 {
  89         struct qstr this;
  90         struct path path;
  91         struct file *file;
  92         int error;
  93
  94         if (IS_ERR(anon_inode_inode))
  95                 return ERR_PTR(-ENODEV);
  96
  97         if (fops->owner && !try_module_get(fops->owner))
  98                 return ERR_PTR(-ENOENT);
  99
 100         /*
 101          * Link the inode to a directory entry by creating a unique name
 102          * using the inode sequence number.
 103          */
 104         error = -ENOMEM;
 105         this.name = name;
 106         this.len = strlen(name);
 107         this.hash = 0;
 108         path.dentry = d_alloc(anon_inode_mnt->mnt_sb->s_root, &this);
 109         if (!path.dentry)
 110                 goto err_module;
 111
 112         path.mnt = mntget(anon_inode_mnt);
 113         /*
 114          * We know the anon_inode inode count is always greater than zero,
 115          * so we can avoid doing an igrab() and we can use an open-coded
 116          * atomic_inc().
 117          */
 118         atomic_inc(&anon_inode_inode->i_count);
 119
 120         path.dentry->d_op = &anon_inodefs_dentry_operations;
 121         d_instantiate(path.dentry, anon_inode_inode);
 122
 123         error = -ENFILE;
 124         file = alloc_file(&path, OPEN_FMODE(flags), fops);
 125         if (!file)
 126                 goto err_dput;
 127         file->f_mapping = anon_inode_inode->i_mapping;
 128
 129         file->f_pos = 0;
 130         file->f_flags = flags & (O_ACCMODE | O_NONBLOCK);
 131         file->f_version = 0;
 132         file->private_data = priv;
 133
 134         return file;
 135
 136 err_dput:
 137         path_put(&path);
 138 err_module:
 139         module_put(fops->owner);
 140         return ERR_PTR(error);
 141 }
 142 EXPORT_SYMBOL_GPL(anon_inode_getfile);
 143
 144 /**
 145  * anon_inode_getfd - creates a new file instance by hooking it up to an
 146  *                    anonymous inode, and a dentry that describe the "class"
 147  *                    of the file
 148  *
 149  * @name:    [in]    name of the "class" of the new file
 150  * @fops:    [in]    file operations for the new file
 151  * @priv:    [in]    private data for the new file (will be file's private_data)
 152  * @flags:   [in]    flags
 153  *
 154  * Creates a new file by hooking it on a single inode. This is useful for files
 155  * that do not need to have a full-fledged inode in order to operate correctly.
 156  * All the files created with anon_inode_getfd() will share a single inode,
 157  * hence saving memory and avoiding code duplication for the file/inode/dentry
 158  * setup.  Returns new descriptor or an error code.
 159  */
 160 int anon_inode_getfd(const char *name, const struct file_operations *fops,
 161                      void *priv, int flags)
 162 {
 163         int error, fd;
 164         struct file *file;
 165
 166         error = get_unused_fd_flags(flags);
 167         if (error < 0)
 168                 return error;
 169         fd = error;
 170
 171         file = anon_inode_getfile(name, fops, priv, flags);
 172         if (IS_ERR(file)) {
 173                 error = PTR_ERR(file);
 174                 goto err_put_unused_fd;
 175         }
 176         fd_install(fd, file);
 177
 178         return fd;
 179
 180 err_put_unused_fd:
 181         put_unused_fd(fd);
 182         return error;
 183 }
 184 EXPORT_SYMBOL_GPL(anon_inode_getfd);
 185
 186 /*
 187  * A single inode exists for all anon_inode files. Contrary to pipes,
 188  * anon_inode inodes have no associated per-instance data, so we need
 189  * only allocate one of them.
 190  */
 191 static struct inode *anon_inode_mkinode(void)
 192 {
 193         struct inode *inode = new_inode(anon_inode_mnt->mnt_sb);
 194
 195         if (!inode)
 196                 return ERR_PTR(-ENOMEM);
 197
 198         inode->i_fop = &anon_inode_fops;
 199
 200         inode->i_mapping->a_ops = &anon_aops;
 201
 202         /*
 203          * Mark the inode dirty from the very beginning,
 204          * that way it will never be moved to the dirty
 205          * list because mark_inode_dirty() will think
 206          * that it already _is_ on the dirty list.
 207          */
 208         inode->i_state = I_DIRTY;
 209         inode->i_mode = S_IRUSR | S_IWUSR;
 210         inode->i_uid = current_fsuid();
 211         inode->i_gid = current_fsgid();
 212         inode->i_flags |= S_PRIVATE;
 213         inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
 214         return inode;
 215 }
 216
 217 static int __init anon_inode_init(void)
 218 {
 219         int error;
 220
 221         error = register_filesystem(&anon_inode_fs_type);
 222         if (error)
 223                 goto err_exit;
 224         anon_inode_mnt = kern_mount(&anon_inode_fs_type);
 225         if (IS_ERR(anon_inode_mnt)) {
 226                 error = PTR_ERR(anon_inode_mnt);
 227                 goto err_unregister_filesystem;
 228         }
 229         anon_inode_inode = anon_inode_mkinode();
 230         if (IS_ERR(anon_inode_inode)) {
 231                 error = PTR_ERR(anon_inode_inode);
 232                 goto err_mntput;
 233         }
 234
 235         return 0;
 236
 237 err_mntput:
 238         mntput(anon_inode_mnt);
 239 err_unregister_filesystem:
 240         unregister_filesystem(&anon_inode_fs_type);
 241 err_exit:
 242         panic(KERN_ERR "anon_inode_init() failed (%d)\n", error);
 243 }
 244
 245 fs_initcall(anon_inode_init);
 246