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         fmode_t mode;
  93         int error;
  94
  95         if (IS_ERR(anon_inode_inode))
  96                 return ERR_PTR(-ENODEV);
  97
  98         switch (flags & O_ACCMODE) {
  99         case O_RDONLY: mode = FMODE_READ;               break;
 100         case O_WRONLY: mode = FMODE_WRITE;              break;
 101         case O_RDWR:   mode = FMODE_READ | FMODE_WRITE; break;
 102         default:       return ERR_PTR(-EINVAL);
 103         }
 104
 105         if (fops->owner && !try_module_get(fops->owner))
 106                 return ERR_PTR(-ENOENT);
 107
 108         /*
 109          * Link the inode to a directory entry by creating a unique name
 110          * using the inode sequence number.
 111          */
 112         error = -ENOMEM;
 113         this.name = name;
 114         this.len = strlen(name);
 115         this.hash = 0;
 116         path.dentry = d_alloc(anon_inode_mnt->mnt_sb->s_root, &this);
 117         if (!path.dentry)
 118                 goto err_module;
 119
 120         path.mnt = mntget(anon_inode_mnt);
 121         /*
 122          * We know the anon_inode inode count is always greater than zero,
 123          * so we can avoid doing an igrab() and we can use an open-coded
 124          * atomic_inc().
 125          */
 126         atomic_inc(&anon_inode_inode->i_count);
 127
 128         path.dentry->d_op = &anon_inodefs_dentry_operations;
 129         d_instantiate(path.dentry, anon_inode_inode);
 130
 131         error = -ENFILE;
 132         file = alloc_file(&path, mode, fops);
 133         if (!file)
 134                 goto err_dput;
 135         file->f_mapping = anon_inode_inode->i_mapping;
 136
 137         file->f_pos = 0;
 138         file->f_flags = flags & (O_ACCMODE | O_NONBLOCK);
 139         file->f_version = 0;
 140         file->private_data = priv;
 141
 142         return file;
 143
 144 err_dput:
 145         path_put(&path);
 146 err_module:
 147         module_put(fops->owner);
 148         return ERR_PTR(error);
 149 }
 150 EXPORT_SYMBOL_GPL(anon_inode_getfile);
 151
 152 /**
 153  * anon_inode_getfd - creates a new file instance by hooking it up to an
 154  *                    anonymous inode, and a dentry that describe the "class"
 155  *                    of the file
 156  *
 157  * @name:    [in]    name of the "class" of the new file
 158  * @fops:    [in]    file operations for the new file
 159  * @priv:    [in]    private data for the new file (will be file's private_data)
 160  * @flags:   [in]    flags
 161  *
 162  * Creates a new file by hooking it on a single inode. This is useful for files
 163  * that do not need to have a full-fledged inode in order to operate correctly.
 164  * All the files created with anon_inode_getfd() will share a single inode,
 165  * hence saving memory and avoiding code duplication for the file/inode/dentry
 166  * setup.  Returns new descriptor or an error code.
 167  */
 168 int anon_inode_getfd(const char *name, const struct file_operations *fops,
 169                      void *priv, int flags)
 170 {
 171         int error, fd;
 172         struct file *file;
 173
 174         error = get_unused_fd_flags(flags);
 175         if (error < 0)
 176                 return error;
 177         fd = error;
 178
 179         file = anon_inode_getfile(name, fops, priv, flags);
 180         if (IS_ERR(file)) {
 181                 error = PTR_ERR(file);
 182                 goto err_put_unused_fd;
 183         }
 184         fd_install(fd, file);
 185
 186         return fd;
 187
 188 err_put_unused_fd:
 189         put_unused_fd(fd);
 190         return error;
 191 }
 192 EXPORT_SYMBOL_GPL(anon_inode_getfd);
 193
 194 /*
 195  * A single inode exists for all anon_inode files. Contrary to pipes,
 196  * anon_inode inodes have no associated per-instance data, so we need
 197  * only allocate one of them.
 198  */
 199 static struct inode *anon_inode_mkinode(void)
 200 {
 201         struct inode *inode = new_inode(anon_inode_mnt->mnt_sb);
 202
 203         if (!inode)
 204                 return ERR_PTR(-ENOMEM);
 205
 206         inode->i_fop = &anon_inode_fops;
 207
 208         inode->i_mapping->a_ops = &anon_aops;
 209
 210         /*
 211          * Mark the inode dirty from the very beginning,
 212          * that way it will never be moved to the dirty
 213          * list because mark_inode_dirty() will think
 214          * that it already _is_ on the dirty list.
 215          */
 216         inode->i_state = I_DIRTY;
 217         inode->i_mode = S_IRUSR | S_IWUSR;
 218         inode->i_uid = current_fsuid();
 219         inode->i_gid = current_fsgid();
 220         inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
 221         return inode;
 222 }
 223
 224 static int __init anon_inode_init(void)
 225 {
 226         int error;
 227
 228         error = register_filesystem(&anon_inode_fs_type);
 229         if (error)
 230                 goto err_exit;
 231         anon_inode_mnt = kern_mount(&anon_inode_fs_type);
 232         if (IS_ERR(anon_inode_mnt)) {
 233                 error = PTR_ERR(anon_inode_mnt);
 234                 goto err_unregister_filesystem;
 235         }
 236         anon_inode_inode = anon_inode_mkinode();
 237         if (IS_ERR(anon_inode_inode)) {
 238                 error = PTR_ERR(anon_inode_inode);
 239                 goto err_mntput;
 240         }
 241
 242         return 0;
 243
 244 err_mntput:
 245         mntput(anon_inode_mnt);
 246 err_unregister_filesystem:
 247         unregister_filesystem(&anon_inode_fs_type);
 248 err_exit:
 249         panic(KERN_ERR "anon_inode_init() failed (%d)\n", error);
 250 }
 251
 252 fs_initcall(anon_inode_init);
 253