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/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
  25 static struct vfsmount *anon_inode_mnt __read_mostly;
  26 static struct inode *anon_inode_inode;
  27 static const struct file_operations anon_inode_fops;
  28
  29 /*
  30  * anon_inodefs_dname() is called from d_path().
  31  */
  32 static 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
  38 static 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  */
  46 static int anon_set_page_dirty(struct page *page)
  47 {
  48         return 0;
  49 };
  50
  51 static 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  */
  60 static 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
  87 static 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
 105 static 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  */
 127 struct 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
 172 err_dput:
 173         path_put(&path);
 174 err_module:
 175         module_put(fops->owner);
 176         return file;
 177 }
 178 EXPORT_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  */
 196 int 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
 216 err_put_unused_fd:
 217         put_unused_fd(fd);
 218         return error;
 219 }
 220 EXPORT_SYMBOL_GPL(anon_inode_getfd);
 221
 222 static 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
 236 err_unregister_filesystem:
 237         unregister_filesystem(&anon_inode_fs_type);
 238 err_exit:
 239         panic(KERN_ERR "anon_inode_init() failed (%d)\n", error);
 240 }
 241
 242 fs_initcall(anon_inode_init);
 243