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 static int anon_inodefs_delete_dentry(struct dentry *dentry)
  39 {
  40         /*
  41          * We faked vfs to believe the dentry was hashed when we created it.
  42          * Now we restore the flag so that dput() will work correctly.
  43          */
  44         dentry->d_flags |= DCACHE_UNHASHED;
  45         return 1;
  46 }
  47
  48 /*
  49  * anon_inodefs_dname() is called from d_path().
  50  */
  51 static char *anon_inodefs_dname(struct dentry *dentry, char *buffer, int buflen)
  52 {
  53         return dynamic_dname(dentry, buffer, buflen, "anon_inode:%s",
  54                                 dentry->d_name.name);
  55 }
  56
  57 static struct file_system_type anon_inode_fs_type = {
  58         .name           = "anon_inodefs",
  59         .get_sb         = anon_inodefs_get_sb,
  60         .kill_sb        = kill_anon_super,
  61 };
  62 static const struct dentry_operations anon_inodefs_dentry_operations = {
  63         .d_delete       = anon_inodefs_delete_dentry,
  64         .d_dname        = anon_inodefs_dname,
  65 };
  66
  67 /*
  68  * nop .set_page_dirty method so that people can use .page_mkwrite on
  69  * anon inodes.
  70  */
  71 static int anon_set_page_dirty(struct page *page)
  72 {
  73         return 0;
  74 };
  75
  76 static const struct address_space_operations anon_aops = {
  77         .set_page_dirty = anon_set_page_dirty,
  78 };
  79
  80 /**
  81  * anon_inode_getfd - creates a new file instance by hooking it up to an
  82  *                    anonymous inode, and a dentry that describe the "class"
  83  *                    of the file
  84  *
  85  * @name:    [in]    name of the "class" of the new file
  86  * @fops:    [in]    file operations for the new file
  87  * @priv:    [in]    private data for the new file (will be file's private_data)
  88  * @flags:   [in]    flags
  89  *
  90  * Creates a new file by hooking it on a single inode. This is useful for files
  91  * that do not need to have a full-fledged inode in order to operate correctly.
  92  * All the files created with anon_inode_getfile() will share a single inode,
  93  * hence saving memory and avoiding code duplication for the file/inode/dentry
  94  * setup.  Returns the newly created file* or an error pointer.
  95  */
  96 struct file *anon_inode_getfile(const char *name,
  97                                 const struct file_operations *fops,
  98                                 void *priv, int flags)
  99 {
 100         struct qstr this;
 101         struct path path;
 102         struct file *file;
 103         int error;
 104
 105         if (IS_ERR(anon_inode_inode))
 106                 return ERR_PTR(-ENODEV);
 107
 108         if (fops->owner && !try_module_get(fops->owner))
 109                 return ERR_PTR(-ENOENT);
 110
 111         /*
 112          * Link the inode to a directory entry by creating a unique name
 113          * using the inode sequence number.
 114          */
 115         error = -ENOMEM;
 116         this.name = name;
 117         this.len = strlen(name);
 118         this.hash = 0;
 119         path.dentry = d_alloc(anon_inode_mnt->mnt_sb->s_root, &this);
 120         if (!path.dentry)
 121                 goto err_module;
 122
 123         path.mnt = mntget(anon_inode_mnt);
 124         /*
 125          * We know the anon_inode inode count is always greater than zero,
 126          * so we can avoid doing an igrab() and we can use an open-coded
 127          * atomic_inc().
 128          */
 129         atomic_inc(&anon_inode_inode->i_count);
 130
 131         path.dentry->d_op = &anon_inodefs_dentry_operations;
 132         /* Do not publish this dentry inside the global dentry hash table */
 133         path.dentry->d_flags &= ~DCACHE_UNHASHED;
 134         d_instantiate(path.dentry, anon_inode_inode);
 135
 136         error = -ENFILE;
 137         file = alloc_file(&path, FMODE_READ | FMODE_WRITE, fops);
 138         if (!file)
 139                 goto err_dput;
 140         file->f_mapping = anon_inode_inode->i_mapping;
 141
 142         file->f_pos = 0;
 143         file->f_flags = O_RDWR | (flags & O_NONBLOCK);
 144         file->f_version = 0;
 145         file->private_data = priv;
 146
 147         return file;
 148
 149 err_dput:
 150         path_put(&path);
 151 err_module:
 152         module_put(fops->owner);
 153         return ERR_PTR(error);
 154 }
 155 EXPORT_SYMBOL_GPL(anon_inode_getfile);
 156
 157 /**
 158  * anon_inode_getfd - creates a new file instance by hooking it up to an
 159  *                    anonymous inode, and a dentry that describe the "class"
 160  *                    of the file
 161  *
 162  * @name:    [in]    name of the "class" of the new file
 163  * @fops:    [in]    file operations for the new file
 164  * @priv:    [in]    private data for the new file (will be file's private_data)
 165  * @flags:   [in]    flags
 166  *
 167  * Creates a new file by hooking it on a single inode. This is useful for files
 168  * that do not need to have a full-fledged inode in order to operate correctly.
 169  * All the files created with anon_inode_getfd() will share a single inode,
 170  * hence saving memory and avoiding code duplication for the file/inode/dentry
 171  * setup.  Returns new descriptor or an error code.
 172  */
 173 int anon_inode_getfd(const char *name, const struct file_operations *fops,
 174                      void *priv, int flags)
 175 {
 176         int error, fd;
 177         struct file *file;
 178
 179         error = get_unused_fd_flags(flags);
 180         if (error < 0)
 181                 return error;
 182         fd = error;
 183
 184         file = anon_inode_getfile(name, fops, priv, flags);
 185         if (IS_ERR(file)) {
 186                 error = PTR_ERR(file);
 187                 goto err_put_unused_fd;
 188         }
 189         fd_install(fd, file);
 190
 191         return fd;
 192
 193 err_put_unused_fd:
 194         put_unused_fd(fd);
 195         return error;
 196 }
 197 EXPORT_SYMBOL_GPL(anon_inode_getfd);
 198
 199 /*
 200  * A single inode exists for all anon_inode files. Contrary to pipes,
 201  * anon_inode inodes have no associated per-instance data, so we need
 202  * only allocate one of them.
 203  */
 204 static struct inode *anon_inode_mkinode(void)
 205 {
 206         struct inode *inode = new_inode(anon_inode_mnt->mnt_sb);
 207
 208         if (!inode)
 209                 return ERR_PTR(-ENOMEM);
 210
 211         inode->i_fop = &anon_inode_fops;
 212
 213         inode->i_mapping->a_ops = &anon_aops;
 214
 215         /*
 216          * Mark the inode dirty from the very beginning,
 217          * that way it will never be moved to the dirty
 218          * list because mark_inode_dirty() will think
 219          * that it already _is_ on the dirty list.
 220          */
 221         inode->i_state = I_DIRTY;
 222         inode->i_mode = S_IRUSR | S_IWUSR;
 223         inode->i_uid = current_fsuid();
 224         inode->i_gid = current_fsgid();
 225         inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
 226         return inode;
 227 }
 228
 229 static int __init anon_inode_init(void)
 230 {
 231         int error;
 232
 233         error = register_filesystem(&anon_inode_fs_type);
 234         if (error)
 235                 goto err_exit;
 236         anon_inode_mnt = kern_mount(&anon_inode_fs_type);
 237         if (IS_ERR(anon_inode_mnt)) {
 238                 error = PTR_ERR(anon_inode_mnt);
 239                 goto err_unregister_filesystem;
 240         }
 241         anon_inode_inode = anon_inode_mkinode();
 242         if (IS_ERR(anon_inode_inode)) {
 243                 error = PTR_ERR(anon_inode_inode);
 244                 goto err_mntput;
 245         }
 246
 247         return 0;
 248
 249 err_mntput:
 250         mntput(anon_inode_mnt);
 251 err_unregister_filesystem:
 252         unregister_filesystem(&anon_inode_fs_type);
 253 err_exit:
 254         panic(KERN_ERR "anon_inode_init() failed (%d)\n", error);
 255 }
 256
 257 fs_initcall(anon_inode_init);
 258