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