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/file.h>
  12 #include <linux/poll.h>
  13 #include <linux/slab.h>
  14 #include <linux/init.h>
  15 #include <linux/fs.h>
  16 #include <linux/mount.h>
  17 #include <linux/module.h>
  18 #include <linux/kernel.h>
  19 #include <linux/magic.h>
  20 #include <linux/anon_inodes.h>
  21
  22 #include <asm/uaccess.h>
  23
  24 static struct vfsmount *anon_inode_mnt __read_mostly;
  25 static struct inode *anon_inode_inode;
  26 static const struct file_operations anon_inode_fops;
  27
  28 static int anon_inodefs_get_sb(struct file_system_type *fs_type, int flags,
  29                                const char *dev_name, void *data,
  30                                struct vfsmount *mnt)
  31 {
  32         return get_sb_pseudo(fs_type, "anon_inode:", NULL, ANON_INODE_FS_MAGIC,
  33                              mnt);
  34 }
  35
  36 static int anon_inodefs_delete_dentry(struct dentry *dentry)
  37 {
  38         /*
  39          * We faked vfs to believe the dentry was hashed when we created it.
  40          * Now we restore the flag so that dput() will work correctly.
  41          */
  42         dentry->d_flags |= DCACHE_UNHASHED;
  43         return 1;
  44 }
  45
  46 static struct file_system_type anon_inode_fs_type = {
  47         .name           = "anon_inodefs",
  48         .get_sb         = anon_inodefs_get_sb,
  49         .kill_sb        = kill_anon_super,
  50 };
  51 static const struct dentry_operations anon_inodefs_dentry_operations = {
  52         .d_delete       = anon_inodefs_delete_dentry,
  53 };
  54
  55 /*
  56  * nop .set_page_dirty method so that people can use .page_mkwrite on
  57  * anon inodes.
  58  */
  59 static int anon_set_page_dirty(struct page *page)
  60 {
  61         return 0;
  62 };
  63
  64 static const struct address_space_operations anon_aops = {
  65         .set_page_dirty = anon_set_page_dirty,
  66 };
  67
  68 /**
  69  * anon_inode_getfd - creates a new file instance by hooking it up to an
  70  *                    anonymous inode, and a dentry that describe the "class"
  71  *                    of the file
  72  *
  73  * @name:    [in]    name of the "class" of the new file
  74  * @fops:    [in]    file operations for the new file
  75  * @priv:    [in]    private data for the new file (will be file's private_data)
  76  * @flags:   [in]    flags
  77  *
  78  * Creates a new file by hooking it on a single inode. This is useful for files
  79  * that do not need to have a full-fledged inode in order to operate correctly.
  80  * All the files created with anon_inode_getfd() will share a single inode,
  81  * hence saving memory and avoiding code duplication for the file/inode/dentry
  82  * setup.  Returns new descriptor or -error.
  83  */
  84 int anon_inode_getfd(const char *name, const struct file_operations *fops,
  85                      void *priv, int flags)
  86 {
  87         struct qstr this;
  88         struct dentry *dentry;
  89         struct file *file;
  90         int error, fd;
  91
  92         if (IS_ERR(anon_inode_inode))
  93                 return -ENODEV;
  94
  95         if (fops->owner && !try_module_get(fops->owner))
  96                 return -ENOENT;
  97
  98         error = get_unused_fd_flags(flags);
  99         if (error < 0)
 100                 goto err_module;
 101         fd = error;
 102
 103         /*
 104          * Link the inode to a directory entry by creating a unique name
 105          * using the inode sequence number.
 106          */
 107         error = -ENOMEM;
 108         this.name = name;
 109         this.len = strlen(name);
 110         this.hash = 0;
 111         dentry = d_alloc(anon_inode_mnt->mnt_sb->s_root, &this);
 112         if (!dentry)
 113                 goto err_put_unused_fd;
 114
 115         /*
 116          * We know the anon_inode inode count is always greater than zero,
 117          * so we can avoid doing an igrab() and we can use an open-coded
 118          * atomic_inc().
 119          */
 120         atomic_inc(&anon_inode_inode->i_count);
 121
 122         dentry->d_op = &anon_inodefs_dentry_operations;
 123         /* Do not publish this dentry inside the global dentry hash table */
 124         dentry->d_flags &= ~DCACHE_UNHASHED;
 125         d_instantiate(dentry, anon_inode_inode);
 126
 127         error = -ENFILE;
 128         file = alloc_file(anon_inode_mnt, dentry,
 129                           FMODE_READ | FMODE_WRITE, fops);
 130         if (!file)
 131                 goto err_dput;
 132         file->f_mapping = anon_inode_inode->i_mapping;
 133
 134         file->f_pos = 0;
 135         file->f_flags = O_RDWR | (flags & O_NONBLOCK);
 136         file->f_version = 0;
 137         file->private_data = priv;
 138
 139         fd_install(fd, file);
 140
 141         return fd;
 142
 143 err_dput:
 144         dput(dentry);
 145 err_put_unused_fd:
 146         put_unused_fd(fd);
 147 err_module:
 148         module_put(fops->owner);
 149         return error;
 150 }
 151 EXPORT_SYMBOL_GPL(anon_inode_getfd);
 152
 153 /*
 154  * A single inode exists for all anon_inode files. Contrary to pipes,
 155  * anon_inode inodes have no associated per-instance data, so we need
 156  * only allocate one of them.
 157  */
 158 static struct inode *anon_inode_mkinode(void)
 159 {
 160         struct inode *inode = new_inode(anon_inode_mnt->mnt_sb);
 161
 162         if (!inode)
 163                 return ERR_PTR(-ENOMEM);
 164
 165         inode->i_fop = &anon_inode_fops;
 166
 167         inode->i_mapping->a_ops = &anon_aops;
 168
 169         /*
 170          * Mark the inode dirty from the very beginning,
 171          * that way it will never be moved to the dirty
 172          * list because mark_inode_dirty() will think
 173          * that it already _is_ on the dirty list.
 174          */
 175         inode->i_state = I_DIRTY;
 176         inode->i_mode = S_IRUSR | S_IWUSR;
 177         inode->i_uid = current_fsuid();
 178         inode->i_gid = current_fsgid();
 179         inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
 180         return inode;
 181 }
 182
 183 static int __init anon_inode_init(void)
 184 {
 185         int error;
 186
 187         error = register_filesystem(&anon_inode_fs_type);
 188         if (error)
 189                 goto err_exit;
 190         anon_inode_mnt = kern_mount(&anon_inode_fs_type);
 191         if (IS_ERR(anon_inode_mnt)) {
 192                 error = PTR_ERR(anon_inode_mnt);
 193                 goto err_unregister_filesystem;
 194         }
 195         anon_inode_inode = anon_inode_mkinode();
 196         if (IS_ERR(anon_inode_inode)) {
 197                 error = PTR_ERR(anon_inode_inode);
 198                 goto err_mntput;
 199         }
 200
 201         return 0;
 202
 203 err_mntput:
 204         mntput(anon_inode_mnt);
 205 err_unregister_filesystem:
 206         unregister_filesystem(&anon_inode_fs_type);
 207 err_exit:
 208         panic(KERN_ERR "anon_inode_init() failed (%d)\n", error);
 209 }
 210
 211 fs_initcall(anon_inode_init);
 212