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 struct dentry_operations anon_inodefs_dentry_operations = {
  52         .d_delete       = anon_inodefs_delete_dentry,
  53 };
  54
  55 /**
  56  * anon_inode_getfd - creates a new file instance by hooking it up to an
  57  *                    anonymous inode, and a dentry that describe the "class"
  58  *                    of the file
  59  *
  60  * @name:    [in]    name of the "class" of the new file
  61  * @fops     [in]    file operations for the new file
  62  * @priv     [in]    private data for the new file (will be file's private_data)
  63  *
  64  * Creates a new file by hooking it on a single inode. This is useful for files
  65  * that do not need to have a full-fledged inode in order to operate correctly.
  66  * All the files created with anon_inode_getfd() will share a single inode,
  67  * hence saving memory and avoiding code duplication for the file/inode/dentry
  68  * setup.  Returns new descriptor or -error.
  69  */
  70 int anon_inode_getfd(const char *name, const struct file_operations *fops,
  71                      void *priv)
  72 {
  73         struct qstr this;
  74         struct dentry *dentry;
  75         struct file *file;
  76         int error, fd;
  77
  78         if (IS_ERR(anon_inode_inode))
  79                 return -ENODEV;
  80
  81         error = get_unused_fd();
  82         if (error < 0)
  83                 return error;
  84         fd = error;
  85
  86         /*
  87          * Link the inode to a directory entry by creating a unique name
  88          * using the inode sequence number.
  89          */
  90         error = -ENOMEM;
  91         this.name = name;
  92         this.len = strlen(name);
  93         this.hash = 0;
  94         dentry = d_alloc(anon_inode_mnt->mnt_sb->s_root, &this);
  95         if (!dentry)
  96                 goto err_put_unused_fd;
  97
  98         /*
  99          * We know the anon_inode inode count is always greater than zero,
 100          * so we can avoid doing an igrab() and we can use an open-coded
 101          * atomic_inc().
 102          */
 103         atomic_inc(&anon_inode_inode->i_count);
 104
 105         dentry->d_op = &anon_inodefs_dentry_operations;
 106         /* Do not publish this dentry inside the global dentry hash table */
 107         dentry->d_flags &= ~DCACHE_UNHASHED;
 108         d_instantiate(dentry, anon_inode_inode);
 109
 110         error = -ENFILE;
 111         file = alloc_file(anon_inode_mnt, dentry,
 112                           FMODE_READ | FMODE_WRITE, fops);
 113         if (!file)
 114                 goto err_dput;
 115         file->f_mapping = anon_inode_inode->i_mapping;
 116
 117         file->f_pos = 0;
 118         file->f_flags = O_RDWR;
 119         file->f_version = 0;
 120         file->private_data = priv;
 121
 122         fd_install(fd, file);
 123
 124         return fd;
 125
 126 err_dput:
 127         dput(dentry);
 128 err_put_unused_fd:
 129         put_unused_fd(fd);
 130         return error;
 131 }
 132 EXPORT_SYMBOL_GPL(anon_inode_getfd);
 133
 134 /*
 135  * A single inode exists for all anon_inode files. Contrary to pipes,
 136  * anon_inode inodes have no associated per-instance data, so we need
 137  * only allocate one of them.
 138  */
 139 static struct inode *anon_inode_mkinode(void)
 140 {
 141         struct inode *inode = new_inode(anon_inode_mnt->mnt_sb);
 142
 143         if (!inode)
 144                 return ERR_PTR(-ENOMEM);
 145
 146         inode->i_fop = &anon_inode_fops;
 147
 148         /*
 149          * Mark the inode dirty from the very beginning,
 150          * that way it will never be moved to the dirty
 151          * list because mark_inode_dirty() will think
 152          * that it already _is_ on the dirty list.
 153          */
 154         inode->i_state = I_DIRTY;
 155         inode->i_mode = S_IRUSR | S_IWUSR;
 156         inode->i_uid = current->fsuid;
 157         inode->i_gid = current->fsgid;
 158         inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
 159         return inode;
 160 }
 161
 162 static int __init anon_inode_init(void)
 163 {
 164         int error;
 165
 166         error = register_filesystem(&anon_inode_fs_type);
 167         if (error)
 168                 goto err_exit;
 169         anon_inode_mnt = kern_mount(&anon_inode_fs_type);
 170         if (IS_ERR(anon_inode_mnt)) {
 171                 error = PTR_ERR(anon_inode_mnt);
 172                 goto err_unregister_filesystem;
 173         }
 174         anon_inode_inode = anon_inode_mkinode();
 175         if (IS_ERR(anon_inode_inode)) {
 176                 error = PTR_ERR(anon_inode_inode);
 177                 goto err_mntput;
 178         }
 179
 180         return 0;
 181
 182 err_mntput:
 183         mntput(anon_inode_mnt);
 184 err_unregister_filesystem:
 185         unregister_filesystem(&anon_inode_fs_type);
 186 err_exit:
 187         panic(KERN_ERR "anon_inode_init() failed (%d)\n", error);
 188 }
 189
 190 fs_initcall(anon_inode_init);
 191