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  * @flags:   [in]    flags
  64  *
  65  * Creates a new file by hooking it on a single inode. This is useful for files
  66  * that do not need to have a full-fledged inode in order to operate correctly.
  67  * All the files created with anon_inode_getfd() will share a single inode,
  68  * hence saving memory and avoiding code duplication for the file/inode/dentry
  69  * setup.  Returns new descriptor or -error.
  70  */
  71 int anon_inode_getfd(const char *name, const struct file_operations *fops,
  72                      void *priv, int flags)
  73 {
  74         struct qstr this;
  75         struct dentry *dentry;
  76         struct file *file;
  77         int error, fd;
  78
  79         if (IS_ERR(anon_inode_inode))
  80                 return -ENODEV;
  81
  82         error = get_unused_fd_flags(flags);
  83         if (error < 0)
  84                 return error;
  85         fd = error;
  86
  87         /*
  88          * Link the inode to a directory entry by creating a unique name
  89          * using the inode sequence number.
  90          */
  91         error = -ENOMEM;
  92         this.name = name;
  93         this.len = strlen(name);
  94         this.hash = 0;
  95         dentry = d_alloc(anon_inode_mnt->mnt_sb->s_root, &this);
  96         if (!dentry)
  97                 goto err_put_unused_fd;
  98
  99         /*
 100          * We know the anon_inode inode count is always greater than zero,
 101          * so we can avoid doing an igrab() and we can use an open-coded
 102          * atomic_inc().
 103          */
 104         atomic_inc(&anon_inode_inode->i_count);
 105
 106         dentry->d_op = &anon_inodefs_dentry_operations;
 107         /* Do not publish this dentry inside the global dentry hash table */
 108         dentry->d_flags &= ~DCACHE_UNHASHED;
 109         d_instantiate(dentry, anon_inode_inode);
 110
 111         error = -ENFILE;
 112         file = alloc_file(anon_inode_mnt, dentry,
 113                           FMODE_READ | FMODE_WRITE, fops);
 114         if (!file)
 115                 goto err_dput;
 116         file->f_mapping = anon_inode_inode->i_mapping;
 117
 118         file->f_pos = 0;
 119         file->f_flags = O_RDWR | (flags & O_NONBLOCK);
 120         file->f_version = 0;
 121         file->private_data = priv;
 122
 123         fd_install(fd, file);
 124
 125         return fd;
 126
 127 err_dput:
 128         dput(dentry);
 129 err_put_unused_fd:
 130         put_unused_fd(fd);
 131         return error;
 132 }
 133 EXPORT_SYMBOL_GPL(anon_inode_getfd);
 134
 135 /*
 136  * A single inode exists for all anon_inode files. Contrary to pipes,
 137  * anon_inode inodes have no associated per-instance data, so we need
 138  * only allocate one of them.
 139  */
 140 static struct inode *anon_inode_mkinode(void)
 141 {
 142         struct inode *inode = new_inode(anon_inode_mnt->mnt_sb);
 143
 144         if (!inode)
 145                 return ERR_PTR(-ENOMEM);
 146
 147         inode->i_fop = &anon_inode_fops;
 148
 149         /*
 150          * Mark the inode dirty from the very beginning,
 151          * that way it will never be moved to the dirty
 152          * list because mark_inode_dirty() will think
 153          * that it already _is_ on the dirty list.
 154          */
 155         inode->i_state = I_DIRTY;
 156         inode->i_mode = S_IRUSR | S_IWUSR;
 157         inode->i_uid = current_fsuid();
 158         inode->i_gid = current_fsgid();
 159         inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
 160         return inode;
 161 }
 162
 163 static int __init anon_inode_init(void)
 164 {
 165         int error;
 166
 167         error = register_filesystem(&anon_inode_fs_type);
 168         if (error)
 169                 goto err_exit;
 170         anon_inode_mnt = kern_mount(&anon_inode_fs_type);
 171         if (IS_ERR(anon_inode_mnt)) {
 172                 error = PTR_ERR(anon_inode_mnt);
 173                 goto err_unregister_filesystem;
 174         }
 175         anon_inode_inode = anon_inode_mkinode();
 176         if (IS_ERR(anon_inode_inode)) {
 177                 error = PTR_ERR(anon_inode_inode);
 178                 goto err_mntput;
 179         }
 180
 181         return 0;
 182
 183 err_mntput:
 184         mntput(anon_inode_mnt);
 185 err_unregister_filesystem:
 186         unregister_filesystem(&anon_inode_fs_type);
 187 err_exit:
 188         panic(KERN_ERR "anon_inode_init() failed (%d)\n", error);
 189 }
 190
 191 fs_initcall(anon_inode_init);
 192