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         if (fops->owner && !try_module_get(fops->owner))
  83                 return -ENOENT;
  84
  85         error = get_unused_fd_flags(flags);
  86         if (error < 0)
  87                 goto err_module;
  88         fd = error;
  89
  90         /*
  91          * Link the inode to a directory entry by creating a unique name
  92          * using the inode sequence number.
  93          */
  94         error = -ENOMEM;
  95         this.name = name;
  96         this.len = strlen(name);
  97         this.hash = 0;
  98         dentry = d_alloc(anon_inode_mnt->mnt_sb->s_root, &this);
  99         if (!dentry)
 100                 goto err_put_unused_fd;
 101
 102         /*
 103          * We know the anon_inode inode count is always greater than zero,
 104          * so we can avoid doing an igrab() and we can use an open-coded
 105          * atomic_inc().
 106          */
 107         atomic_inc(&anon_inode_inode->i_count);
 108
 109         dentry->d_op = &anon_inodefs_dentry_operations;
 110         /* Do not publish this dentry inside the global dentry hash table */
 111         dentry->d_flags &= ~DCACHE_UNHASHED;
 112         d_instantiate(dentry, anon_inode_inode);
 113
 114         error = -ENFILE;
 115         file = alloc_file(anon_inode_mnt, dentry,
 116                           FMODE_READ | FMODE_WRITE, fops);
 117         if (!file)
 118                 goto err_dput;
 119         file->f_mapping = anon_inode_inode->i_mapping;
 120
 121         file->f_pos = 0;
 122         file->f_flags = O_RDWR | (flags & O_NONBLOCK);
 123         file->f_version = 0;
 124         file->private_data = priv;
 125
 126         fd_install(fd, file);
 127
 128         return fd;
 129
 130 err_dput:
 131         dput(dentry);
 132 err_put_unused_fd:
 133         put_unused_fd(fd);
 134 err_module:
 135         module_put(fops->owner);
 136         return error;
 137 }
 138 EXPORT_SYMBOL_GPL(anon_inode_getfd);
 139
 140 /*
 141  * A single inode exists for all anon_inode files. Contrary to pipes,
 142  * anon_inode inodes have no associated per-instance data, so we need
 143  * only allocate one of them.
 144  */
 145 static struct inode *anon_inode_mkinode(void)
 146 {
 147         struct inode *inode = new_inode(anon_inode_mnt->mnt_sb);
 148
 149         if (!inode)
 150                 return ERR_PTR(-ENOMEM);
 151
 152         inode->i_fop = &anon_inode_fops;
 153
 154         /*
 155          * Mark the inode dirty from the very beginning,
 156          * that way it will never be moved to the dirty
 157          * list because mark_inode_dirty() will think
 158          * that it already _is_ on the dirty list.
 159          */
 160         inode->i_state = I_DIRTY;
 161         inode->i_mode = S_IRUSR | S_IWUSR;
 162         inode->i_uid = current_fsuid();
 163         inode->i_gid = current_fsgid();
 164         inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
 165         return inode;
 166 }
 167
 168 static int __init anon_inode_init(void)
 169 {
 170         int error;
 171
 172         error = register_filesystem(&anon_inode_fs_type);
 173         if (error)
 174                 goto err_exit;
 175         anon_inode_mnt = kern_mount(&anon_inode_fs_type);
 176         if (IS_ERR(anon_inode_mnt)) {
 177                 error = PTR_ERR(anon_inode_mnt);
 178                 goto err_unregister_filesystem;
 179         }
 180         anon_inode_inode = anon_inode_mkinode();
 181         if (IS_ERR(anon_inode_inode)) {
 182                 error = PTR_ERR(anon_inode_inode);
 183                 goto err_mntput;
 184         }
 185
 186         return 0;
 187
 188 err_mntput:
 189         mntput(anon_inode_mnt);
 190 err_unregister_filesystem:
 191         unregister_filesystem(&anon_inode_fs_type);
 192 err_exit:
 193         panic(KERN_ERR "anon_inode_init() failed (%d)\n", error);
 194 }
 195
 196 fs_initcall(anon_inode_init);
 197