xref: /openbmc/linux/fs/anon_inodes.c (revision e868d61272caa648214046a096e5a6bfc068dc8c)
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 and
57  *                    anonymous inode, and a dentry that describe the "class"
58  *                    of the file
59  *
60  * @pfd:     [out]   pointer to the file descriptor
61  * @dpinode: [out]   pointer to the inode
62  * @pfile:   [out]   pointer to the file struct
63  * @name:    [in]    name of the "class" of the new file
64  * @fops     [in]    file operations for the new file
65  * @priv     [in]    private data for the new file (will be file's private_data)
66  *
67  * Creates a new file by hooking it on a single inode. This is useful for files
68  * that do not need to have a full-fledged inode in order to operate correctly.
69  * All the files created with anon_inode_getfd() will share a single inode, by
70  * hence saving memory and avoiding code duplication for the file/inode/dentry
71  * setup.
72  */
73 int anon_inode_getfd(int *pfd, struct inode **pinode, struct file **pfile,
74 		     const char *name, const struct file_operations *fops,
75 		     void *priv)
76 {
77 	struct qstr this;
78 	struct dentry *dentry;
79 	struct inode *inode;
80 	struct file *file;
81 	int error, fd;
82 
83 	if (IS_ERR(anon_inode_inode))
84 		return -ENODEV;
85 	file = get_empty_filp();
86 	if (!file)
87 		return -ENFILE;
88 
89 	inode = igrab(anon_inode_inode);
90 	if (IS_ERR(inode)) {
91 		error = PTR_ERR(inode);
92 		goto err_put_filp;
93 	}
94 
95 	error = get_unused_fd();
96 	if (error < 0)
97 		goto err_iput;
98 	fd = error;
99 
100 	/*
101 	 * Link the inode to a directory entry by creating a unique name
102 	 * using the inode sequence number.
103 	 */
104 	error = -ENOMEM;
105 	this.name = name;
106 	this.len = strlen(name);
107 	this.hash = 0;
108 	dentry = d_alloc(anon_inode_mnt->mnt_sb->s_root, &this);
109 	if (!dentry)
110 		goto err_put_unused_fd;
111 	dentry->d_op = &anon_inodefs_dentry_operations;
112 	/* Do not publish this dentry inside the global dentry hash table */
113 	dentry->d_flags &= ~DCACHE_UNHASHED;
114 	d_instantiate(dentry, inode);
115 
116 	file->f_path.mnt = mntget(anon_inode_mnt);
117 	file->f_path.dentry = dentry;
118 	file->f_mapping = inode->i_mapping;
119 
120 	file->f_pos = 0;
121 	file->f_flags = O_RDWR;
122 	file->f_op = fops;
123 	file->f_mode = FMODE_READ | FMODE_WRITE;
124 	file->f_version = 0;
125 	file->private_data = priv;
126 
127 	fd_install(fd, file);
128 
129 	*pfd = fd;
130 	*pinode = inode;
131 	*pfile = file;
132 	return 0;
133 
134 err_put_unused_fd:
135 	put_unused_fd(fd);
136 err_iput:
137 	iput(inode);
138 err_put_filp:
139 	put_filp(file);
140 	return error;
141 }
142 
143 /*
144  * A single inode exist for all anon_inode files. Contrary to pipes,
145  * anon_inode inodes has no per-instance data associated, so we can avoid
146  * the allocation of multiple of them.
147  */
148 static struct inode *anon_inode_mkinode(void)
149 {
150 	struct inode *inode = new_inode(anon_inode_mnt->mnt_sb);
151 
152 	if (!inode)
153 		return ERR_PTR(-ENOMEM);
154 
155 	inode->i_fop = &anon_inode_fops;
156 
157 	/*
158 	 * Mark the inode dirty from the very beginning,
159 	 * that way it will never be moved to the dirty
160 	 * list because mark_inode_dirty() will think
161 	 * that it already _is_ on the dirty list.
162 	 */
163 	inode->i_state = I_DIRTY;
164 	inode->i_mode = S_IRUSR | S_IWUSR;
165 	inode->i_uid = current->fsuid;
166 	inode->i_gid = current->fsgid;
167 	inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
168 	return inode;
169 }
170 
171 static int __init anon_inode_init(void)
172 {
173 	int error;
174 
175 	error = register_filesystem(&anon_inode_fs_type);
176 	if (error)
177 		goto err_exit;
178 	anon_inode_mnt = kern_mount(&anon_inode_fs_type);
179 	if (IS_ERR(anon_inode_mnt)) {
180 		error = PTR_ERR(anon_inode_mnt);
181 		goto err_unregister_filesystem;
182 	}
183 	anon_inode_inode = anon_inode_mkinode();
184 	if (IS_ERR(anon_inode_inode)) {
185 		error = PTR_ERR(anon_inode_inode);
186 		goto err_mntput;
187 	}
188 
189 	return 0;
190 
191 err_mntput:
192 	mntput(anon_inode_mnt);
193 err_unregister_filesystem:
194 	unregister_filesystem(&anon_inode_fs_type);
195 err_exit:
196 	panic(KERN_ERR "anon_inode_init() failed (%d)\n", error);
197 }
198 
199 fs_initcall(anon_inode_init);
200 
201