debugfs: add support for more elaborate ->d_fsdata

Currently, the user provided fops, "real_fops", are stored directly into
->d_fsdata.

In order to be able to store more per-file state and thus prepare for more
granular file removal protection, wrap the real_fops into a dynamically
allocated container struct, debugfs_fsdata.

A struct debugfs_fsdata gets allocated at file creation and freed from the
newly intoduced ->d_release().

Finally, move the implementation of debugfs_real_fops() out of the public
debugfs header such that struct debugfs_fsdata's declaration can be kept
private.

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/debugfs/file.c

@@ -97,6 +97,18 @@ EXPORT_SYMBOL_GPL(debugfs_use_file_finish);
 
 #define F_DENTRY(filp) ((filp)->f_path.dentry)
 
+const struct file_operations *debugfs_real_fops(const struct file *filp)
+	__must_hold(&debugfs_srcu)
+{
+	struct debugfs_fsdata *fsd = F_DENTRY(filp)->d_fsdata;
+	/*
+	 * Neither the pointer to the struct file_operations, nor its
+	 * contents ever change -- srcu_dereference() is not needed here.
+	 */
+	return fsd->real_fops;
+}
+EXPORT_SYMBOL_GPL(debugfs_real_fops);
+
 static int open_proxy_open(struct inode *inode, struct file *filp)
 {
 	const struct dentry *dentry = F_DENTRY(filp);

fs/debugfs/inode.c

@@ -185,6 +185,11 @@ static const struct super_operations debugfs_super_operations = {
 	.evict_inode	= debugfs_evict_inode,
 };
 
+static void debugfs_release_dentry(struct dentry *dentry)
+{
+	kfree(dentry->d_fsdata);
+}
+
 static struct vfsmount *debugfs_automount(struct path *path)
 {
 	debugfs_automount_t f;
@@ -194,6 +199,7 @@ static struct vfsmount *debugfs_automount(struct path *path)
 
 static const struct dentry_operations debugfs_dops = {
 	.d_delete = always_delete_dentry,
+	.d_release = debugfs_release_dentry,
 	.d_automount = debugfs_automount,
 };
 
@@ -341,24 +347,34 @@ static struct dentry *__debugfs_create_file(const char *name, umode_t mode,
 {
 	struct dentry *dentry;
 	struct inode *inode;
+	struct debugfs_fsdata *fsd;
+
+	fsd = kmalloc(sizeof(*fsd), GFP_KERNEL);
+	if (!fsd)
+		return NULL;
 
 	if (!(mode & S_IFMT))
 		mode |= S_IFREG;
 	BUG_ON(!S_ISREG(mode));
 	dentry = start_creating(name, parent);
 
-	if (IS_ERR(dentry))
+	if (IS_ERR(dentry)) {
+		kfree(fsd);
 		return NULL;
+	}
 
 	inode = debugfs_get_inode(dentry->d_sb);
-	if (unlikely(!inode))
+	if (unlikely(!inode)) {
+		kfree(fsd);
 		return failed_creating(dentry);
+	}
 
 	inode->i_mode = mode;
 	inode->i_private = data;
 
 	inode->i_fop = proxy_fops;
-	dentry->d_fsdata = (void *)real_fops;
+	fsd->real_fops = real_fops;
+	dentry->d_fsdata = fsd;
 
 	d_instantiate(dentry, inode);
 	fsnotify_create(d_inode(dentry->d_parent), dentry);

fs/debugfs/internal.h

@@ -19,4 +19,8 @@ extern const struct file_operations debugfs_noop_file_operations;
 extern const struct file_operations debugfs_open_proxy_file_operations;
 extern const struct file_operations debugfs_full_proxy_file_operations;
 
+struct debugfs_fsdata {
+	const struct file_operations *real_fops;
+};
+
 #endif /* _DEBUGFS_INTERNAL_H_ */

include/linux/debugfs.h

@@ -45,23 +45,6 @@ extern struct dentry *arch_debugfs_dir;
 
 extern struct srcu_struct debugfs_srcu;
 
-/**
- * debugfs_real_fops - getter for the real file operation
- * @filp: a pointer to a struct file
- *
- * Must only be called under the protection established by
- * debugfs_use_file_start().
- */
-static inline const struct file_operations *debugfs_real_fops(const struct file *filp)
-	__must_hold(&debugfs_srcu)
-{
-	/*
-	 * Neither the pointer to the struct file_operations, nor its
-	 * contents ever change -- srcu_dereference() is not needed here.
-	 */
-	return filp->f_path.dentry->d_fsdata;
-}
-
 #define DEFINE_DEBUGFS_ATTRIBUTE(__fops, __get, __set, __fmt)		\
 static int __fops ## _open(struct inode *inode, struct file *file)	\
 {									\
@@ -112,6 +95,9 @@ int debugfs_use_file_start(const struct dentry *dentry, int *srcu_idx)
 
 void debugfs_use_file_finish(int srcu_idx) __releases(&debugfs_srcu);
 
+const struct file_operations *debugfs_real_fops(const struct file *filp)
+	__must_hold(&debugfs_srcu);
+
 ssize_t debugfs_attr_read(struct file *file, char __user *buf,
 			size_t len, loff_t *ppos);
 ssize_t debugfs_attr_write(struct file *file, const char __user *buf,