sysfs: Shadow directory support

The problem.  When implementing a network namespace I need to be able
to have multiple network devices with the same name.  Currently this
is a problem for /sys/class/net/*. 

What I want is a separate /sys/class/net directory in sysfs for each
network namespace, and I want to name each of them /sys/class/net.

I looked and the VFS actually allows that.  All that is needed is
for /sys/class/net to implement a follow link method to redirect
lookups to the real directory you want. 

Implementing a follow link method that is sensitive to the current
network namespace turns out to be 3 lines of code so it looks like a
clean approach.  Modifying sysfs so it doesn't get in my was is a bit
trickier. 

I am calling the concept of multiple directories all at the same path
in the filesystem shadow directories.  With the directory entry really
at that location the shadow master. 

The following patch modifies sysfs so it can handle a directory
structure slightly different from the kobject tree so I can implement
the shadow directories for handling /sys/class/net/.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Maneesh Soni <maneesh@in.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

fs/sysfs/dir.c

@@ -33,8 +33,7 @@ static struct dentry_operations sysfs_dentry_ops = {
 /*
  * Allocates a new sysfs_dirent and links it to the parent sysfs_dirent
  */
-static struct sysfs_dirent * sysfs_new_dirent(struct sysfs_dirent * parent_sd,
-						void * element)
+static struct sysfs_dirent * __sysfs_new_dirent(void * element)
 {
 	struct sysfs_dirent * sd;
 
@@ -46,12 +45,28 @@ static struct sysfs_dirent * sysfs_new_dirent(struct sysfs_dirent * parent_sd,
 	atomic_set(&sd->s_count, 1);
 	atomic_set(&sd->s_event, 1);
 	INIT_LIST_HEAD(&sd->s_children);
-	list_add(&sd->s_sibling, &parent_sd->s_children);
+	INIT_LIST_HEAD(&sd->s_sibling);
 	sd->s_element = element;
 
 	return sd;
 }
 
+static void __sysfs_list_dirent(struct sysfs_dirent *parent_sd,
+			      struct sysfs_dirent *sd)
+{
+	if (sd)
+		list_add(&sd->s_sibling, &parent_sd->s_children);
+}
+
+static struct sysfs_dirent * sysfs_new_dirent(struct sysfs_dirent *parent_sd,
+						void * element)
+{
+	struct sysfs_dirent *sd;
+	sd = __sysfs_new_dirent(element);
+	__sysfs_list_dirent(parent_sd, sd);
+	return sd;
+}
+
 /*
  *
  * Return -EEXIST if there is already a sysfs element with the same name for
@@ -78,14 +93,14 @@ int sysfs_dirent_exist(struct sysfs_dirent *parent_sd,
 }
 
 
-int sysfs_make_dirent(struct sysfs_dirent * parent_sd, struct dentry * dentry,
-			void * element, umode_t mode, int type)
+static struct sysfs_dirent *
+__sysfs_make_dirent(struct dentry *dentry, void *element, mode_t mode, int type)
 {
 	struct sysfs_dirent * sd;
 
-	sd = sysfs_new_dirent(parent_sd, element);
+	sd = __sysfs_new_dirent(element);
 	if (!sd)
-		return -ENOMEM;
+		goto out;
 
 	sd->s_mode = mode;
 	sd->s_type = type;
@@ -95,7 +110,19 @@ int sysfs_make_dirent(struct sysfs_dirent * parent_sd, struct dentry * dentry,
 		dentry->d_op = &sysfs_dentry_ops;
 	}
 
-	return 0;
+out:
+	return sd;
+}
+
+int sysfs_make_dirent(struct sysfs_dirent * parent_sd, struct dentry * dentry,
+			void * element, umode_t mode, int type)
+{
+	struct sysfs_dirent *sd;
+
+	sd = __sysfs_make_dirent(dentry, element, mode, type);
+	__sysfs_list_dirent(parent_sd, sd);
+
+	return sd ? 0 : -ENOMEM;
 }
 
 static int init_dir(struct inode * inode)
@@ -166,19 +193,21 @@ int sysfs_create_subdir(struct kobject * k, const char * n, struct dentry ** d)
 
 /**
  *	sysfs_create_dir - create a directory for an object.
- *	@parent:	parent parent object.
  *	@kobj:		object we're creating directory for. 
+ *	@shadow_parent:	parent parent object.
  */
 
-int sysfs_create_dir(struct kobject * kobj)
+int sysfs_create_dir(struct kobject * kobj, struct dentry *shadow_parent)
 {
 	struct dentry * dentry = NULL;
 	struct dentry * parent;
 	int error = 0;
 
 	BUG_ON(!kobj);
 
-	if (kobj->parent)
+	if (shadow_parent)
+		parent = shadow_parent;
+	else if (kobj->parent)
 		parent = kobj->parent->dentry;
 	else if (sysfs_mount && sysfs_mount->mnt_sb)
 		parent = sysfs_mount->mnt_sb->s_root;
@@ -299,21 +328,12 @@ void sysfs_remove_subdir(struct dentry * d)
 }
 
 
-/**
- *	sysfs_remove_dir - remove an object's directory.
- *	@kobj:	object. 
- *
- *	The only thing special about this is that we remove any files in 
- *	the directory before we remove the directory, and we've inlined
- *	what used to be sysfs_rmdir() below, instead of calling separately.
- */
-
-void sysfs_remove_dir(struct kobject * kobj)
+static void __sysfs_remove_dir(struct dentry *dentry)
 {
-	struct dentry * dentry = dget(kobj->dentry);
 	struct sysfs_dirent * parent_sd;
 	struct sysfs_dirent * sd, * tmp;
 
+	dget(dentry);
 	if (!dentry)
 		return;
 
@@ -334,40 +354,68 @@ void sysfs_remove_dir(struct kobject * kobj)
 	 * Drop reference from dget() on entrance.
 	 */
 	dput(dentry);
+}
+
+/**
+ *	sysfs_remove_dir - remove an object's directory.
+ *	@kobj:	object.
+ *
+ *	The only thing special about this is that we remove any files in
+ *	the directory before we remove the directory, and we've inlined
+ *	what used to be sysfs_rmdir() below, instead of calling separately.
+ */
+
+void sysfs_remove_dir(struct kobject * kobj)
+{
+	__sysfs_remove_dir(kobj->dentry);
 	kobj->dentry = NULL;
 }
 
-int sysfs_rename_dir(struct kobject * kobj, const char *new_name)
+int sysfs_rename_dir(struct kobject * kobj, struct dentry *new_parent,
+		     const char *new_name)
 {
 	int error = 0;
-	struct dentry * new_dentry, * parent;
+	struct dentry * new_dentry;
 
-	if (!strcmp(kobject_name(kobj), new_name))
-		return -EINVAL;
-
-	if (!kobj->parent)
-		return -EINVAL;
+	if (!new_parent)
+		return -EFAULT;
 
 	down_write(&sysfs_rename_sem);
-	parent = kobj->parent->dentry;
+	mutex_lock(&new_parent->d_inode->i_mutex);
 
-	mutex_lock(&parent->d_inode->i_mutex);
-
-	new_dentry = lookup_one_len(new_name, parent, strlen(new_name));
+	new_dentry = lookup_one_len(new_name, new_parent, strlen(new_name));
 	if (!IS_ERR(new_dentry)) {
-  		if (!new_dentry->d_inode) {
+		/* By allowing two different directories with the
+		 * same d_parent we allow this routine to move
+		 * between different shadows of the same directory
+		 */
+		if (kobj->dentry->d_parent->d_inode != new_parent->d_inode)
+			return -EINVAL;
+		else if (new_dentry->d_parent->d_inode != new_parent->d_inode)
+			error = -EINVAL;
+		else if (new_dentry == kobj->dentry)
+			error = -EINVAL;
+		else if (!new_dentry->d_inode) {
 			error = kobject_set_name(kobj, "%s", new_name);
 			if (!error) {
+				struct sysfs_dirent *sd, *parent_sd;
+
 				d_add(new_dentry, NULL);
 				d_move(kobj->dentry, new_dentry);
+
+				sd = kobj->dentry->d_fsdata;
+				parent_sd = new_parent->d_fsdata;
+
+				list_del_init(&sd->s_sibling);
+				list_add(&sd->s_sibling, &parent_sd->s_children);
 			}
 			else
 				d_drop(new_dentry);
 		} else
 			error = -EEXIST;
 		dput(new_dentry);
 	}
-	mutex_unlock(&parent->d_inode->i_mutex);
+	mutex_unlock(&new_parent->d_inode->i_mutex);
 	up_write(&sysfs_rename_sem);
 
 	return error;
@@ -546,6 +594,95 @@ static loff_t sysfs_dir_lseek(struct file * file, loff_t offset, int origin)
 	return offset;
 }
 
+
+/**
+ *	sysfs_make_shadowed_dir - Setup so a directory can be shadowed
+ *	@kobj:	object we're creating shadow of.
+ */
+
+int sysfs_make_shadowed_dir(struct kobject *kobj,
+	void * (*follow_link)(struct dentry *, struct nameidata *))
+{
+	struct inode *inode;
+	struct inode_operations *i_op;
+
+	inode = kobj->dentry->d_inode;
+	if (inode->i_op != &sysfs_dir_inode_operations)
+		return -EINVAL;
+
+	i_op = kmalloc(sizeof(*i_op), GFP_KERNEL);
+	if (!i_op)
+		return -ENOMEM;
+
+	memcpy(i_op, &sysfs_dir_inode_operations, sizeof(*i_op));
+	i_op->follow_link = follow_link;
+
+	/* Locking of inode->i_op?
+	 * Since setting i_op is a single word write and they
+	 * are atomic we should be ok here.
+	 */
+	inode->i_op = i_op;
+	return 0;
+}
+
+/**
+ *	sysfs_create_shadow_dir - create a shadow directory for an object.
+ *	@kobj:	object we're creating directory for.
+ *
+ *	sysfs_make_shadowed_dir must already have been called on this
+ *	directory.
+ */
+
+struct dentry *sysfs_create_shadow_dir(struct kobject *kobj)
+{
+	struct sysfs_dirent *sd;
+	struct dentry *parent, *dir, *shadow;
+	struct inode *inode;
+
+	dir = kobj->dentry;
+	inode = dir->d_inode;
+	parent = dir->d_parent;
+	shadow = ERR_PTR(-EINVAL);
+	if (!sysfs_is_shadowed_inode(inode))
+		goto out;
+
+	shadow = d_alloc(parent, &dir->d_name);
+	if (!shadow)
+		goto nomem;
+
+	sd = __sysfs_make_dirent(shadow, kobj, inode->i_mode, SYSFS_DIR);
+	if (!sd)
+		goto nomem;
+
+	d_instantiate(shadow, igrab(inode));
+	inc_nlink(inode);
+	inc_nlink(parent->d_inode);
+	shadow->d_op = &sysfs_dentry_ops;
+
+	dget(shadow);		/* Extra count - pin the dentry in core */
+
+out:
+	return shadow;
+nomem:
+	dput(shadow);
+	shadow = ERR_PTR(-ENOMEM);
+	goto out;
+}
+
+/**
+ *	sysfs_remove_shadow_dir - remove an object's directory.
+ *	@shadow: dentry of shadow directory
+ *
+ *	The only thing special about this is that we remove any files in
+ *	the directory before we remove the directory, and we've inlined
+ *	what used to be sysfs_rmdir() below, instead of calling separately.
+ */
+
+void sysfs_remove_shadow_dir(struct dentry *shadow)
+{
+	__sysfs_remove_dir(shadow);
+}
+
 const struct file_operations sysfs_dir_operations = {
 	.open		= sysfs_dir_open,
 	.release	= sysfs_dir_close,

fs/sysfs/group.c

@@ -13,6 +13,7 @@
 #include <linux/dcache.h>
 #include <linux/namei.h>
 #include <linux/err.h>
+#include <linux/fs.h>
 #include <asm/semaphore.h>
 #include "sysfs.h"
 

fs/sysfs/inode.c

@@ -33,6 +33,16 @@ static struct inode_operations sysfs_inode_operations ={
 	.setattr	= sysfs_setattr,
 };
 
+void sysfs_delete_inode(struct inode *inode)
+{
+	/* Free the shadowed directory inode operations */
+	if (sysfs_is_shadowed_inode(inode)) {
+		kfree(inode->i_op);
+		inode->i_op = NULL;
+	}
+	return generic_delete_inode(inode);
+}
+
 int sysfs_setattr(struct dentry * dentry, struct iattr * iattr)
 {
 	struct inode * inode = dentry->d_inode;

fs/sysfs/mount.c

@@ -23,7 +23,7 @@ static void sysfs_clear_inode(struct inode *inode);
 
 static struct super_operations sysfs_ops = {
 	.statfs		= simple_statfs,
-	.drop_inode	= generic_delete_inode,
+	.drop_inode	= sysfs_delete_inode,
 	.clear_inode	= sysfs_clear_inode,
 };
 

fs/sysfs/sysfs.h

@@ -2,6 +2,7 @@
 extern struct vfsmount * sysfs_mount;
 extern struct kmem_cache *sysfs_dir_cachep;
 
+extern void sysfs_delete_inode(struct inode *inode);
 extern struct inode * sysfs_new_inode(mode_t mode, struct sysfs_dirent *);
 extern int sysfs_create(struct dentry *, int mode, int (*init)(struct inode *));
 
@@ -112,3 +113,7 @@ static inline void sysfs_put(struct sysfs_dirent * sd)
 		release_sysfs_dirent(sd);
 }
 
+static inline int sysfs_is_shadowed_inode(struct inode *inode)
+{
+	return S_ISDIR(inode->i_mode) && inode->i_op->follow_link;
+}

include/linux/kobject.h

@@ -74,9 +74,13 @@ extern void kobject_init(struct kobject *);
 extern void kobject_cleanup(struct kobject *);
 
 extern int __must_check kobject_add(struct kobject *);
+extern int __must_check kobject_shadow_add(struct kobject *, struct dentry *);
 extern void kobject_del(struct kobject *);
 
 extern int __must_check kobject_rename(struct kobject *, const char *new_name);
+extern int __must_check kobject_shadow_rename(struct kobject *kobj,
+						struct dentry *new_parent,
+						const char *new_name);
 extern int __must_check kobject_move(struct kobject *, struct kobject *);
 
 extern int __must_check kobject_register(struct kobject *);