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>

1 files changed, 37 insertions, 5 deletions

diff --git a/lib/kobject.c b/lib/kobject.c
index 74b8dbca150..c2917ffe8bf 100644
--- a/lib/kobject.c
+++ b/lib/kobject.c
@@ -44,11 +44,11 @@ static int populate_dir(struct kobject * kobj)
 	return error;
 }
 
-static int create_dir(struct kobject * kobj)
+static int create_dir(struct kobject * kobj, struct dentry *shadow_parent)
 {
 	int error = 0;
 	if (kobject_name(kobj)) {
-		error = sysfs_create_dir(kobj);
+		error = sysfs_create_dir(kobj, shadow_parent);
 		if (!error) {
 			if ((error = populate_dir(kobj)))
 				sysfs_remove_dir(kobj);
@@ -158,9 +158,10 @@ static void unlink(struct kobject * kobj)
 /**
  *	kobject_add - add an object to the hierarchy.
  *	@kobj:	object.
+ *	@shadow_parent: sysfs directory to add to.
  */
 
-int kobject_add(struct kobject * kobj)
+int kobject_shadow_add(struct kobject * kobj, struct dentry *shadow_parent)
 {
 	int error = 0;
 	struct kobject * parent;
@@ -191,7 +192,7 @@ int kobject_add(struct kobject * kobj)
 	}
 	kobj->parent = parent;
 
-	error = create_dir(kobj);
+	error = create_dir(kobj, shadow_parent);
 	if (error) {
 		/* unlink does the kobject_put() for us */
 		unlink(kobj);
@@ -212,6 +213,15 @@ int kobject_add(struct kobject * kobj)
 	return error;
 }
 
+/**
+ *	kobject_add - add an object to the hierarchy.
+ *	@kobj:	object.
+ */
+int kobject_add(struct kobject * kobj)
+{
+	return kobject_shadow_add(kobj, NULL);
+}
+
 
 /**
  *	kobject_register - initialize and add an object.
@@ -304,7 +314,29 @@ int kobject_rename(struct kobject * kobj, const char *new_name)
 	kobj = kobject_get(kobj);
 	if (!kobj)
 		return -EINVAL;
-	error = sysfs_rename_dir(kobj, new_name);
+	if (!kobj->parent)
+		return -EINVAL;
+	error = sysfs_rename_dir(kobj, kobj->parent->dentry, new_name);
+	kobject_put(kobj);
+
+	return error;
+}
+
+/**
+ *	kobject_rename - change the name of an object
+ *	@kobj:	object in question.
+ *	@new_name: object's new name
+ */
+
+int kobject_shadow_rename(struct kobject * kobj, struct dentry *new_parent,
+			  const char *new_name)
+{
+	int error = 0;
+
+	kobj = kobject_get(kobj);
+	if (!kobj)
+		return -EINVAL;
+	error = sysfs_rename_dir(kobj, new_parent, new_name);
 	kobject_put(kobj);
 
 	return error;