kernfs: implement kernfs_get_parent(), kernfs_name/path() and friends

kernfs_node->parent and ->name are currently marked as "published"
indicating that kernfs users may access them directly; however, those
fields may get updated by kernfs_rename[_ns]() and unrestricted access
may lead to erroneous values or oops.

Protect ->parent and ->name updates with a irq-safe spinlock
kernfs_rename_lock and implement the following accessors for these
fields.

* kernfs_name()		- format the node's name into the specified buffer
* kernfs_path()		- format the node's path into the specified buffer
* pr_cont_kernfs_name()	- pr_cont a node's name (doesn't need buffer)
* pr_cont_kernfs_path()	- pr_cont a node's path (doesn't need buffer)
* kernfs_get_parent()	- pin and return a node's parent

All can be called under any context.  The recursive sysfs_pathname()
in fs/sysfs/dir.c is replaced with kernfs_path() and
sysfs_rename_dir_ns() is updated to use kernfs_get_parent() instead of
dereferencing parent directly.

v2: Dummy definition of kernfs_path() for !CONFIG_KERNFS was missing
    static inline making it cause a lot of build warnings.  Add it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 13 insertions, 31 deletions

diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
index ee0d761c317..0b45ff42f37 100644
--- a/fs/sysfs/dir.c
+++ b/fs/sysfs/dir.c
@@ -19,39 +19,18 @@
 
 DEFINE_SPINLOCK(sysfs_symlink_target_lock);
 
-/**
- *	sysfs_pathname - return full path to sysfs dirent
- *	@kn: kernfs_node whose path we want
- *	@path: caller allocated buffer of size PATH_MAX
- *
- *	Gives the name "/" to the sysfs_root entry; any path returned
- *	is relative to wherever sysfs is mounted.
- */
-static char *sysfs_pathname(struct kernfs_node *kn, char *path)
-{
-	if (kn->parent) {
-		sysfs_pathname(kn->parent, path);
-		strlcat(path, "/", PATH_MAX);
-	}
-	strlcat(path, kn->name, PATH_MAX);
-	return path;
-}
-
 void sysfs_warn_dup(struct kernfs_node *parent, const char *name)
 {
-	char *path;
+	char *buf, *path = NULL;
 
-	path = kzalloc(PATH_MAX, GFP_KERNEL);
-	if (path) {
-		sysfs_pathname(parent, path);
-		strlcat(path, "/", PATH_MAX);
-		strlcat(path, name, PATH_MAX);
-	}
+	buf = kzalloc(PATH_MAX, GFP_KERNEL);
+	if (buf)
+		path = kernfs_path(parent, buf, PATH_MAX);
 
-	WARN(1, KERN_WARNING "sysfs: cannot create duplicate filename '%s'\n",
-	     path ? path : name);
+	WARN(1, KERN_WARNING "sysfs: cannot create duplicate filename '%s/%s'\n",
+	     path, name);
 
-	kfree(path);
+	kfree(buf);
 }
 
 /**
@@ -122,9 +101,13 @@ void sysfs_remove_dir(struct kobject *kobj)
 int sysfs_rename_dir_ns(struct kobject *kobj, const char *new_name,
 			const void *new_ns)
 {
-	struct kernfs_node *parent = kobj->sd->parent;
+	struct kernfs_node *parent;
+	int ret;
 
-	return kernfs_rename_ns(kobj->sd, parent, new_name, new_ns);
+	parent = kernfs_get_parent(kobj->sd);
+	ret = kernfs_rename_ns(kobj->sd, parent, new_name, new_ns);
+	kernfs_put(parent);
+	return ret;
 }
 
 int sysfs_move_dir_ns(struct kobject *kobj, struct kobject *new_parent_kobj,
@@ -133,7 +116,6 @@ int sysfs_move_dir_ns(struct kobject *kobj, struct kobject *new_parent_kobj,
 	struct kernfs_node *kn = kobj->sd;
 	struct kernfs_node *new_parent;
 
-	BUG_ON(!kn->parent);
 	new_parent = new_parent_kobj && new_parent_kobj->sd ?
 		new_parent_kobj->sd : sysfs_root_kn;