sysfs: reimplement symlink using sysfs_dirent tree

sysfs symlink is implemented by referencing dentry and kobject from
sysfs_dirent - symlink entry references kobject, dentry is used to
walk the tree.  This complicates object lifetimes rules and is
dangerous - for example, there is no way to tell to which module the
target of a symlink belongs and referencing that kobject can make it
linger after the module is gone.

This patch reimplements symlink using only sysfs_dirent tree.  sd for
a symlink points and holds reference to the target sysfs_dirent and
all walking is done using sysfs_dirent tree.  Simpler and safer.

Please read the following message for more info.

  http://article.gmane.org/gmane.linux.kernel/510293

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 5 insertions, 4 deletions

diff --git a/fs/sysfs/sysfs.h b/fs/sysfs/sysfs.h
index 718e2e1..6071766 100644
--- a/fs/sysfs/sysfs.h
+++ b/fs/sysfs/sysfs.h
@@ -3,7 +3,7 @@ struct sysfs_elem_dir {
 };
 
 struct sysfs_elem_symlink {
-	struct kobject		* target_kobj;
+	struct sysfs_dirent	* target_sd;
 };
 
 struct sysfs_elem_attr {
@@ -100,10 +100,11 @@ static inline struct kobject *sysfs_get_kobject(struct dentry *dentry)
 	spin_lock(&dcache_lock);
 	if (!d_unhashed(dentry)) {
 		struct sysfs_dirent * sd = dentry->d_fsdata;
+
 		if (sd->s_type & SYSFS_KOBJ_LINK)
-			kobj = kobject_get(sd->s_elem.symlink.target_kobj);
-		else
-			kobj = kobject_get(sd->s_elem.dir.kobj);
+			sd = sd->s_elem.symlink.target_sd;
+
+		kobj = kobject_get(sd->s_elem.dir.kobj);
 	}
 	spin_unlock(&dcache_lock);