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>

3 files changed, 53 insertions, 46 deletions

diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
index e9fddcc59447..2a94dc36d166 100644
--- a/fs/sysfs/dir.c
+++ b/fs/sysfs/dir.c
@@ -54,7 +54,7 @@ void release_sysfs_dirent(struct sysfs_dirent * sd)
         parent_sd = sd->s_parent;
 
         if (sd->s_type & SYSFS_KOBJ_LINK)
-                kobject_put(sd->s_elem.symlink.target_kobj);
+                sysfs_put(sd->s_elem.symlink.target_sd);
         if (sd->s_type & SYSFS_COPY_NAME)
                 kfree(sd->s_name);
         kfree(sd->s_iattr);
diff --git a/fs/sysfs/symlink.c b/fs/sysfs/symlink.c
index 27df635b786a..ff605d3f4d33 100644
--- a/fs/sysfs/symlink.c
+++ b/fs/sysfs/symlink.c
@@ -11,50 +11,49 @@
 
 #include "sysfs.h"
 
-static int object_depth(struct kobject * kobj)
+static int object_depth(struct sysfs_dirent *sd)
 {
-        struct kobject * p = kobj;
         int depth = 0;
-        do { depth++; } while ((p = p->parent));
+
+        for (; sd->s_parent; sd = sd->s_parent)
+                depth++;
+
         return depth;
 }
 
-static int object_path_length(struct kobject * kobj)
+static int object_path_length(struct sysfs_dirent * sd)
 {
-        struct kobject * p = kobj;
         int length = 1;
-        do {
-                length += strlen(kobject_name(p)) + 1;
-                p = p->parent;
-        } while (p);
+
+        for (; sd->s_parent; sd = sd->s_parent)
+                length += strlen(sd->s_name) + 1;
+
         return length;
 }
 
-static void fill_object_path(struct kobject * kobj, char * buffer, int length)
+static void fill_object_path(struct sysfs_dirent *sd, char *buffer, int length)
 {
-        struct kobject * p;
-
         --length;
-        for (p = kobj; p; p = p->parent) {
-                int cur = strlen(kobject_name(p));
+        for (; sd->s_parent; sd = sd->s_parent) {
+                int cur = strlen(sd->s_name);
 
                 /* back up enough to print this bus id with '/' */
                 length -= cur;
-                strncpy(buffer + length,kobject_name(p),cur);
+                strncpy(buffer + length, sd->s_name, cur);
                 *(buffer + --length) = '/';
         }
 }
 
-static int sysfs_add_link(struct dentry * parent, const char * name, struct kobject * target)
+static int sysfs_add_link(struct sysfs_dirent * parent_sd, const char * name,
+                          struct sysfs_dirent * target_sd)
 {
-        struct sysfs_dirent * parent_sd = parent->d_fsdata;
         struct sysfs_dirent * sd;
 
         sd = sysfs_new_dirent(name, S_IFLNK|S_IRWXUGO, SYSFS_KOBJ_LINK);
         if (!sd)
                 return -ENOMEM;
 
-        sd->s_elem.symlink.target_kobj = kobject_get(target);
+        sd->s_elem.symlink.target_sd = target_sd;
         sysfs_attach_dirent(sd, parent_sd, NULL);
         return 0;
 }
@@ -68,6 +67,8 @@ static int sysfs_add_link(struct dentry * parent, const char * name, struct kobj
 int sysfs_create_link(struct kobject * kobj, struct kobject * target, const char * name)
 {
         struct dentry *dentry = NULL;
+        struct sysfs_dirent *parent_sd = NULL;
+        struct sysfs_dirent *target_sd = NULL;
         int error = -EEXIST;
 
         BUG_ON(!name);
@@ -80,11 +81,27 @@ int sysfs_create_link(struct kobject * kobj, struct kobject * target, const char
 
         if (!dentry)
                 return -EFAULT;
+        parent_sd = dentry->d_fsdata;
+
+        /* target->dentry can go away beneath us but is protected with
+         * kobj_sysfs_assoc_lock.  Fetch target_sd from it.
+         */
+        spin_lock(&kobj_sysfs_assoc_lock);
+        if (target->dentry)
+                target_sd = sysfs_get(target->dentry->d_fsdata);
+        spin_unlock(&kobj_sysfs_assoc_lock);
+
+        if (!target_sd)
+                return -ENOENT;
 
         mutex_lock(&dentry->d_inode->i_mutex);
         if (!sysfs_dirent_exist(dentry->d_fsdata, name))
-                error = sysfs_add_link(dentry, name, target);
+                error = sysfs_add_link(parent_sd, name, target_sd);
         mutex_unlock(&dentry->d_inode->i_mutex);
+
+        if (error)
+                sysfs_put(target_sd);
+
         return error;
 }
 
@@ -100,14 +117,14 @@ void sysfs_remove_link(struct kobject * kobj, const char * name)
         sysfs_hash_and_remove(kobj->dentry,name);
 }
 
-static int sysfs_get_target_path(struct kobject * kobj, struct kobject * target,
-                                 char *path)
+static int sysfs_get_target_path(struct sysfs_dirent * parent_sd,
+                                 struct sysfs_dirent * target_sd, char *path)
 {
         char * s;
         int depth, size;
 
-        depth = object_depth(kobj);
-        size = object_path_length(target) + depth * 3 - 1;
+        depth = object_depth(parent_sd);
+        size = object_path_length(target_sd) + depth * 3 - 1;
         if (size > PATH_MAX)
                 return -ENAMETOOLONG;
 
@@ -116,7 +133,7 @@ static int sysfs_get_target_path(struct kobject * kobj, struct kobject * target,
         for (s = path; depth--; s += 3)
                 strcpy(s,"../");
 
-        fill_object_path(target, path, size);
+        fill_object_path(target_sd, path, size);
         pr_debug("%s: path = '%s'\n", __FUNCTION__, path);
 
         return 0;
@@ -124,27 +141,16 @@ static int sysfs_get_target_path(struct kobject * kobj, struct kobject * target,
 
 static int sysfs_getlink(struct dentry *dentry, char * path)
 {
-        struct kobject *kobj, *target_kobj;
-        int error = 0;
-
-        kobj = sysfs_get_kobject(dentry->d_parent);
-        if (!kobj)
-                return -EINVAL;
-
-        target_kobj = sysfs_get_kobject(dentry);
-        if (!target_kobj) {
-                kobject_put(kobj);
-                return -EINVAL;
-        }
+        struct sysfs_dirent *sd = dentry->d_fsdata;
+        struct sysfs_dirent *parent_sd = sd->s_parent;
+        struct sysfs_dirent *target_sd = sd->s_elem.symlink.target_sd;
+        int error;
 
         down_read(&sysfs_rename_sem);
-        error = sysfs_get_target_path(kobj, target_kobj, path);
+        error = sysfs_get_target_path(parent_sd, target_sd, path);
         up_read(&sysfs_rename_sem);
-        
-        kobject_put(kobj);
-        kobject_put(target_kobj);
-        return error;
 
+        return error;
 }
 
 static void *sysfs_follow_link(struct dentry *dentry, struct nameidata *nd)
diff --git a/fs/sysfs/sysfs.h b/fs/sysfs/sysfs.h
index 718e2e123fae..60717660ac55 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);