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>

diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
index e9fddcc..2a94dc3 100644 (file)
--- 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 27df635..ff605d3 100644 (file)
--- 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 718e2e1..6071766 100644 (file)
--- 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);