[PATCH 10/14] sysfs: reimplement symlink using sysfs_dirent tree

Tejun Heo
2007-04-07T08:29:12Z

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>
---
fs/sysfs/dir.c | 9 +++--
fs/sysfs/symlink.c | 88 +++++++++++++++++++++++++++------------------------
fs/sysfs/sysfs.h | 2 +-
3 files changed, 53 insertions(+), 46 deletions(-)

diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
index 8372c0c..b7d0e0e 100644
--- a/fs/sysfs/dir.c
+++ b/fs/sysfs/dir.c
@@ -22,10 +22,11 @@ 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);

@@ -40,7 +41,7 @@ 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
--- 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 9dcd0b0..b9acf0e 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 {
--
1.5.0.3