[RFC] fix sysfs symlinks

Maneesh Soni
2004-04-13T12:36:18Z

Hi,

As pointed by Al Viro, the current symlinks support in sysfs is incorrect as
it always sees the old target if target is renamed and obviously does not
follow the new target. The page symlink operations as used by current sysfs
code always see the target information at the time of creation.

Thanks
Maneesh

o The following patch implements ->readlink and ->follow_link operations
for sysfs. The pointer to target kobject is saved in the link dentry's
d_fsdata field. The target path is generated everytime we do ->readlink
and ->follow_link.

o Apart from being correct this patch also saves some memory by not using
a whole page for saving the target information.

fs/sysfs/symlink.c | 136 +++++++++++++++++++++++++++++++++++++----------------
fs/sysfs/sysfs.h | 16 ++++++
2 files changed, 112 insertions(+), 40 deletions(-)

diff -puN fs/sysfs/symlink.c~sysfs-symlinks-fix fs/sysfs/symlink.c
--- linux-2.6.5/fs/sysfs/symlink.c~sysfs-symlinks-fix 2004-04-12 18:43:15.000000000 +0530
+++ linux-2.6.5-maneesh/fs/sysfs/symlink.c 2004-04-13 17:58:15.000000000 +0530
@@ -8,27 +8,17 @@

#include "sysfs.h"

+static struct inode_operations sysfs_symlink_inode_operations = {
+ .readlink = sysfs_readlink,
+ .follow_link = sysfs_follow_link,
+};

static int init_symlink(struct inode * inode)
{
- inode->i_op = &page_symlink_inode_operations;
+ inode->i_op = &sysfs_symlink_inode_operations;
return 0;
}

-static int sysfs_symlink(struct inode * dir, struct dentry *dentry, const char * symname)
-{
- int error;
-
- error = sysfs_create(dentry, S_IFLNK|S_IRWXUGO, init_symlink);
- if (!error) {
- int l = strlen(symname)+1;
- error = page_symlink(dentry->d_inode, symname, l);
- if (error)
- iput(dentry->d_inode);
- }
- return error;
-}
-
static int object_depth(struct kobject * kobj)
{
struct kobject * p = kobj;
@@ -74,37 +64,17 @@ int sysfs_create_link(struct kobject * k
struct dentry * dentry = kobj->dentry;
struct dentry * d;
int error = 0;
- int size;
- int depth;
- char * path;
- char * s;
-
- depth = object_depth(kobj);
- size = object_path_length(target) + depth * 3 - 1;
- if (size > PATH_MAX)
- return -ENAMETOOLONG;
- pr_debug("%s: depth = %d, size = %d\n",__FUNCTION__,depth,size);
-
- path = kmalloc(size,GFP_KERNEL);
- if (!path)
- return -ENOMEM;
- memset(path,0,size);
-
- for (s = path; depth--; s += 3)
- strcpy(s,"../");
-
- fill_object_path(target,path,size);
- pr_debug("%s: path = '%s'\n",__FUNCTION__,path);

down(&dentry->d_inode->i_sem);
d = sysfs_get_dentry(dentry,name);
- if (!IS_ERR(d))
- error = sysfs_symlink(dentry->d_inode,d,path);
- else
+ if (!IS_ERR(d)) {
+ error = sysfs_create(d, S_IFLNK|S_IRWXUGO, init_symlink);
+ if (!error)
+ d->d_fsdata = target;
+ } else
error = PTR_ERR(d);
dput(d);
up(&dentry->d_inode->i_sem);
- kfree(path);
return error;
}

@@ -120,6 +90,92 @@ void sysfs_remove_link(struct kobject *
sysfs_hash_and_remove(kobj->dentry,name);
}

+static int sysfs_get_target_path(struct kobject * kobj, struct kobject * target,
+ char **path)
+{
+ char * s;
+ int depth, size;
+ int error = 0;
+
+ depth = object_depth(kobj);
+ size = object_path_length(target) + depth * 3 - 1;
+ if (size > PATH_MAX)
+ return -ENAMETOOLONG;
+
+ pr_debug("%s: depth = %d, size = %d\n",__FUNCTION__,depth,size);
+
+ *path = kmalloc(size, GFP_KERNEL);
+ if (!*path)
+ return -ENOMEM;
+
+ memset(*path, 0, size);
+
+ for (s = *path; depth--; s += 3)
+ strcpy(s,"../");
+
+ fill_object_path(target, *path, size);
+ pr_debug("%s: path = '%s'\n",__FUNCTION__, *path);
+
+ return error;
+}
+
+static char * sysfs_getlink(struct dentry *dentry)
+{
+ struct kobject *kobj, *target_kobj;
+ struct dentry * target_parent;
+ char *path;
+ int error = 0;
+
+ kobj = sysfs_get_kobject(dentry->d_parent);
+ if (!kobj)
+ return ERR_PTR(-EINVAL);
+
+ target_kobj = sysfs_get_kobject(dentry);
+ if (!target_kobj) {
+ kobject_put(kobj);
+ return ERR_PTR(-EINVAL);
+ }
+ target_parent = target_kobj->dentry->d_parent;
+
+ down(&target_parent->d_inode->i_sem);
+ error = sysfs_get_target_path(kobj, target_kobj, &path);
+ up(&target_parent->d_inode->i_sem);
+
+ kobject_put(kobj);
+ kobject_put(target_kobj);
+ if (error)
+ return ERR_PTR(error);
+
+ return path;
+}
+
+int sysfs_readlink(struct dentry *dentry, char __user *buffer, int buflen)
+{
+ int res = 0;
+ char * link = sysfs_getlink(dentry);
+
+ if (!IS_ERR(link)) {
+ res = vfs_readlink(dentry, buffer, buflen, link);
+ kfree(link);
+ return res;
+ }
+
+ return PTR_ERR(link);
+}
+
+int sysfs_follow_link(struct dentry *dentry, struct nameidata *nd)
+{
+ int res = 0;
+ char *link = sysfs_getlink(dentry);
+
+ if (!IS_ERR(link)) {
+ res = vfs_follow_link(nd, link);
+ kfree(link);
+ return res;
+ }
+
+ return PTR_ERR(link);
+}

EXPORT_SYMBOL(sysfs_create_link);
EXPORT_SYMBOL(sysfs_remove_link);
diff -puN fs/sysfs/sysfs.h~sysfs-symlinks-fix fs/sysfs/sysfs.h
--- linux-2.6.5/fs/sysfs/sysfs.h~sysfs-symlinks-fix 2004-04-13 12:26:45.000000000 +0530
+++ linux-2.6.5-maneesh/fs/sysfs/sysfs.h 2004-04-13 17:58:24.000000000 +0530
@@ -11,3 +11,19 @@ extern void sysfs_hash_and_remove(struct

extern int sysfs_create_subdir(struct kobject *, const char *, struct dentry **);
extern void sysfs_remove_subdir(struct dentry *);
+
+extern int sysfs_readlink(struct dentry *dentry, char __user *buffer, int buflen);
+extern int sysfs_follow_link(struct dentry *dentry, struct nameidata *nd);
+
+static inline struct kobject * sysfs_get_kobject(struct dentry * dentry)
+{
+ struct kobject * kobj = NULL;
+
+ spin_lock(&dentry->d_lock);
+ if (!d_unhashed(dentry))
+ kobj = kobject_get(dentry->d_fsdata);
+ spin_unlock(&dentry->d_lock);
+
+ return kobj;
+}
+

_
--
Maneesh Soni
Linux Technology Center,
IBM Software Lab, Bangalore, India
email: maneesh@in.ibm.com
Phone: 91-80-25044999 Fax: 91-80-25268553
T/L : 9243696