Re: [RFC] fix sysfs symlinks

From: Maneesh Soni
Date: Fri Apr 30 2004 - 05:05:35 EST

On Thu, Apr 29, 2004 at 04:41:04PM +0100, viro@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx wrote:
> On Thu, Apr 29, 2004 at 06:33:53PM +0530, Maneesh Soni wrote:
>
> [snip]
>
> > @@ -167,10 +175,14 @@ void sysfs_rename_dir(struct kobject * k
> > parent = kobj->parent->dentry;
> >
> > down(&parent->d_inode->i_sem);
> > -
> > new_dentry = sysfs_get_dentry(parent, new_name);
> > - d_move(kobj->dentry, new_dentry);
> > - kobject_set_name(kobj,new_name);
> > + if (!IS_ERR(new_dentry)) {
> > + down_write(&sysfs_rename_sem);
> > + d_move(kobj->dentry, new_dentry);
> > + kobject_set_name(kobj,new_name);
> > + up_write(&sysfs_rename_sem);
> > + dput(new_dentry);
> > + }
> > up(&parent->d_inode->i_sem);
> > }
>
> I would probably lift that rwsem all way up - in front of any other locks
> in sysfs_rename_dir(). Note that kobject_set_name() can very well lead to
> allocations, so just to make the lock hierarchy cleaner... Another thing:
> please, check that new_dentry is negative here. Other than that, I've
> got no problems with the patch.

Ok... the corrected patch is appended. But I see more work to be done here
which if required can be done as a patch over this one which is not related
to sysfs symlinks. Please see the next mail.

Thanks
Maneesh



o The symlinks code in sysfs doesnot point to the correct target kobject
whenever target kobject is renamed and suffers from dangling symlinks
if target kobject is removed.

o The following patch implements ->readlink and ->follow_link operations
for sysfs instead of using the page_symlink_inode_operations.
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.
This results in generating the correct target path during readlink and
follow_link operations inspite of renamed target kobject.

o This also pins the target kobject during link creation and the ref. is
released when the link is removed.

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

o Used a rw_semaphor sysfs_rename_sem to avoid clashing with renaming of
ancestors while the target path is generated.

o Used dcache_lock in fs/sysfs/sysfs.h:sysfs_get_kobject() because of using
d_drop() while removing dentries.


fs/sysfs/dir.c | 22 +++++++-
fs/sysfs/inode.c | 7 ++
fs/sysfs/symlink.c | 135 ++++++++++++++++++++++++++++++++++++-----------------
fs/sysfs/sysfs.h | 7 +-
4 files changed, 123 insertions(+), 48 deletions(-)

diff -puN fs/sysfs/sysfs.h~sysfs-symlinks-fix fs/sysfs/sysfs.h
--- linux-2.6.6-rc2-mm2/fs/sysfs/sysfs.h~sysfs-symlinks-fix 2004-04-30 11:10:09.000000000 +0530
+++ linux-2.6.6-rc2-mm2-maneesh/fs/sysfs/sysfs.h 2004-04-30 11:10:09.000000000 +0530
@@ -12,15 +12,18 @@ 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 *, char __user *, int );
+extern int sysfs_follow_link(struct dentry *, struct nameidata *);
+extern struct rw_semaphore sysfs_rename_sem;

static inline struct kobject *sysfs_get_kobject(struct dentry *dentry)
{
struct kobject * kobj = NULL;

- spin_lock(&dentry->d_lock);
+ spin_lock(&dcache_lock);
if (!d_unhashed(dentry))
kobj = kobject_get(dentry->d_fsdata);
- spin_unlock(&dentry->d_lock);
+ spin_unlock(&dcache_lock);

return kobj;
}
diff -puN fs/sysfs/dir.c~sysfs-symlinks-fix fs/sysfs/dir.c
--- linux-2.6.6-rc2-mm2/fs/sysfs/dir.c~sysfs-symlinks-fix 2004-04-30 11:10:09.000000000 +0530
+++ linux-2.6.6-rc2-mm2-maneesh/fs/sysfs/dir.c 2004-04-30 11:39:16.000000000 +0530
@@ -10,6 +10,8 @@
#include <linux/kobject.h>
#include "sysfs.h"

+DECLARE_RWSEM(sysfs_rename_sem);
+
static int init_dir(struct inode * inode)
{
inode->i_op = &simple_dir_inode_operations;
@@ -134,8 +136,14 @@ restart:
/**
* Unlink and unhash.
*/
+ __d_drop(d);
spin_unlock(&dcache_lock);
- d_delete(d);
+ /* release the target kobject in case of
+ * a symlink
+ */
+ if (S_ISLNK(d->d_inode->i_mode))
+ kobject_put(d->d_fsdata);
+
simple_unlink(dentry->d_inode,d);
dput(d);
pr_debug(" done\n");
@@ -164,14 +172,20 @@ void sysfs_rename_dir(struct kobject * k
if (!kobj->parent)
return;

+ down_write(&sysfs_rename_sem);
parent = kobj->parent->dentry;

down(&parent->d_inode->i_sem);
-
new_dentry = sysfs_get_dentry(parent, new_name);
- d_move(kobj->dentry, new_dentry);
- kobject_set_name(kobj,new_name);
+ if (!IS_ERR(new_dentry)) {
+ if (!new_dentry->d_inode) {
+ d_move(kobj->dentry, new_dentry);
+ kobject_set_name(kobj,new_name);
+ }
+ dput(new_dentry);
+ }
up(&parent->d_inode->i_sem);
+ up_write(&sysfs_rename_sem);
}

EXPORT_SYMBOL(sysfs_create_dir);
diff -puN fs/sysfs/inode.c~sysfs-symlinks-fix fs/sysfs/inode.c
--- linux-2.6.6-rc2-mm2/fs/sysfs/inode.c~sysfs-symlinks-fix 2004-04-30 11:10:09.000000000 +0530
+++ linux-2.6.6-rc2-mm2-maneesh/fs/sysfs/inode.c 2004-04-30 11:10:09.000000000 +0530
@@ -96,7 +96,12 @@ void sysfs_hash_and_remove(struct dentry
pr_debug("sysfs: Removing %s (%d)\n", victim->d_name.name,
atomic_read(&victim->d_count));

- d_delete(victim);
+ d_drop(victim);
+ /* release the target kobject in case of
+ * a symlink
+ */
+ if (S_ISLNK(victim->d_inode->i_mode))
+ kobject_put(victim->d_fsdata);
simple_unlink(dir->d_inode,victim);
}
/*
diff -puN fs/sysfs/symlink.c~sysfs-symlinks-fix fs/sysfs/symlink.c
--- linux-2.6.6-rc2-mm2/fs/sysfs/symlink.c~sysfs-symlinks-fix 2004-04-30 11:10:09.000000000 +0530
+++ linux-2.6.6-rc2-mm2-maneesh/fs/sysfs/symlink.c 2004-04-30 11:10:09.000000000 +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 +64,20 @@ 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)
+ /*
+ * associate the link dentry with the target kobject
+ */
+ d->d_fsdata = kobject_get(target);
+ dput(d);
+ } else
error = PTR_ERR(d);
- dput(d);
up(&dentry->d_inode->i_sem);
- kfree(path);
return error;
}

@@ -120,6 +93,86 @@ 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;
+
+ 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);
+
+ for (s = path; depth--; s += 3)
+ strcpy(s,"../");
+
+ fill_object_path(target, path, size);
+ pr_debug("%s: path = '%s'\n", __FUNCTION__, path);
+
+ return 0;
+}
+
+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;
+ }
+
+ down_read(&sysfs_rename_sem);
+ error = sysfs_get_target_path(kobj, target_kobj, path);
+ up_read(&sysfs_rename_sem);
+
+ kobject_put(kobj);
+ kobject_put(target_kobj);
+ return error;
+
+}
+
+int sysfs_readlink(struct dentry *dentry, char __user *buffer, int buflen)
+{
+ int error = 0;
+ unsigned long page = get_zeroed_page(GFP_KERNEL);
+
+ if (!page)
+ return -ENOMEM;
+
+ error = sysfs_getlink(dentry, (char *) page);
+ if (!error)
+ error = vfs_readlink(dentry, buffer, buflen, (char *) page);
+
+ free_page(page);
+
+ return error;
+}
+
+int sysfs_follow_link(struct dentry *dentry, struct nameidata *nd)
+{
+ int error = 0;
+ unsigned long page = get_zeroed_page(GFP_KERNEL);
+
+ if (!page)
+ return -ENOMEM;
+
+ error = sysfs_getlink(dentry, (char *) page);
+ if (!error)
+ error = vfs_follow_link(nd, (char *) page);
+
+ free_page(page);
+
+ return error;
+}

EXPORT_SYMBOL(sysfs_create_link);
EXPORT_SYMBOL(sysfs_remove_link);

_


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