android_kernel_samsung_univ.../fs/sysfs/symlink.c
Tejun Heo 0cae60f914 sysfs: rename sysfs_assoc_lock and explain what it's about
sysfs_assoc_lock is an odd piece of locking.  In general, whoever owns
a kobject is responsible for synchronizing sysfs operations and sysfs
proper assumes that, for example, removal won't race with any other
operation; however, this doesn't work for symlinking because an entity
performing symlink doesn't usually own the target kobject and thus has
no control over its removal.

sysfs_assoc_lock synchronizes symlink operations against kobj->sd
disassociation so that symlink code doesn't end up dereferencing
already freed sysfs_dirent by racing with removal of the target
kobject.

This is quite obscure and the generic name of the lock and lack of
comments make it difficult to understand its role.  Let's rename it to
sysfs_symlink_target_lock and add comments explaining what's going on.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2013-11-01 12:13:37 -07:00

316 lines
7.4 KiB
C

/*
* fs/sysfs/symlink.c - sysfs symlink implementation
*
* Copyright (c) 2001-3 Patrick Mochel
* Copyright (c) 2007 SUSE Linux Products GmbH
* Copyright (c) 2007 Tejun Heo <teheo@suse.de>
*
* This file is released under the GPLv2.
*
* Please see Documentation/filesystems/sysfs.txt for more information.
*/
#include <linux/fs.h>
#include <linux/gfp.h>
#include <linux/mount.h>
#include <linux/module.h>
#include <linux/kobject.h>
#include <linux/namei.h>
#include <linux/mutex.h>
#include <linux/security.h>
#include "sysfs.h"
static int sysfs_do_create_link_sd(struct sysfs_dirent *parent_sd,
struct kobject *target,
const char *name, int warn)
{
struct sysfs_dirent *target_sd = NULL;
struct sysfs_dirent *sd = NULL;
struct sysfs_addrm_cxt acxt;
int error;
BUG_ON(!name || !parent_sd);
/*
* We don't own @target and it may be removed at any time.
* Synchronize using sysfs_symlink_target_lock. See
* sysfs_remove_dir() for details.
*/
spin_lock(&sysfs_symlink_target_lock);
if (target->sd)
target_sd = sysfs_get(target->sd);
spin_unlock(&sysfs_symlink_target_lock);
error = -ENOENT;
if (!target_sd)
goto out_put;
error = -ENOMEM;
sd = sysfs_new_dirent(name, S_IFLNK|S_IRWXUGO, SYSFS_KOBJ_LINK);
if (!sd)
goto out_put;
sd->s_ns = target_sd->s_ns;
sd->s_symlink.target_sd = target_sd;
target_sd = NULL; /* reference is now owned by the symlink */
sysfs_addrm_start(&acxt);
if (warn)
error = sysfs_add_one(&acxt, sd, parent_sd);
else
error = __sysfs_add_one(&acxt, sd, parent_sd);
sysfs_addrm_finish(&acxt);
if (error)
goto out_put;
return 0;
out_put:
sysfs_put(target_sd);
sysfs_put(sd);
return error;
}
/**
* sysfs_create_link_sd - create symlink to a given object.
* @sd: directory we're creating the link in.
* @target: object we're pointing to.
* @name: name of the symlink.
*/
int sysfs_create_link_sd(struct sysfs_dirent *sd, struct kobject *target,
const char *name)
{
return sysfs_do_create_link_sd(sd, target, name, 1);
}
static int sysfs_do_create_link(struct kobject *kobj, struct kobject *target,
const char *name, int warn)
{
struct sysfs_dirent *parent_sd = NULL;
if (!kobj)
parent_sd = &sysfs_root;
else
parent_sd = kobj->sd;
if (!parent_sd)
return -EFAULT;
return sysfs_do_create_link_sd(parent_sd, target, name, warn);
}
/**
* sysfs_create_link - create symlink between two objects.
* @kobj: object whose directory we're creating the link in.
* @target: object we're pointing to.
* @name: name of the symlink.
*/
int sysfs_create_link(struct kobject *kobj, struct kobject *target,
const char *name)
{
return sysfs_do_create_link(kobj, target, name, 1);
}
EXPORT_SYMBOL_GPL(sysfs_create_link);
/**
* sysfs_create_link_nowarn - create symlink between two objects.
* @kobj: object whose directory we're creating the link in.
* @target: object we're pointing to.
* @name: name of the symlink.
*
* This function does the same as sysfs_create_link(), but it
* doesn't warn if the link already exists.
*/
int sysfs_create_link_nowarn(struct kobject *kobj, struct kobject *target,
const char *name)
{
return sysfs_do_create_link(kobj, target, name, 0);
}
/**
* sysfs_delete_link - remove symlink in object's directory.
* @kobj: object we're acting for.
* @targ: object we're pointing to.
* @name: name of the symlink to remove.
*
* Unlike sysfs_remove_link sysfs_delete_link has enough information
* to successfully delete symlinks in tagged directories.
*/
void sysfs_delete_link(struct kobject *kobj, struct kobject *targ,
const char *name)
{
const void *ns = NULL;
/*
* We don't own @target and it may be removed at any time.
* Synchronize using sysfs_symlink_target_lock. See
* sysfs_remove_dir() for details.
*/
spin_lock(&sysfs_symlink_target_lock);
if (targ->sd)
ns = targ->sd->s_ns;
spin_unlock(&sysfs_symlink_target_lock);
sysfs_hash_and_remove(kobj->sd, name, ns);
}
/**
* sysfs_remove_link - remove symlink in object's directory.
* @kobj: object we're acting for.
* @name: name of the symlink to remove.
*/
void sysfs_remove_link(struct kobject *kobj, const char *name)
{
struct sysfs_dirent *parent_sd = NULL;
if (!kobj)
parent_sd = &sysfs_root;
else
parent_sd = kobj->sd;
sysfs_hash_and_remove(parent_sd, name, NULL);
}
EXPORT_SYMBOL_GPL(sysfs_remove_link);
/**
* sysfs_rename_link_ns - rename symlink in object's directory.
* @kobj: object we're acting for.
* @targ: object we're pointing to.
* @old: previous name of the symlink.
* @new: new name of the symlink.
* @new_ns: new namespace of the symlink.
*
* A helper function for the common rename symlink idiom.
*/
int sysfs_rename_link_ns(struct kobject *kobj, struct kobject *targ,
const char *old, const char *new, const void *new_ns)
{
struct sysfs_dirent *parent_sd, *sd = NULL;
const void *old_ns = NULL;
int result;
if (!kobj)
parent_sd = &sysfs_root;
else
parent_sd = kobj->sd;
if (targ->sd)
old_ns = targ->sd->s_ns;
result = -ENOENT;
sd = sysfs_get_dirent_ns(parent_sd, old, old_ns);
if (!sd)
goto out;
result = -EINVAL;
if (sysfs_type(sd) != SYSFS_KOBJ_LINK)
goto out;
if (sd->s_symlink.target_sd->s_dir.kobj != targ)
goto out;
result = sysfs_rename(sd, parent_sd, new, new_ns);
out:
sysfs_put(sd);
return result;
}
EXPORT_SYMBOL_GPL(sysfs_rename_link_ns);
static int sysfs_get_target_path(struct sysfs_dirent *parent_sd,
struct sysfs_dirent *target_sd, char *path)
{
struct sysfs_dirent *base, *sd;
char *s = path;
int len = 0;
/* go up to the root, stop at the base */
base = parent_sd;
while (base->s_parent) {
sd = target_sd->s_parent;
while (sd->s_parent && base != sd)
sd = sd->s_parent;
if (base == sd)
break;
strcpy(s, "../");
s += 3;
base = base->s_parent;
}
/* determine end of target string for reverse fillup */
sd = target_sd;
while (sd->s_parent && sd != base) {
len += strlen(sd->s_name) + 1;
sd = sd->s_parent;
}
/* check limits */
if (len < 2)
return -EINVAL;
len--;
if ((s - path) + len > PATH_MAX)
return -ENAMETOOLONG;
/* reverse fillup of target string from target to base */
sd = target_sd;
while (sd->s_parent && sd != base) {
int slen = strlen(sd->s_name);
len -= slen;
strncpy(s + len, sd->s_name, slen);
if (len)
s[--len] = '/';
sd = sd->s_parent;
}
return 0;
}
static int sysfs_getlink(struct dentry *dentry, char *path)
{
struct sysfs_dirent *sd = dentry->d_fsdata;
struct sysfs_dirent *parent_sd = sd->s_parent;
struct sysfs_dirent *target_sd = sd->s_symlink.target_sd;
int error;
mutex_lock(&sysfs_mutex);
error = sysfs_get_target_path(parent_sd, target_sd, path);
mutex_unlock(&sysfs_mutex);
return error;
}
static void *sysfs_follow_link(struct dentry *dentry, struct nameidata *nd)
{
int error = -ENOMEM;
unsigned long page = get_zeroed_page(GFP_KERNEL);
if (page) {
error = sysfs_getlink(dentry, (char *) page);
if (error < 0)
free_page((unsigned long)page);
}
nd_set_link(nd, error ? ERR_PTR(error) : (char *)page);
return NULL;
}
static void sysfs_put_link(struct dentry *dentry, struct nameidata *nd,
void *cookie)
{
char *page = nd_get_link(nd);
if (!IS_ERR(page))
free_page((unsigned long)page);
}
const struct inode_operations sysfs_symlink_inode_operations = {
.setxattr = sysfs_setxattr,
.readlink = generic_readlink,
.follow_link = sysfs_follow_link,
.put_link = sysfs_put_link,
.setattr = sysfs_setattr,
.getattr = sysfs_getattr,
.permission = sysfs_permission,
};