[PATCH 55/61] sysfs: only allow one scheduled removal callback per kobj

Greg Kroah-Hartman
2009-03-25T00:50:04Z

From: Alex Chiang <achiang@hp.com>

The only way for a sysfs attribute to remove itself (without
deadlock) is to use the sysfs_schedule_callback() interface.

Vegard Nossum discovered that a poorly written sysfs ->store
callback can repeatedly schedule remove callbacks on the same
device over and over, e.g.

$ while true ; do echo 1 > /sys/devices/.../remove ; done

If the 'remove' attribute uses the sysfs_schedule_callback API
and also does not protect itself from concurrent accesses, its
callback handler will be called multiple times, and will
eventually attempt to perform operations on a freed kobject,
leading to many problems.

Instead of requiring all callers of sysfs_schedule_callback to
implement their own synchronization, provide the protection in
the infrastructure.

Now, sysfs_schedule_callback will only allow one scheduled
callback per kobject. On subsequent calls with the same kobject,
return -EAGAIN.

This is a short term fix. The long term fix is to allow sysfs
attributes to remove themselves directly, without any of this
callback hokey pokey.

[cornelia.huck@de.ibm.com: s390 ccwgroup bits]

Reported-by: vegard.nossum@gmail.com
Signed-off-by: Alex Chiang <achiang@hp.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
drivers/s390/cio/ccwgroup.c | 5 +++--
fs/sysfs/file.c | 26 +++++++++++++++++++++++---
2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/drivers/s390/cio/ccwgroup.c b/drivers/s390/cio/ccwgroup.c
index 918e6fc..b91c171 100644
--- a/drivers/s390/cio/ccwgroup.c
+++ b/drivers/s390/cio/ccwgroup.c
@@ -104,8 +104,9 @@ ccwgroup_ungroup_store(struct device *dev, struct device_attribute *attr, const
rc = device_schedule_callback(dev, ccwgroup_ungroup_callback);
out:
if (rc) {
- /* Release onoff "lock" when ungrouping failed. */
- atomic_set(&gdev->onoff, 0);
+ if (rc != -EAGAIN)
+ /* Release onoff "lock" when ungrouping failed. */
+ atomic_set(&gdev->onoff, 0);
return rc;
}
return count;
diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 1f4a3f8..289c43a 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -659,13 +659,16 @@ void sysfs_remove_file_from_group(struct kobject *kobj,
EXPORT_SYMBOL_GPL(sysfs_remove_file_from_group);

struct sysfs_schedule_callback_struct {
- struct kobject *kobj;
+ struct list_head workq_list;
+ struct kobject *kobj;
void (*func)(void *);
void *data;
struct module *owner;
struct work_struct work;
};

+static DEFINE_MUTEX(sysfs_workq_mutex);
+static LIST_HEAD(sysfs_workq);
static void sysfs_schedule_callback_work(struct work_struct *work)
{
struct sysfs_schedule_callback_struct *ss = container_of(work,
@@ -674,6 +677,9 @@ static void sysfs_schedule_callback_work(struct work_struct *work)
(ss->func)(ss->data);
kobject_put(ss->kobj);
module_put(ss->owner);
+ mutex_lock(&sysfs_workq_mutex);
+ list_del(&ss->workq_list);
+ mutex_unlock(&sysfs_workq_mutex);
kfree(ss);
}

@@ -695,15 +701,25 @@ static void sysfs_schedule_callback_work(struct work_struct *work)
* until @func returns.
*
* Returns 0 if the request was submitted, -ENOMEM if storage could not
- * be allocated, -ENODEV if a reference to @owner isn't available.
+ * be allocated, -ENODEV if a reference to @owner isn't available,
+ * -EAGAIN if a callback has already been scheduled for @kobj.
*/
int sysfs_schedule_callback(struct kobject *kobj, void (*func)(void *),
void *data, struct module *owner)
{
- struct sysfs_schedule_callback_struct *ss;
+ struct sysfs_schedule_callback_struct *ss, *tmp;

if (!try_module_get(owner))
return -ENODEV;
+
+ mutex_lock(&sysfs_workq_mutex);
+ list_for_each_entry_safe(ss, tmp, &sysfs_workq, workq_list)
+ if (ss->kobj == kobj) {
+ mutex_unlock(&sysfs_workq_mutex);
+ return -EAGAIN;
+ }
+ mutex_unlock(&sysfs_workq_mutex);
+
ss = kmalloc(sizeof(*ss), GFP_KERNEL);
if (!ss) {
module_put(owner);
@@ -715,6 +731,10 @@ int sysfs_schedule_callback(struct kobject *kobj, void (*func)(void *),
ss->data = data;
ss->owner = owner;
INIT_WORK(&ss->work, sysfs_schedule_callback_work);
+ INIT_LIST_HEAD(&ss->workq_list);
+ mutex_lock(&sysfs_workq_mutex);
+ list_add_tail(&ss->workq_list, &sysfs_workq);
+ mutex_unlock(&sysfs_workq_mutex);
schedule_work(&ss->work);
return 0;
}
--
1.6.2