linux_dsm_epyc7002/drivers/media/video/v4l2-event.c
Hans de Goede e3e72f39b6 [media] v4l2-event: Don't set sev->fh to NULL on unsubscribe
Setting sev->fh to NULL causes problems for the del op added in the next
patch of this series, since this op needs a way to get to its own data
structures, and typically this will be done by using container_of on an
embedded v4l2_fh struct.

The reason the original code is setting sev->fh to NULL is to signal
to users of the event framework that the unsubscription has happened,
but since their is no shared lock between the event framework and users
of it, this is inherently racy, and it also turns out to be unnecessary
as long as both the event framework and the user of the framework do their
own locking properly and the user guarantees that it holds no references
to the subcribed_event structure after its del operation has been called.

This is best explained by looking at the only code currently checking for
sev->fh being set to NULL on unsubscribe, which is the v4l2-ctrls.c send_event
function. Here is the relevant code from v4l2-ctrls: send_event():

	if (sev->fh && (sev->fh != fh ||
			(sev->flags & V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK)))
		v4l2_event_queue_fh(sev->fh, &ev);

Now lets say that v4l2_event_unsubscribe and v4l2-ctrls: send_event() race
on the same sev, then the following could happens:

1) send_event checks sev->fh, finds it is not NULL
<thread switch>
2) v4l2_event_unsubscribe sets sev->fh NULL
3) v4l2_event_unsubscribe calls v4l2_ctrls del_event function, this blocks
   as the thread calling send_event holds the ctrl_lock
<thread switch>
4) send_event calls v4l2_event_queue_fh(sev->fh, &ev) which not is equivalent
   to calling: v4l2_event_queue_fh(NULL, &ev)
5) oops, NULL pointer deref.

Now again without setting sev->fh to NULL in v4l2_event_unsubscribe and
without the (now senseless since always true) sev->fh != NULL check in

1) send_event is about to call v4l2_event_queue_fh(sev->fh, &ev)
<thread switch>
2) v4l2_event_unsubscribe removes sev->list from the fh->subscribed list
<thread switch>
3) send_event calls v4l2_event_queue_fh(sev->fh, &ev)
4) v4l2_event_queue_fh blocks on the fh_lock spinlock
<thread switch>
5) v4l2_event_unsubscribe unlocks the fh_lock spinlock
6) v4l2_event_unsubscribe calls v4l2_ctrls del_event function, this blocks
   as the thread calling send_event holds the ctrl_lock
<thread switch>
8) v4l2_event_queue_fh takes the fh_lock
7) v4l2_event_queue_fh calls v4l2_event_subscribed, does not find it since
   sev->list has been removed from fh->subscribed already -> does nothing
9) v4l2_event_queue_fh releases the fh_lock
10) the caller of send_event releases the ctrl lock (mutex)
<thread switch>
11) v4l2_ctrls del_event takes the ctrl lock
12) v4l2_ctrls del_event removes sev->node from the ev_subs list
13) v4l2_ctrls del_event releases the ctrl lock
14) v4l2_event_unsubscribe frees the sev, to which no references are being
    held anymore

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
2011-11-08 10:29:51 -02:00

320 lines
7.5 KiB
C

/*
* v4l2-event.c
*
* V4L2 events.
*
* Copyright (C) 2009--2010 Nokia Corporation.
*
* Contact: Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* version 2 as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
* 02110-1301 USA
*/
#include <media/v4l2-dev.h>
#include <media/v4l2-fh.h>
#include <media/v4l2-event.h>
#include <media/v4l2-ctrls.h>
#include <linux/sched.h>
#include <linux/slab.h>
static unsigned sev_pos(const struct v4l2_subscribed_event *sev, unsigned idx)
{
idx += sev->first;
return idx >= sev->elems ? idx - sev->elems : idx;
}
static int __v4l2_event_dequeue(struct v4l2_fh *fh, struct v4l2_event *event)
{
struct v4l2_kevent *kev;
unsigned long flags;
spin_lock_irqsave(&fh->vdev->fh_lock, flags);
if (list_empty(&fh->available)) {
spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
return -ENOENT;
}
WARN_ON(fh->navailable == 0);
kev = list_first_entry(&fh->available, struct v4l2_kevent, list);
list_del(&kev->list);
fh->navailable--;
kev->event.pending = fh->navailable;
*event = kev->event;
kev->sev->first = sev_pos(kev->sev, 1);
kev->sev->in_use--;
spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
return 0;
}
int v4l2_event_dequeue(struct v4l2_fh *fh, struct v4l2_event *event,
int nonblocking)
{
int ret;
if (nonblocking)
return __v4l2_event_dequeue(fh, event);
/* Release the vdev lock while waiting */
if (fh->vdev->lock)
mutex_unlock(fh->vdev->lock);
do {
ret = wait_event_interruptible(fh->wait,
fh->navailable != 0);
if (ret < 0)
break;
ret = __v4l2_event_dequeue(fh, event);
} while (ret == -ENOENT);
if (fh->vdev->lock)
mutex_lock(fh->vdev->lock);
return ret;
}
EXPORT_SYMBOL_GPL(v4l2_event_dequeue);
/* Caller must hold fh->vdev->fh_lock! */
static struct v4l2_subscribed_event *v4l2_event_subscribed(
struct v4l2_fh *fh, u32 type, u32 id)
{
struct v4l2_subscribed_event *sev;
assert_spin_locked(&fh->vdev->fh_lock);
list_for_each_entry(sev, &fh->subscribed, list)
if (sev->type == type && sev->id == id)
return sev;
return NULL;
}
static void __v4l2_event_queue_fh(struct v4l2_fh *fh, const struct v4l2_event *ev,
const struct timespec *ts)
{
struct v4l2_subscribed_event *sev;
struct v4l2_kevent *kev;
bool copy_payload = true;
/* Are we subscribed? */
sev = v4l2_event_subscribed(fh, ev->type, ev->id);
if (sev == NULL)
return;
/* Increase event sequence number on fh. */
fh->sequence++;
/* Do we have any free events? */
if (sev->in_use == sev->elems) {
/* no, remove the oldest one */
kev = sev->events + sev_pos(sev, 0);
list_del(&kev->list);
sev->in_use--;
sev->first = sev_pos(sev, 1);
fh->navailable--;
if (sev->elems == 1) {
if (sev->replace) {
sev->replace(&kev->event, ev);
copy_payload = false;
}
} else if (sev->merge) {
struct v4l2_kevent *second_oldest =
sev->events + sev_pos(sev, 0);
sev->merge(&kev->event, &second_oldest->event);
}
}
/* Take one and fill it. */
kev = sev->events + sev_pos(sev, sev->in_use);
kev->event.type = ev->type;
if (copy_payload)
kev->event.u = ev->u;
kev->event.id = ev->id;
kev->event.timestamp = *ts;
kev->event.sequence = fh->sequence;
sev->in_use++;
list_add_tail(&kev->list, &fh->available);
fh->navailable++;
wake_up_all(&fh->wait);
}
void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event *ev)
{
struct v4l2_fh *fh;
unsigned long flags;
struct timespec timestamp;
ktime_get_ts(&timestamp);
spin_lock_irqsave(&vdev->fh_lock, flags);
list_for_each_entry(fh, &vdev->fh_list, list)
__v4l2_event_queue_fh(fh, ev, &timestamp);
spin_unlock_irqrestore(&vdev->fh_lock, flags);
}
EXPORT_SYMBOL_GPL(v4l2_event_queue);
void v4l2_event_queue_fh(struct v4l2_fh *fh, const struct v4l2_event *ev)
{
unsigned long flags;
struct timespec timestamp;
ktime_get_ts(&timestamp);
spin_lock_irqsave(&fh->vdev->fh_lock, flags);
__v4l2_event_queue_fh(fh, ev, &timestamp);
spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
}
EXPORT_SYMBOL_GPL(v4l2_event_queue_fh);
int v4l2_event_pending(struct v4l2_fh *fh)
{
return fh->navailable;
}
EXPORT_SYMBOL_GPL(v4l2_event_pending);
static void ctrls_replace(struct v4l2_event *old, const struct v4l2_event *new)
{
u32 old_changes = old->u.ctrl.changes;
old->u.ctrl = new->u.ctrl;
old->u.ctrl.changes |= old_changes;
}
static void ctrls_merge(const struct v4l2_event *old, struct v4l2_event *new)
{
new->u.ctrl.changes |= old->u.ctrl.changes;
}
int v4l2_event_subscribe(struct v4l2_fh *fh,
struct v4l2_event_subscription *sub, unsigned elems)
{
struct v4l2_subscribed_event *sev, *found_ev;
struct v4l2_ctrl *ctrl = NULL;
unsigned long flags;
unsigned i;
if (sub->type == V4L2_EVENT_ALL)
return -EINVAL;
if (elems < 1)
elems = 1;
if (sub->type == V4L2_EVENT_CTRL) {
ctrl = v4l2_ctrl_find(fh->ctrl_handler, sub->id);
if (ctrl == NULL)
return -EINVAL;
}
sev = kzalloc(sizeof(*sev) + sizeof(struct v4l2_kevent) * elems, GFP_KERNEL);
if (!sev)
return -ENOMEM;
for (i = 0; i < elems; i++)
sev->events[i].sev = sev;
sev->type = sub->type;
sev->id = sub->id;
sev->flags = sub->flags;
sev->fh = fh;
sev->elems = elems;
if (ctrl) {
sev->replace = ctrls_replace;
sev->merge = ctrls_merge;
}
spin_lock_irqsave(&fh->vdev->fh_lock, flags);
found_ev = v4l2_event_subscribed(fh, sub->type, sub->id);
if (!found_ev)
list_add(&sev->list, &fh->subscribed);
spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
/* v4l2_ctrl_add_event uses a mutex, so do this outside the spin lock */
if (found_ev)
kfree(sev);
else if (ctrl)
v4l2_ctrl_add_event(ctrl, sev);
return 0;
}
EXPORT_SYMBOL_GPL(v4l2_event_subscribe);
void v4l2_event_unsubscribe_all(struct v4l2_fh *fh)
{
struct v4l2_event_subscription sub;
struct v4l2_subscribed_event *sev;
unsigned long flags;
do {
sev = NULL;
spin_lock_irqsave(&fh->vdev->fh_lock, flags);
if (!list_empty(&fh->subscribed)) {
sev = list_first_entry(&fh->subscribed,
struct v4l2_subscribed_event, list);
sub.type = sev->type;
sub.id = sev->id;
}
spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
if (sev)
v4l2_event_unsubscribe(fh, &sub);
} while (sev);
}
EXPORT_SYMBOL_GPL(v4l2_event_unsubscribe_all);
int v4l2_event_unsubscribe(struct v4l2_fh *fh,
struct v4l2_event_subscription *sub)
{
struct v4l2_subscribed_event *sev;
unsigned long flags;
int i;
if (sub->type == V4L2_EVENT_ALL) {
v4l2_event_unsubscribe_all(fh);
return 0;
}
spin_lock_irqsave(&fh->vdev->fh_lock, flags);
sev = v4l2_event_subscribed(fh, sub->type, sub->id);
if (sev != NULL) {
/* Remove any pending events for this subscription */
for (i = 0; i < sev->in_use; i++) {
list_del(&sev->events[sev_pos(sev, i)].list);
fh->navailable--;
}
list_del(&sev->list);
}
spin_unlock_irqrestore(&fh->vdev->fh_lock, flags);
if (sev && sev->type == V4L2_EVENT_CTRL) {
struct v4l2_ctrl *ctrl = v4l2_ctrl_find(fh->ctrl_handler, sev->id);
if (ctrl)
v4l2_ctrl_del_event(ctrl, sev);
}
kfree(sev);
return 0;
}
EXPORT_SYMBOL_GPL(v4l2_event_unsubscribe);