Re: [PATCH 4/4] [meida] media-device: dynamically allocate struct media_devnode

Mauro Carvalho Chehab
2016-03-24T11:37:18Z

Em Thu, 24 Mar 2016 10:24:44 +0200
Laurent Pinchart <laurent.pinchart@ideasonboard.com> escreveu:

> On Wednesday 23 Mar 2016 16:27:46 Mauro Carvalho Chehab wrote:
> > struct media_devnode is currently embedded at struct media_device.
> >
> > While this works fine during normal usage, it leads to a race
> > condition during devnode unregister. the problem is that drivers
> > assume that, after calling media_device_unregister(), the struct
> > that contains media_device can be freed. This is not true, as it
> > can't be freed until userspace closes all opened /dev/media devnodes.
> >
> > In other words, if the media devnode is still open, and media_device
> > gets freed, any call to an ioctl will make the core to try to access
> > struct media_device, with will cause an use-after-free and even GPF.
> >
> > Fix this by dynamically allocating the struct media_devnode and only
> > freeing it when it is safe.
>
> We have the exact same issue with video_device, and there we've solved the
> problem by passing the release call all the way up to the driver. I'm open to
> discuss what the best solution is, but I don't think we should special-case
> media_device unless there are compelling arguments regarding why different
> solutions are needed for media_device and video_device.

The relationship between a video driver and video_device/v4l2_dev is
different. On V4L2 we have:
- One driver using video_device resources;
- multiple video_device devnodes.

For media devices, the relationship is the opposite:
- multiple independent drivers using media_devnode.
- One media device node;

On media devices, when multiple drivers are sharing the same devnode, the
.probe() order can be different than the .release() order.

So, we don't need to use the same solution as we did for video_device
on media controller. Actually, the V4L2 solution won't work.

On V4L2, a video device is typically initialized with:

video-dev->release = video_device_release;
err = video_register_device(video_dev,VFL_TYPE_GRABBER,
video_nr[dev->nr]);

And video_device_release is simply a kfree:

void video_device_release(struct video_device *vdev)
{
kfree(vdev);
}

The caller driver may opt to use its own code to free the resources
instead of the core one, but it needs to free vdev in the end
(or some struct that embedds it).

In the specific case of media, drivers don't need to touch or even
be aware of media_devnode, as the creation of the media devnode is
handled internally by the core. Also, there's no good reason to
make the caller drivers to be aware of that.

So, the approach taken by this patch is actually simpler, as the
kfree() is internal to the core, and it doesn't require
any callbacks. This patch provides all that it is needed to make devnode
destroy safe.

On the common case where one driver allocates one /dev/media devnode,
using the standard media_device_register()/media_device_unregister(),
grants that a media_devnode instance will only be freed after all uses
have gone, including open() descriptors. It also grants that the caller
can free its own resources after media_device_unregister(), because
media_devnode won't use media_device anymore.

This happens because media_devnode_is_registered() will return
false after media_device_unregister(), and the media_ioctl logic
will return an error in this case:
__media_ioctl(struct file *filp, unsigned int cmd, unsigned long arg,
long (*ioctl_func)(struct file *filp, unsigned int cmd,
unsigned long arg))
{
struct media_devnode *devnode = media_devnode_data(filp);

if (!ioctl_func)
return -ENOTTY;

if (!media_devnode_is_registered(devnode))
return -EIO;
/* IMHO, it should be -ENODEV here */

return ioctl_func(filp, cmd, arg);
}

all other syscalls have a similar test.

When more than one driver shares the same media devnode - e. g. the
case that it is currently using media_device_*_devres(), the V4L2
solution of exposing the .release() callback to the caller driver
won't work, as the unbind order can be different than the binding
one. So, it is not possible to have .release() callbacks.

On the multiple drivers scenario, a kref is used to identify when
all drivers called media_device_unregister_devres(). Only when the
last driver called it, it will do the actual media_device cleanups
and will wait for userspace to close all opened file descriptors,
calling kfree(media_devnode) only after that. It is also safe for
a device driver to cleanup its own resources after
media_device_release_devres(), as, if the driver is not the last
one, media_device and media_devnode will still be allocated, and,
if it is the last one, this will fallback on the case of a single
driver.

I can't think on any other race-free solution than the one implemented
by this patch, and still being simple.

> I also suspect we will need to consider dynamic pipeline management sooner
> than later to solve the problem properly if we don't want to create code today
> that will need to be completely reworked tomorrow.

On the stress testing we're doing, we're removing/recreating part of the
graph, by unbinding/rebinding one one of the drivers, while keep calling
G_TOPOLOGY on an endless loop.

It is working quite well. The change from semaphore->mutex, suggested
by Sakari seemed to solve all the locking issues we had before.

Ok, I didn't test SETUP_LINK, but, as it is now protected by the same
mutex, except for some hidden bug, I guess it will work just fine.

So, I don't see any need to change the locking schema at the core,
to avoid race issues.

>
> > Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
> > ---
> > drivers/media/media-device.c | 38 +++++++++++++++++++------------
> > drivers/media/media-devnode.c | 7 ++++++-
> > drivers/media/usb/au0828/au0828-core.c | 4 ++--
> > drivers/media/usb/uvc/uvc_driver.c | 2 +-
> > include/media/media-device.h | 5 +----
> > include/media/media-devnode.h | 15 ++++++++++++--
> > sound/usb/media.c | 8 +++----
> > 7 files changed, 52 insertions(+), 27 deletions(-)
> >
> > diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
> > index 10cc4766de10..d10dc615e7a8 100644
> > --- a/drivers/media/media-device.c
> > +++ b/drivers/media/media-device.c
> > @@ -428,7 +428,7 @@ static long media_device_ioctl(struct file *filp,
> > unsigned int cmd, unsigned long arg)
> > {
> > struct media_devnode *devnode = media_devnode_data(filp);
> > - struct media_device *dev = to_media_device(devnode);
> > + struct media_device *dev = devnode->media_dev;
> > long ret;
> >
> > switch (cmd) {
> > @@ -504,7 +504,7 @@ static long media_device_compat_ioctl(struct file *filp,
> > unsigned int cmd, unsigned long arg)
> > {
> > struct media_devnode *devnode = media_devnode_data(filp);
> > - struct media_device *dev = to_media_device(devnode);
> > + struct media_device *dev = devnode->media_dev;
> > long ret;
> >
> > switch (cmd) {
> > @@ -540,7 +540,8 @@ static const struct media_file_operations
> > media_device_fops = { static ssize_t show_model(struct device *cd,
> > struct device_attribute *attr, char *buf)
> > {
> > - struct media_device *mdev = to_media_device(to_media_devnode(cd));
> > + struct media_devnode *devnode = to_media_devnode(cd);
> > + struct media_device *mdev = devnode->media_dev;
> >
> > return sprintf(buf, "%.*s\n", (int)sizeof(mdev->model), mdev->model);
> > }
> > @@ -718,25 +719,36 @@ EXPORT_SYMBOL_GPL(media_device_cleanup);
> > int __must_check __media_device_register(struct media_device *mdev,
> > struct module *owner)
> > {
> > + struct media_devnode *devnode;
> > int ret;
> >
> > mutex_lock(&mdev->graph_mutex);
> >
> > + devnode = kzalloc(sizeof(*devnode), GFP_KERNEL);
> > + if (!devnode)
> > + return -ENOMEM;
> > +
> > /* Register the device node. */
> > - mdev->devnode.fops = &media_device_fops;
> > - mdev->devnode.parent = mdev->dev;
> > - mdev->devnode.release = media_device_release;
> > + mdev->devnode = devnode;
> > + devnode->fops = &media_device_fops;
> > + devnode->parent = mdev->dev;
> > + devnode->release = media_device_release;
> >
> > /* Set version 0 to indicate user-space that the graph is static */
> > mdev->topology_version = 0;
> >
> > - ret = media_devnode_register(&mdev->devnode, owner);
> > - if (ret < 0)
> > + ret = media_devnode_register(mdev, devnode, owner);
> > + if (ret < 0) {
> > + mdev->devnode = NULL;
> > + kfree(devnode);
> > goto err;
> > + }
> >
> > - ret = device_create_file(&mdev->devnode.dev, &dev_attr_model);
> > + ret = device_create_file(&devnode->dev, &dev_attr_model);
> > if (ret < 0) {
> > - media_devnode_unregister(&mdev->devnode);
> > + mdev->devnode = NULL;
> > + media_devnode_unregister(devnode);
> > + kfree(devnode);
> > goto err;
> > }
> >
> > @@ -800,9 +812,9 @@ static void __media_device_unregister(struct
> > media_device *mdev) }
> >
> > /* Check if mdev devnode was registered */
> > - if (media_devnode_is_registered(&mdev->devnode)) {
> > - device_remove_file(&mdev->devnode.dev, &dev_attr_model);
> > - media_devnode_unregister(&mdev->devnode);
> > + if (media_devnode_is_registered(mdev->devnode)) {
> > + device_remove_file(&mdev->devnode->dev, &dev_attr_model);
> > + media_devnode_unregister(mdev->devnode);
> > }
> >
> > dev_dbg(mdev->dev, "Media device unregistered\n");
> > diff --git a/drivers/media/media-devnode.c b/drivers/media/media-devnode.c
> > index ae2bc0b7a368..db47063d8801 100644
> > --- a/drivers/media/media-devnode.c
> > +++ b/drivers/media/media-devnode.c
> > @@ -44,6 +44,7 @@
> > #include <linux/uaccess.h>
> >
> > #include <media/media-devnode.h>
> > +#include <media/media-device.h>
> >
> > #define MEDIA_NUM_DEVICES 256
> > #define MEDIA_NAME "media"
> > @@ -74,6 +75,8 @@ static void media_devnode_release(struct device *cd)
> > /* Release media_devnode and perform other cleanups as needed. */
> > if (devnode->release)
> > devnode->release(devnode);
> > +
> > + kfree(devnode);
> > }
> >
> > static struct bus_type media_bus_type = {
> > @@ -218,7 +221,8 @@ static const struct file_operations media_devnode_fops =
> > { .llseek = no_llseek,
> > };
> >
> > -int __must_check media_devnode_register(struct media_devnode *devnode,
> > +int __must_check media_devnode_register(struct media_device *mdev,
> > + struct media_devnode *devnode,
> > struct module *owner)
> > {
> > int minor;
> > @@ -237,6 +241,7 @@ int __must_check media_devnode_register(struct
> > media_devnode *devnode, mutex_unlock(&media_devnode_lock);
> >
> > devnode->minor = minor;
> > + devnode->media_dev = mdev;
> >
> > /* Part 2: Initialize and register the character device */
> > cdev_init(&devnode->cdev, &media_devnode_fops);
> > diff --git a/drivers/media/usb/au0828/au0828-core.c
> > b/drivers/media/usb/au0828/au0828-core.c index 85c13ca5178f..598a85388d77
> > 100644
> > --- a/drivers/media/usb/au0828/au0828-core.c
> > +++ b/drivers/media/usb/au0828/au0828-core.c
> > @@ -142,7 +142,7 @@ static void au0828_unregister_media_device(struct
> > au0828_dev *dev) struct media_device *mdev = dev->media_dev;
> > struct media_entity_notify *notify, *nextp;
> >
> > - if (!mdev || !media_devnode_is_registered(&mdev->devnode))
> > + if (!mdev || !media_devnode_is_registered(mdev->devnode))
> > return;
> >
> > /* Remove au0828 entity_notify callbacks */
> > @@ -480,7 +480,7 @@ static int au0828_media_device_register(struct
> > au0828_dev *dev, if (!dev->media_dev)
> > return 0;
> >
> > - if (!media_devnode_is_registered(&dev->media_dev->devnode)) {
> > + if (!media_devnode_is_registered(dev->media_dev->devnode)) {
> >
> > /* register media device */
> > ret = media_device_register(dev->media_dev);
> > diff --git a/drivers/media/usb/uvc/uvc_driver.c
> > b/drivers/media/usb/uvc/uvc_driver.c index 451e84e962e2..302e284a95eb
> > 100644
> > --- a/drivers/media/usb/uvc/uvc_driver.c
> > +++ b/drivers/media/usb/uvc/uvc_driver.c
> > @@ -1674,7 +1674,7 @@ static void uvc_delete(struct uvc_device *dev)
> > if (dev->vdev.dev)
> > v4l2_device_unregister(&dev->vdev);
> > #ifdef CONFIG_MEDIA_CONTROLLER
> > - if (media_devnode_is_registered(&dev->mdev.devnode))
> > + if (media_devnode_is_registered(dev->mdev.devnode))
> > media_device_unregister(&dev->mdev);
> > media_device_cleanup(&dev->mdev);
> > #endif
> > diff --git a/include/media/media-device.h b/include/media/media-device.h
> > index e59772ed8494..b04cfa907350 100644
> > --- a/include/media/media-device.h
> > +++ b/include/media/media-device.h
> > @@ -347,7 +347,7 @@ struct media_entity_notify {
> > struct media_device {
> > /* dev->driver_data points to this struct. */
> > struct device *dev;
> > - struct media_devnode devnode;
> > + struct media_devnode *devnode;
> >
> > char model[32];
> > char driver_name[32];
> > @@ -403,9 +403,6 @@ struct usb_device;
> > #define MEDIA_DEV_NOTIFY_PRE_LINK_CH 0
> > #define MEDIA_DEV_NOTIFY_POST_LINK_CH 1
> >
> > -/* media_devnode to media_device */
> > -#define to_media_device(node) container_of(node, struct media_device,
> > devnode) -
> > /**
> > * media_entity_enum_init - Initialise an entity enumeration
> > *
> > diff --git a/include/media/media-devnode.h b/include/media/media-devnode.h
> > index e1d5af077adb..cc2b3155593c 100644
> > --- a/include/media/media-devnode.h
> > +++ b/include/media/media-devnode.h
> > @@ -33,6 +33,8 @@
> > #include <linux/device.h>
> > #include <linux/cdev.h>
> >
> > +struct media_device;
> > +
> > /*
> > * Flag to mark the media_devnode struct as registered. Drivers must not
> > touch * this flag directly, it will be set and cleared by
> > media_devnode_register and @@ -81,6 +83,8 @@ struct media_file_operations {
> > * before registering the node.
> > */
> > struct media_devnode {
> > + struct media_device *media_dev;
> > +
> > /* device ops */
> > const struct media_file_operations *fops;
> >
> > @@ -103,7 +107,8 @@ struct media_devnode {
> > /**
> > * media_devnode_register - register a media device node
> > *
> > - * @devnode: media device node structure we want to register
> > + * @media_dev: struct media_device we want to register a device node
> > + * @devnode: the device node to unregister
> > * @owner: should be filled with %THIS_MODULE
> > *
> > * The registration code assigns minor numbers and registers the new device
> > node @@ -116,7 +121,8 @@ struct media_devnode {
> > * the media_devnode structure is *not* called, so the caller is
> > responsible for * freeing any data.
> > */
> > -int __must_check media_devnode_register(struct media_devnode *devnode,
> > +int __must_check media_devnode_register(struct media_device *mdev,
> > + struct media_devnode *devnode,
> > struct module *owner);
> >
> > /**
> > @@ -146,9 +152,14 @@ static inline struct media_devnode
> > *media_devnode_data(struct file *filp) * false otherwise.
> > *
> > * @devnode: pointer to struct &media_devnode.
> > + *
> > + * Note: If mdev is NULL, it also returns false.
> > */
> > static inline int media_devnode_is_registered(struct media_devnode
> > *devnode) {
> > + if (!devnode)
> > + return false;
> > +
> > return test_bit(MEDIA_FLAG_REGISTERED, &devnode->flags);
> > }
> >
> > diff --git a/sound/usb/media.c b/sound/usb/media.c
> > index 6db4878045e5..8fed0adec9e1 100644
> > --- a/sound/usb/media.c
> > +++ b/sound/usb/media.c
> > @@ -136,7 +136,7 @@ void media_snd_stream_delete(struct snd_usb_substream
> > *subs) struct media_device *mdev;
> >
> > mdev = mctl->media_dev;
> > - if (mdev && media_devnode_is_registered(&mdev->devnode)) {
> > + if (mdev && media_devnode_is_registered(mdev->devnode)) {
> > media_devnode_remove(mctl->intf_devnode);
> > media_device_unregister_entity(&mctl->media_entity);
> > media_entity_cleanup(&mctl->media_entity);
> > @@ -241,14 +241,14 @@ static void media_snd_mixer_delete(struct
> > snd_usb_audio *chip) if (!mixer->media_mixer_ctl)
> > continue;
> >
> > - if (media_devnode_is_registered(&mdev->devnode)) {
> > + if (media_devnode_is_registered(mdev->devnode)) {
> > media_device_unregister_entity(&mctl->media_entity);
> > media_entity_cleanup(&mctl->media_entity);
> > }
> > kfree(mctl);
> > mixer->media_mixer_ctl = NULL;
> > }
> > - if (media_devnode_is_registered(&mdev->devnode))
> > + if (media_devnode_is_registered(mdev->devnode))
> > media_devnode_remove(chip->ctl_intf_media_devnode);
> > chip->ctl_intf_media_devnode = NULL;
> > }
> > @@ -268,7 +268,7 @@ int media_snd_device_create(struct snd_usb_audio *chip,
> > if (!mdev->dev)
> > media_device_usb_init(mdev, usbdev, NULL);
> >
> > - if (!media_devnode_is_registered(&mdev->devnode)) {
> > + if (!media_devnode_is_registered(mdev->devnode)) {
> > ret = media_device_register(mdev);
> > if (ret) {
> > dev_err(&usbdev->dev,
>

--
Thanks,
Mauro