At Thu, 4 Dec 2014 16:38:57 +0000,
Chris Wilson wrote:
>
> On Thu, Dec 04, 2014 at 04:31:15PM +0000, Chris Wilson wrote:
> > On Thu, Dec 04, 2014 at 01:28:39PM +0100, Daniel Vetter wrote:
> > > On Thu, Dec 04, 2014 at 11:51:14AM +0000, Chris Wilson wrote:
> > > > On Thu, Dec 04, 2014 at 11:56:42AM +0100, Takashi Iwai wrote:
> > > > > Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> > > > > ---
> > > > > drivers/gpu/drm/drm_fops.c | 1 +
> > > > > 1 file changed, 1 insertion(+)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/drm_fops.c b/drivers/gpu/drm/drm_fops.c
> > > > > index ed7bc68f7e87..a82dc28d54f3 100644
> > > > > --- a/drivers/gpu/drm/drm_fops.c
> > > > > +++ b/drivers/gpu/drm/drm_fops.c
> > > > > @@ -525,6 +525,7 @@ ssize_t drm_read(struct file *filp, char __user *buffer,
> > > > > if (copy_to_user(buffer + total,
> > > > > e->event, e->event->length)) {
> > > > > total = -EFAULT;
> > > > > + e->destroy(e);
> > > >
> > > > We shouldn't just be throwing away the event here, but put the event
> > > > back at the front of the queue. Poses an interesting race issue. Seems
> > > > like we want to hold the spinlock until the copy is complete so that we
> > > > can fix up the failure correctly.
> > >
> > > I've read the manpage for read and it explicitly states that when you get
> > > an error it's undefined what happens to the read position. Since -EFAULT
> > > is really just a userspace bug I think we can happily drop the event on
> > > the floor, no reason to bend over in the kernel.
> >
> > Hmm. Actually the code is buggy is the provided buffer is too short for
> > the first event in O_NONBLOCK mode.
>
> Just inlining drm_dequeue_event is simple enough to fix the issue:
Right, this avoids the dequeuing at error.
But still there is a race between wait_event_interruptible() and
dequeuing. So, the more comprehensive code should look like:
if (!access_ok(VERIFY_WRITE, buffer, count))
return -EFAULT;
spin_lock_irq(&dev->event_lock);
for (;;) {
if (list_empty(&file_priv->event_list)) {
if (ret)
break;
if (!(filp->f_flags & O_NONBLOCK)) {
ret = -EAGAIN;
break;
}
spin_unlock_irq(&dev->event_lock);
ret = wait_event_interruptible(....);
spin_lock_irq(&dev->event_lock);
if (ret < 0)
break;
ret = 0;
continue;
}
e = list_first_entry(&file_priv->event_list,
struct drm_pending_event, link);
if (e->event->length + ret > count)
break;
if (__copy_to_user_inatomic(buffer + ret,
e->event, e->event->length)) {
if (!ret)
ret = -EFAULT;
break;
}
file_priv->event_space += e->event->length;
ret += e->event->length;
list_del(&e->link);
e->destroy(e);
}
spin_unlock_irq(&dev->event_lock);
return ret;
thanks,
Takashi
>
> diff --git a/drivers/gpu/drm/drm_fops.c b/drivers/gpu/drm/drm_fops.c
> index 91e1105..7af6676 100644
> --- a/drivers/gpu/drm/drm_fops.c
> +++ b/drivers/gpu/drm/drm_fops.c
> @@ -478,43 +478,17 @@ int drm_release(struct inode *inode, struct file *filp)
> }
> EXPORT_SYMBOL(drm_release);
>
> -static bool
> -drm_dequeue_event(struct drm_file *file_priv,
> - size_t total, size_t max, struct drm_pending_event **out)
> -{
> - struct drm_device *dev = file_priv->minor->dev;
> - struct drm_pending_event *e;
> - unsigned long flags;
> - bool ret = false;
> -
> - spin_lock_irqsave(&dev->event_lock, flags);
> -
> - *out = NULL;
> - if (list_empty(&file_priv->event_list))
> - goto out;
> - e = list_first_entry(&file_priv->event_list,
> - struct drm_pending_event, link);
> - if (e->event->length + total > max)
> - goto out;
> -
> - file_priv->event_space += e->event->length;
> - list_del(&e->link);
> - *out = e;
> - ret = true;
> -
> -out:
> - spin_unlock_irqrestore(&dev->event_lock, flags);
> - return ret;
> -}
> -
> ssize_t drm_read(struct file *filp, char __user *buffer,
> size_t count, loff_t *offset)
> {
> struct drm_file *file_priv = filp->private_data;
> - struct drm_pending_event *e;
> - size_t total;
> + struct drm_device *dev = file_priv->minor->dev;
> + unsigned long flags;
> ssize_t ret;
>
> + if (!access_ok(VERIFY_WRITE, buffer, count))
> + return -EFAULT;
> +
> if ((filp->f_flags & O_NONBLOCK) == 0) {
> ret = wait_event_interruptible(file_priv->event_wait,
> !list_empty(&file_priv->event_list));
> @@ -522,19 +496,35 @@ ssize_t drm_read(struct file *filp, char __user *buffer,
> return ret;
> }
>
> - total = 0;
> - while (drm_dequeue_event(file_priv, total, count, &e)) {
> - if (copy_to_user(buffer + total,
> - e->event, e->event->length)) {
> - total = -EFAULT;
> - break;
> - }
> + spin_lock_irqsave(&dev->event_lock, flags);
> + if (list_empty(&file_priv->event_list)) {
> + ret = -EAGAIN;
> + } else {
> + ret = 0;
> + do {
> + struct drm_pending_event *e;
> +
> + e = list_first_entry(&file_priv->event_list,
> + struct drm_pending_event, link);
> + if (e->event->length + ret > count)
> + break;
> +
> + if (__copy_to_user_inatomic(buffer + ret,
> + e->event, e->event->length)) {
> + if (ret == 0)
> + ret = -EFAULT;
> + break;
> + }
>
> - total += e->event->length;
> - e->destroy(e);
> + file_priv->event_space += e->event->length;
> + ret += e->event->length;
> + list_del(&e->link);
> + e->destroy(e);
> + } while (!list_empty(&file_priv->event_list));
> }
> + spin_unlock_irqrestore(&dev->event_lock, flags);
>
> - return total ?: -EAGAIN;
> + return ret;
> }
> EXPORT_SYMBOL(drm_read);
>
>
> which looks familar to the previous attempt to get O_NONBLOCK fixed.
> -Chris
>
> --
> Chris Wilson, Intel Open Source Technology Centre
>
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