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:
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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