[bug report] drm/xen-front: Add support for Xen PV display frontend

[Dan Carpenter]

On Tue, Apr 21, 2020 at 05:29:02PM +0200, Julia Lawall wrote:
> 
> 
> On Tue, 21 Apr 2020, Dan Carpenter wrote:
> 
> > Hi Kernel Janitors,
> >
> > Here is another idea that someone could work on, fixing the
> > IS_ERR_OR_NULL() checks in the xen driver.
> >
> > The patch c575b7eeb89f: "drm/xen-front: Add support for Xen PV
> > display frontend" from Apr 3, 2018, leads to the following static
> > checker warning:
> >
> > 	drivers/gpu/drm/xen/xen_drm_front_gem.c:140 xen_drm_front_gem_create()
> > 	warn: passing zero to 'ERR_CAST'
> >
> > drivers/gpu/drm/xen/xen_drm_front_gem.c
> >    133  struct drm_gem_object *xen_drm_front_gem_create(struct drm_device *dev,
> >    134                                                  size_t size)
> >    135  {
> >    136          struct xen_gem_object *xen_obj;
> >    137
> >    138          xen_obj = gem_create(dev, size);
> >    139          if (IS_ERR_OR_NULL(xen_obj))
> >    140                  return ERR_CAST(xen_obj);
> 
> Are the other occurrences of this also a possible problem?  There are a
> few others outside of xen.

We sometimes check a parameter for IS_ERR_OR_NULL().

void free_function(struct something *p)
{
	if (IS_ERR_OR_NULL(p))
		return;
}

That's fine, absolutely harmless and not a bug.  But if we are checking
a return value like this then probably most of the time it's invalid
code.  Normally it's again like this code where we're dealing with an
impossible thing because the return is never NULL.  The common bugs are
that it returns NULL to a caller which only expects error pointers or it
returns success instead of failure.  But sometimes returning success can
be valid:

	obj = get_feature(dev);
	if (IS_ERR_OR_NULL(obj))
		return PTR_ERR(obj);

It deliberately returns success because the rest of the function is
useless when we don't have the feature.

regards,
dan carpenter