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

[Julia Lawall]

On Tue, 21 Apr 2020, Dan Carpenter wrote:

> 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.

The other cases are also with ERR_CAST:

drivers/infiniband/hw/usnic/usnic_ib_qp_grp.c in create_udp_flow
fs/overlayfs/namei.c in ovl_index_upper
sound/soc/qcom/qdsp6/q6adm.c in q6adm_open
drivers/clk/clk.c in clk_hw_create_clk

julia