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

[Julia Lawall]

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.

julia

>
> This warning is old and it's actually the result of a bug I had in my
> devel version of Smatch yesterday.  xen_obj can't really be NULL, but
> if it were then the caller would return success which would probably
> create some issues.
>
> When a function returns both error pointers and NULL then NULL is a
> special case of success.  Like say you have:  "p = start_feature();".
> If there is an allocation failure, then the code should return -ENOMEM
> and the whole thing should fail.  But if the feature is optional and
> the user has disabled it in the config then we return NULL and the
> caller has to be able to accept that.  There are a lot of these
> IS_ERR_OR_NULL() checks in the xen driver...
>
> The one here is clearly buggy because returning NULL would lead to a
> run time failure.  All these IS_ERR_OR_NULL() should be checked and
> probably changed to just IS_ERR().
>
> This sort of change is probably change is probably easiest if you build
> the Smatch DB and you can check if Smatch thinks the functions return
> NULL.
>
> ~/smatch/smatch_data/db/smdb.py return_states gem_create | grep INTERNAL
> drivers/gpu/drm/xen/xen_drm_front_gem.c | gem_create | 58 |  (-4095)-(-1) |      INTERNAL |  -1 |                      | struct xen_gem_object*(*)(struct drm_device*, ulong) |
> drivers/gpu/drm/xen/xen_drm_front_gem.c | gem_create | 62 | 4065035897849303040 |      INTERNAL |  -1 |                      | struct xen_gem_object*(*)(struct drm_device*, ulong) |
> drivers/gpu/drm/xen/xen_drm_front_gem.c | gem_create | 66 | 4065035897849303040 |      INTERNAL |  -1 |                      | struct xen_gem_object*(*)(struct drm_device*, ulong) |
> drivers/gpu/drm/xen/xen_drm_front_gem.c | gem_create | 68 | 0,(-4095)-(-1) |      INTERNAL |  -1 |                      | struct xen_gem_object*(*)(struct drm_device*, ulong) |
>
> Smatch thinks that gem_create() sometimes returns NULL or error pointers
> but that's because of a bug in the unreleased version of Smatch and
> because gem_create() uses IS_ERR_OR_NULL().
>
>    141
>    142          return &xen_obj->base;
>    143  }
>
> regards,
> dan carpenter
>