Re: [PATCH v1 2/2] drm/gem: Check whether object is NULL in drm_gem_vunmap()

[Christian König]

Am 27.10.22 um 09:28 schrieb Dan Carpenter:
> On Thu, Oct 27, 2022 at 08:17:31AM +0200, Christian König wrote:
> > Am 27.10.22 um 00:46 schrieb Dmitry Osipenko:
> > > The drm_gem_vunmap() will crash with a NULL dereference if the passed
> > > object pointer is NULL. It wasn't a problem before we added the locking
> > > support to drm_gem_vunmap function because the mapping argument was always
> > > NULL together with the object. Make drm_gem_vunmap() functions to handle
> > > the NULL pointers better.
> > >
> > > Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > > Link: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fdri-devel%2FY1kFEGxT8MVlf32V%40kili%2F&amp;data=05%7C01%7Cchristian.koenig%40amd.com%7C50d3b7e10eca4c7fb72108dab7ecd646%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638024525139985844%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=ET0TRHscb1bQVVHgBkmYvTQyV2Q6WqcMC83LDlrY5ZM%3D&amp;reserved=0  Fixes:
> > > 79e2cf2e7a19 ("drm/gem: Take reservation lock for vmap/vunmap
> > > operations")
> > > Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
> > > ---
> > >    drivers/gpu/drm/drm_gem.c | 9 ++++++---
> > >    1 file changed, 6 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> > > index b8db675e7fb5..ee0a246ff4ac 100644
> > > --- a/drivers/gpu/drm/drm_gem.c
> > > +++ b/drivers/gpu/drm/drm_gem.c
> > > @@ -1175,11 +1175,11 @@ EXPORT_SYMBOL(drm_gem_vmap);
> > >    void drm_gem_vunmap(struct drm_gem_object *obj, struct iosys_map *map)
> > >    {
> > > -	dma_resv_assert_held(obj->resv);
> > > -
> > > -	if (iosys_map_is_null(map))
> > > +	if (!obj || iosys_map_is_null(map))
> > >    		return;
> > I'm not very keen about that. Calling a function with all parameters NULL
> > doesn't make much sense and is clearly a coding error. Hiding that somehow
> > doesn't help but rather makes things worse.
> >
> > The only execption to that are things like kfree() or *_put() which work
> > with the lifetime of objects.
> >
> > Why is the static checker complaining about that in the first place?
> drivers/gpu/drm/drm_client.c:240 drm_client_buffer_delete()
> warn: variable dereferenced before check 'buffer->gem' (see line 238)

Well than rather modify drm_client_buffer_delete() instead.

Calling drm_gem_vunmap() with obj NULL is certainly not a good idea if 
we even check that for drm_gem_object_put().

Interesting that the static checkers complain about that :)

Regards,
Christian.

> regards,
> dan carpenter