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

[Dan Carpenter <dan.carpenter@oracle.com>]

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@oracle.com>
> > Link: https://lore.kernel.org/dri-devel/Y1kFEGxT8MVlf32V@kili/  Fixes:
> > 79e2cf2e7a19 ("drm/gem: Take reservation lock for vmap/vunmap
> > operations")
> > Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> > ---
> >   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)

regards,
dan carpenter