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

[Christian König]

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://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@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?

Regards,
Christian.

>   
> +	dma_resv_assert_held(obj->resv);
> +
>   	if (obj->funcs->vunmap)
>   		obj->funcs->vunmap(obj, map);
>   
> @@ -1202,6 +1202,9 @@ EXPORT_SYMBOL(drm_gem_vmap_unlocked);
>   
>   void drm_gem_vunmap_unlocked(struct drm_gem_object *obj, struct iosys_map *map)
>   {
> +	if (!obj || iosys_map_is_null(map))
> +		return;
> +
>   	dma_resv_lock(obj->resv, NULL);
>   	drm_gem_vunmap(obj, map);
>   	dma_resv_unlock(obj->resv);