linux-kernel - Re: [PATCH v1 2/2] drm/gem: Check whether object is NULL in drm_gem

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Date:   Thu, 27 Oct 2022 08:17:31 +0200
From:   Christian König <christian.koenig@....com>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, noralf@...nnes.org,
        Dan Carpenter <dan.carpenter@...cle.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] drm/gem: Check whether object is NULL in
 drm_gem_vunmap()



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@...cle.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@...labora.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?

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);