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

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Message-ID: <7fccb1d3-cd05-d1f6-fa68-6c1828925d4c@amd.com>
Date:   Thu, 27 Oct 2022 09:40:27 +0200
From:   Christian König <christian.koenig@....com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     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,
        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 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@...cle.com>
>>> 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@...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?
>>
> 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
>