linux-kernel - Re: [PATCH] drm/tegra: submit: No need for Null pointer check before kfree

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Message-ID: <65468c84-fc40-e4e1-0adb-ddfc23ec4fb9@kapsi.fi>
Date:   Fri, 30 Dec 2022 12:01:23 +0200
From:   Mikko Perttunen <cyndis@...si.fi>
To:     Stanislaw Gruszka <stanislaw.gruszka@...ux.intel.com>
Cc:     Deepak R Varma <drv@...lo.com>,
        Praveen Kumar <kumarpraveen@...ux.microsoft.com>,
        Saurabh Singh Sengar <ssengar@...rosoft.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Jonathan Hunter <jonathanh@...dia.com>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-tegra@...r.kernel.org
Subject: Re: [PATCH] drm/tegra: submit: No need for Null pointer check before
 kfree

On 12/30/22 11:15, Stanislaw Gruszka wrote:
> On Wed, Dec 28, 2022 at 03:17:59PM +0200, Mikko Perttunen wrote:
>> On 12/28/22 15:08, Deepak R Varma wrote:
>>> On Wed, Dec 28, 2022 at 02:28:54PM +0200, Mikko Perttunen wrote:
>>>> On 12/27/22 19:14, Deepak R Varma wrote:
>>>>> kfree() & vfree() internally perform NULL check on the pointer handed
>>>>> to it and take no action if it indeed is NULL. Hence there is no need
>>>>> for a pre-check of the memory pointer before handing it to
>>>>> kfree()/vfree().
>>>>>
>>>>> Issue reported by ifnullfree.cocci Coccinelle semantic patch script.
>>>>>
>>>>> Signed-off-by: Deepak R Varma <drv@...lo.com>
>>>>> ---
>>>>>     drivers/gpu/drm/tegra/submit.c | 4 ++--
>>>>>     1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/tegra/submit.c b/drivers/gpu/drm/tegra/submit.c
>>>>> index 066f88564169..06f836db99d0 100644
>>>>> --- a/drivers/gpu/drm/tegra/submit.c
>>>>> +++ b/drivers/gpu/drm/tegra/submit.c
>>>>> @@ -680,8 +680,8 @@ int tegra_drm_ioctl_channel_submit(struct drm_device *drm, void *data,
>>>>>     		kfree(job_data->used_mappings);
>>>>>     	}
>>>>>
>>>>> -	if (job_data)
>>>>> -		kfree(job_data);
>>>>> +	kfree(job_data);
>>>>> +
>>>>>     put_bo:
>>>>>     	gather_bo_put(&bo->base);
>>>>>     unlock:
>>>>> --
>>>>> 2.34.1
>>>>>
>>>>>
>>>>>
>>>>
>>>> It continues to be the case that I think this transform is bad. Same applies
>>>> to the host1x patch.
>>>
>>> Hello Mikko,
>>> Thank you for responding to the patch proposal. Could you please explain why is
>>> this bad?
>>>
>>> Regards,
>>> ./drv
>>>
>>>>
>>>> Mikko
>>>
>>>
>>
>> Hi,
>>
>> it gets rid of visual hints on code paths indicating the possible liveness
>> of pointer variables. I.e., after the change, whether the pointer can be
>> NULL or not is more difficult to reason about locally, instead requiring
>> more global reasoning which is mentally more taxing.
>>
>> Since C's type system doesn't help with tracking these kinds of things, I
>> believe it is important to have these kinds of local contextual cues to help
>> the programmer.
> 
> I agree with your point of view. But regarding this particular patch,
> at least on code base I can see, after free_job_data label job_done
> can not be NULL. So patch seems to be ok, but maybe changelog need to
> be different
> 
> Regards
> Stanislaw

It can be NULL; see:

         job->user_data = job_data;
         job->release = release_job;
         job->timeout = 10000;

         /*
          * job_data is now part of job reference counting, so don't release
          * it from here.
          */
         job_data = NULL;

If we go into free_job_data after this code (which happens if there is 
no error, or if host1x_job_submit fails), job_data will be NULL.

The memory is instead released in the 'put_job' label; host1x_job_put 
ends up calling release_job, which does the kfree.

(Yes, it is rather complicated..)

Thanks,
Mikko