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

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Date:   Fri, 30 Dec 2022 11:15:32 +0100
From:   Stanislaw Gruszka <stanislaw.gruszka@...ux.intel.com>
To:     Mikko Perttunen <cyndis@...si.fi>
Cc:     Deepak R Varma <drv@...lo.com>,
        Saurabh Singh Sengar <ssengar@...rosoft.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Jonathan Hunter <jonathanh@...dia.com>,
        Praveen Kumar <kumarpraveen@...ux.microsoft.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 Fri, Dec 30, 2022 at 12:01:23PM +0200, Mikko Perttunen wrote:
> 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..)

Ok, then better to keep the check.

Regards
Stanislaw