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

[Mikko Perttunen]

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@xxxxxxxxx>
> > > > > ---
> > > > >     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