linux-kernel - Re: [PATCH 1/2] drm/sched: memset() 'job' in drm_sched_job

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Message-ID: <3b7b2e9f324d3a40f290392f56569154a3d6395f.camel@redhat.com>
Date: Tue, 22 Oct 2024 16:17:29 +0200
From: Philipp Stanner <pstanner@...hat.com>
To: Christian König <christian.koenig@....com>, Luben
 Tuikov <ltuikov89@...il.com>, Matthew Brost <matthew.brost@...el.com>,
 Danilo Krummrich <dakr@...nel.org>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>,  Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
 Simona Vetter <simona@...ll.ch>, Tvrtko Ursulin <tursulin@...lia.com>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] drm/sched: memset() 'job' in drm_sched_job_init()

On Mon, 2024-10-21 at 15:05 +0200, Christian König wrote:
> Am 21.10.24 um 12:50 schrieb Philipp Stanner:
> > drm_sched_job_init() has no control over how users allocate struct
> > drm_sched_job. Unfortunately, the function can also not set some
> > struct
> > members such as job->sched.
> > 
> > This could theoretically lead to UB by users dereferencing the
> > struct's
> > pointer members too early.
> > 
> > It is easier to debug such issues if these pointers are initialized
> > to
> > NULL, so dereferencing them causes a NULL pointer exception.
> > Accordingly, drm_sched_entity_init() does precisely that and
> > initializes
> > its struct with memset().
> > 
> > Initialize parameter "job" to 0 in drm_sched_job_init().
> > 
> > Signed-off-by: Philipp Stanner <pstanner@...hat.com>
> > ---
> > No changes in v2.
> > 
> > +CC Christian and Tvrtko in this thread.
> > Would be cool if someone can do a review.
> > ---
> >   drivers/gpu/drm/scheduler/sched_main.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/scheduler/sched_main.c
> > b/drivers/gpu/drm/scheduler/sched_main.c
> > index dab8cca79eb7..2e0e5a9577d1 100644
> > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > @@ -796,6 +796,14 @@ int drm_sched_job_init(struct drm_sched_job
> > *job,
> >   		return -EINVAL;
> >   	}
> >   
> > +	/*
> > +	 * We don't know for sure how the user has allocated.
> > Thus, zero the
> > +	 * struct so that unallowed (i.e., too early) usage of
> > pointers that
> > +	 * this function does not set is guaranteed to lead to a
> > NULL pointer
> > +	 * exception instead of UB.
> > +	 */
> > +	memset(job, 0, sizeof(*job));
> > +
> 
> Maybe just implicitly set the sched pointer to NULL here?
> 
> On the other hand compilers these days are really good at optimizing 
> that away anyway, so feel free to add Reviewed-by: Christian König 
> <christian.koenig@....com> to the series as is as well.

(I had performance-tested it with several million jobs and couldn't
detect a performance regression that was measurable)

Applied #1 to drm-misc-next, thanks.

Regarding patch #2, I just noticed that it violates the docstring
style. I therefore hereby reject my own patch and will resubmit it in a
cleaner form ^^'

P.

> 
> Regards,
> Christian.
> 
> >   	job->entity = entity;
> >   	job->credits = credits;
> >   	job->s_fence = drm_sched_fence_alloc(entity, owner);
>