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

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Message-ID: <46046055be8fcf29949d36778bdb4ee7a7b6ed67.camel@redhat.com>
Date: Tue, 20 Aug 2024 12:23:49 +0200
From: Philipp Stanner <pstanner@...hat.com>
To: Luben Tuikov <ltuikov89@...il.com>, Matthew Brost
 <matthew.brost@...el.com>,  Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Daniel Vetter <daniel@...ll.ch>, Danilo Krummrich <dakr@...hat.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()

*PING*


On Tue, 2024-08-06 at 16:38 +0200, Philipp Stanner wrote:
> 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>
> ---
> Hi all,
> I did some experiments with the scheduler recently and am trying to
> make
> the documentation and bits of the code more bullet proof.
> 
> I tested the performance of v6.11-rc2 with and without this memset()
> by
> creating 1e6 jobs and found no performance regression.
> 
> Cheers,
> P.
> ---
>  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 76969f9c59c2..1498ee3cbf39 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -936,6 +936,14 @@ 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));
> +
>  	job->entity = entity;
>  	job->credits = credits;
>  	job->s_fence = drm_sched_fence_alloc(entity, owner);