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

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Message-ID: <9c1f72ea-06d9-4615-b2f8-d98a0aa78aa1@amd.com>
Date: Mon, 21 Oct 2024 15:05:52 +0200
From: Christian König <christian.koenig@....com>
To: Philipp Stanner <pstanner@...hat.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()

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.

Regards,
Christian.

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