linux-kernel - Re: [PATCH v2 1/2] drm/sched: memset() 'job' in drm_sched_job

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Message-ID: <92a123f1-73c9-45a1-8e0a-af72dd0a70e7@ursulin.net>
Date: Fri, 13 Sep 2024 12:56:25 +0100
From: Tvrtko Ursulin <tursulin@...ulin.net>
To: Philipp Stanner <pstanner@...hat.com>, 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 v2 1/2] drm/sched: memset() 'job' in drm_sched_job_init()


Hi,

On 28/08/2024 10:41, 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.

job->sched usage from within looks like a bug. But not related to the 
memset you add.

For this one something like this looks easiest for a start:

diff --git a/drivers/gpu/drm/scheduler/sched_main.c 
b/drivers/gpu/drm/scheduler/sched_main.c
index ab53ab486fe6..877113b01af2 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -788,7 +788,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
                  * or worse--a blank screen--leave a trail in the
                  * logs, so this can be debugged easier.
                  */
-               drm_err(job->sched, "%s: entity has no rq!\n", __func__);
+               pr_err("%s: entity has no rq!\n", __func__);
                 return -ENOENT;
         }

Fixes: 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable 
number of run-queues")
Cc: <stable@...r.kernel.org> # v6.7+

> This could theoretically lead to UB by users dereferencing the struct's
> pointer members too early.

Hmm if drm_sched_job_init returned an error callers should not 
dereference anything. What was actually the issue you were debugging?

Adding a memset is I think not the best solution since it is very likely 
redundant to someone doing a kzalloc in the first place.

Regards,

Tvrtko

> 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.
> ---
>   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 356c30fa24a8..b0c8ad10b419 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -806,6 +806,14 @@ 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);