linux-kernel - Re: [PATCH] drm/sched: Don't crash kernel on wrong params

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Message-ID: <f39d38c8-8a07-4a63-a09f-3319b5ebb5f6@ursulin.net>
Date: Fri, 7 Nov 2025 15:18:08 +0000
From: Tvrtko Ursulin <tursulin@...ulin.net>
To: Philipp Stanner <phasta@...nel.org>,
 Matthew Brost <matthew.brost@...el.com>, Danilo Krummrich <dakr@...nel.org>,
 Christian König <ckoenig.leichtzumerken@...il.com>,
 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>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/sched: Don't crash kernel on wrong params


On 07/11/2025 14:42, Tvrtko Ursulin wrote:
> 
> On 07/11/2025 12:55, Philipp Stanner wrote:
>> drm_sched_job_arm() just panics the kernel with BUG_ON() in case of an
>> entity being NULL. While drm_sched_job_arm() crashing or not effectively
>> arming jobs is certainly a huge issue that needs to be noticed,
>> completely shooting down the kernel reduces the probability of reaching
>> and debugging a system to 0.
>>
>> Moreover, the checkpatch script by now strongly discourages all new uses
>> of BUG_ON() for this reason.
>>
>> Replace the BUG_ON() in drm_sched_job_arm() with a WARN_ON().
>>
>> Signed-off-by: Philipp Stanner <phasta@...nel.org>
>> ---
>>   drivers/gpu/drm/scheduler/sched_main.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/ 
>> scheduler/sched_main.c
>> index 1d4f1b822e7b..3bf4ae0ca4bc 100644
>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>> @@ -857,7 +857,7 @@ void drm_sched_job_arm(struct drm_sched_job *job)
>>       struct drm_gpu_scheduler *sched;
>>       struct drm_sched_entity *entity = job->entity;
>> -    BUG_ON(!entity);
>> +    WARN_ON(!entity);
>>       drm_sched_entity_select_rq(entity);
> 
> void drm_sched_entity_select_rq(struct drm_sched_entity *entity)
> {
>      struct dma_fence *fence;
>      struct drm_gpu_scheduler *sched;
>      struct drm_sched_rq *rq;
> 
>      /* single possible engine and already selected */
>      if (!entity->sched_list)
> 
> Still the same end result.
> 
> I believe best practice is to simply not have BUG_ON's followed by null 

Sorry I meant WARN_ONs. If we think there is scope for bad things to 
happen after letting the execution proceed then I would add an early 
return to the WARN_ON (_ONCE?). That probably means to push job as well.

Regards,

Tvrtko

> pointer dereferences since they add no value. Ie. it should just be 
> removed and not replaced.
> 
> Regards,
> 
> Tvrtko
> 
> Regards,
> 
> Tvrtko
> 
>>       sched = entity->rq->sched;
>