[PATCH] drm/sched: fix potential page fault in drm_sched_job_init()

[Danilo Krummrich]

On 11/8/23 06:46, Luben Tuikov wrote:
> Hi,
> 
> Could you please use my gmail address, the one one I'm responding from--I don't want
> to miss any DRM scheduler patches. BTW, the luben.tuikov at amd.com email should bounce
> as undeliverable.
> 
> On 2023-11-07 21:26, Danilo Krummrich wrote:
>> Commit 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable
>> number of run-queues") introduces drm_err() in drm_sched_job_init(), in
>> order to indicate that the given entity has no runq, however at this
>> time job->sched is not yet set, likely to be NULL initialized, and hence
>> shouldn't be used.
>>
>> Replace the corresponding drm_err() call with pr_err() to avoid a
>> potential page fault.
>>
>> While at it, extend the documentation of drm_sched_job_init() to
>> indicate that job->sched is not a valid pointer until
>> drm_sched_job_arm() has been called.
>>
>> Fixes: 56e449603f0a ("drm/sched: Convert the GPU scheduler to variable number of run-queues")
>> Signed-off-by: Danilo Krummrich <dakr at redhat.com>
>> ---
>>   drivers/gpu/drm/scheduler/sched_main.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>> index 27843e37d9b7..dd28389f0ddd 100644
>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>> @@ -680,6 +680,9 @@ EXPORT_SYMBOL(drm_sched_resubmit_jobs);
>>    * This function returns -ENOENT in this case (which probably should be -EIO as
>>    * a more meanigful return value).
>>    *
>> + * Note that job->sched is not a valid pointer until drm_sched_job_arm() has
>> + * been called.
>> + *
> 
> Good catch!
> 
> Did you actually get this to page-fault and have a kernel log?

No, I just found it because I was about to make the same mistake.

> 
> I'm asking because we see it correctly set in this kernel log coming from AMD,

I think that's because amdgpu just sets job->sched to *some* scheduler instance after
job allocation [1].

[1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/amd/amdgpu/amdgpu_job.c#L108

> 
> [   11.886024] amdgpu 0000:0a:00.0: [drm] *ERROR* drm_sched_job_init: entity has no rq!
> 
> in this email,
> https://lore.kernel.org/r/CADnq5_PS64jYS_Y3kGW27m-kuWP+FQFiaVcOaZiB=JLSgPnXBQ@mail.gmail.com
> 
>>    * Returns 0 for success, negative error code otherwise.
>>    */
>>   int drm_sched_job_init(struct drm_sched_job *job,
>> @@ -691,7 +694,7 @@ 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__);
> 
> Is it feasible to do something like the following?
> 
> 		dev_err(job->sched ? job->sched->dev : NULL, "%s: entity has no rq!\n", __func__);

I don't think that's a good idea. Although I'd assume that every driver zero-initializes its job
structures, I can't see a rule enforcing that. Hence, job->sched can be a random value until
drm_sched_job_arm() is called.

However, I notice there are quite a view more fields of struct drm_sched_job that are never
initialized, hence there are either a couple more potential bugs or missing documentation that
drivers *must* ensure that a job is zero-initialized.

Not quite sure if we really want to rely on the latter for core infrastructure...

> 
>>   		return -ENOENT;
>>   	}
>>   
>>
>> base-commit: c015fb6d01adb616fb54824feb55ce5ab18e8ca1
>