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

[Danilo Krummrich]

On 11/9/23 01:09, Danilo Krummrich wrote:
> 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@xxxxxxx 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@xxxxxxxxxx>
> > > ---
> > >   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@xxxxxxxxxxxxxx
> >
> > >    * 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.

Any opinions on that? Otherwise I'd probably go ahead and send a fix for the other bugs too.

> Not quite sure if we really want to rely on the latter for core infrastructure...
>
> > >           return -ENOENT;
> > >       }
> > >
> > > base-commit: c015fb6d01adb616fb54824feb55ce5ab18e8ca1