Re: [PATCH v8 2/2] drm/gem: Don't map imported GEMs

[Rob Clark]

On Fri, Aug 12, 2022 at 4:26 AM Dmitry Osipenko
<dmitry.osipenko@xxxxxxxxxxxxx> wrote:
>
> On 8/11/22 02:19, Rob Clark wrote:
> > On Wed, Aug 10, 2022 at 3:23 PM Dmitry Osipenko
> > <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> >>
> >> On 8/11/22 01:03, Rob Clark wrote:
> >>> On Wed, Aug 10, 2022 at 12:26 PM Dmitry Osipenko
> >>> <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> >>>>
> >>>> On 8/10/22 18:08, Rob Clark wrote:
> >>>>> On Wed, Aug 10, 2022 at 4:47 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
> >>>>>>
> >>>>>> On Wed, Jul 06, 2022 at 10:02:07AM +0300, Dmitry Osipenko wrote:
> >>>>>>> On 7/6/22 00:48, Rob Clark wrote:
> >>>>>>>> On Tue, Jul 5, 2022 at 4:51 AM Christian König <christian.koenig@xxxxxxx> wrote:
> >>>>>>>>>
> >>>>>>>>> Am 01.07.22 um 11:02 schrieb Dmitry Osipenko:
> >>>>>>>>>> Drivers that use drm_gem_mmap() and drm_gem_mmap_obj() helpers don't
> >>>>>>>>>> handle imported dma-bufs properly, which results in mapping of something
> >>>>>>>>>> else than the imported dma-buf. On NVIDIA Tegra we get a hard lockup when
> >>>>>>>>>> userspace writes to the memory mapping of a dma-buf that was imported into
> >>>>>>>>>> Tegra's DRM GEM.
> >>>>>>>>>>
> >>>>>>>>>> Majority of DRM drivers prohibit mapping of the imported GEM objects.
> >>>>>>>>>> Mapping of imported GEMs require special care from userspace since it
> >>>>>>>>>> should sync dma-buf because mapping coherency of the exporter device may
> >>>>>>>>>> not match the DRM device. Let's prohibit the mapping for all DRM drivers
> >>>>>>>>>> for consistency.
> >>>>>>>>>>
> >>>>>>>>>> Suggested-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> >>>>>>>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
> >>>>>>>>>
> >>>>>>>>> I'm pretty sure that this is the right approach, but it's certainly more
> >>>>>>>>> than possible that somebody abused this already.
> >>>>>>>>
> >>>>>>>> I suspect that this is abused if you run deqp cts on android.. ie. all
> >>>>>>>> winsys buffers are dma-buf imports from gralloc.  And then when you
> >>>>>>>> hit readpix...
> >>>>>>>>
> >>>>>>>> You might only hit this in scenarios with separate gpu and display (or
> >>>>>>>> dGPU+iGPU) because self-imports are handled differently in
> >>>>>>>> drm_gem_prime_import_dev().. and maybe not in cases where you end up
> >>>>>>>> with a blit from tiled/compressed to linear.. maybe that narrows the
> >>>>>>>> scope enough to just fix it in userspace?
> >>>>>>>
> >>>>>>> Given that that only drivers which use DRM-SHMEM potentially could've
> >>>>>>> map imported dma-bufs (Panfrost, Lima) and they already don't allow to
> >>>>>>> do that, I think we're good.
> >>>>>>
> >>>>>> So can I have an ack from Rob here or are there still questions that this
> >>>>>> might go boom?
> >>>>>>
> >>>>>> Dmitry, since you have a bunch of patches merged now I think would also be
> >>>>>> good to get commit rights so you can drive this more yourself. I've asked
> >>>>>> Daniel Stone to help you out with getting that.
> >>>>>
> >>>>> I *think* we'd be ok with this on msm, mostly just by dumb luck.
> >>>>> Because the dma-buf's we import will be self-import.  I'm less sure
> >>>>> about panfrost (src/panfrost/lib/pan_bo.c doesn't seem to have a
> >>>>> special path for imported dma-bufs either, and in that case they won't
> >>>>> be self-imports.. but I guess no one has tried to run android cts on
> >>>>> panfrost).
> >>>>
> >>>> The last time I tried to mmap dma-buf imported to Panfrost didn't work
> >>>> because Panfrost didn't implement something needed for that. I'll need
> >>>> to take a look again because can't recall what it was.
> Upd: I re-checked Panfrost using today's linux-next and mapping of
> imported dma-buf works, I mapped imported buf from video decoder.
> Apparently previously I had some local kernel change that broke the mapping.
>
> >>>>> What about something less drastic to start, like (apologies for
> >>>>> hand-edited patch):
> >>>>>
> >>>>> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> >>>>> index 86d670c71286..fc9ec42fa0ab 100644
> >>>>> --- a/drivers/gpu/drm/drm_gem.c
> >>>>> +++ b/drivers/gpu/drm/drm_gem.c
> >>>>> @@ -1034,6 +1034,10 @@ int drm_gem_mmap_obj(struct drm_gem_object
> >>>>> *obj, unsigned long obj_size,
> >>>>>  {
> >>>>>         int ret;
> >>>>>
> >>>>> +       WARN_ON_ONCE(obj->import_attach);
> >>>>
> >>>> This will hang NVIDIA Tegra, which is what this patch fixed initially.
> >>>> If neither of upstream DRM drivers need to map imported dma-bufs and
> >>>> never needed, then why do we need this?
> >>>
> >>> oh, tegra isn't using shmem helpers?  I assumed it was.  Well my point
> >>> was to make a more targeted fail on tegra, and a WARN_ON for everyone
> >>> else to make it clear that what they are doing is undefined behavior.
> >>> Because so far existing userspace (or well, panfrost and freedreno at
> >>> least, those are the two I know or checked) don't make special cases
> >>> for mmap'ing against the dmabuf fd against the dmabuf fd instead of
> >>> the drm device fd.
> >>
> >> It's not clear to me what bad Android does form yours comments. Does it
> >> export dma-buf from GPU and then import it to GPU? If yes, then DRM core
> >> has a check for the self-importing [1].
> >>
> >> [1]
> >> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/drm_prime.c#L918
> >>
> >> If you're meaning something else, then please explain in a more details.
> >
> > So, android/gralloc allocates buffers externally to the driver and
> > imports them into driver.  (And that seems to not just be window
> > surfaces, but in cases random textures, etc)
> >
> > In the normal case these should be allocated from drm/msm so it should
> > hit [1].. this is the "dumb luck" I mentioned earlier.  But I'm not
> > confident enough to say that there is no other case.
> >
> >>
> >>> I *think* it should work out that we don't hit this path with
> >>> freedreno but on android I can't really guarantee or prove it.  So
> >>> your patch would potentially break existing working userspace.  Maybe
> >>> it is userspace that isn't portable (but OTOH it isn't like you are
> >>> going to be using freedreno on tegra).  So why don't you go for a more
> >>> targeted fix that only returns an error on hw where this is
> >>> problematic?
> >>
> >> That's what the first versions of the patch did and Christian suggested
> >> that it's not a good approach. In fact it should be not only Tegra that
> >> has a broken dma-buf mapping, but apparently OMAP driver too.
> >
> > Hmm, I guess I'm a bit more conservative when it comes to potentially
> > breaking userspace.
>
> If such userspace exists, then of course the mapping should continue to
> work. Still will be great to know what that userpsace is.

Definitely existing mesa does not have a special mmap path for
imported dma-bufs (both in the case of panfrost and freedreno, I
didn't check any others).  The only question is whether there is a
case where some app/test/etc imports a foreign dma-buf fd and then
does something that would trigger mmap'ing, like readpix.

BR,
-R

> Alright, let's keep the dma-buf mapping as-is for now. I'll fix just the
> Tegra driver then.
>
> --
> Best regards,
> Dmitry