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

Rob Clark
2022-08-12T15:02:04Z

On Fri, Aug 12, 2022 at 7:57 AM Rob Clark <robdclark@gmail.com> wrote:
>
> On Fri, Aug 12, 2022 at 4:26 AM Dmitry Osipenko
> <dmitry.osipenko@collabora.com> wrote:
> >
> > On 8/11/22 02:19, Rob Clark wrote:
> > > On Wed, Aug 10, 2022 at 3:23 PM Dmitry Osipenko
> > > <dmitry.osipenko@collabora.com> wrote:
> > >>
> > >> On 8/11/22 01:03, Rob Clark wrote:
> > >>> On Wed, Aug 10, 2022 at 12:26 PM Dmitry Osipenko
> > >>> <dmitry.osipenko@collabora.com> wrote:
> > >>>>
> > >>>> On 8/10/22 18:08, Rob Clark wrote:
> > >>>>> On Wed, Aug 10, 2022 at 4:47 AM Daniel Vetter <daniel@ffwll.ch> 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@amd.com> 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@linux.intel.com>
> > >>>>>>>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> > >>>>>>>>>
> > >>>>>>>>> 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.

The other complication I noticed is that we don't seem to keep around
the fd after importing to a GEM handle. And I could imagine that
doing so could cause issues with too many fd's. So I guess the best
thing is to keep the status quo and let drivers that cannot mmap
imported buffers just fail mmap?

BR,
-R

> 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