Date:   Wed, 10 Aug 2022 15:03:55 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc:     Christian König <christian.koenig@....com>,
        David Airlie <airlied@...ux.ie>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Emil Velikov <emil.l.velikov@...il.com>,
        Thomas Hellström <thomas_os@...pmail.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@...ts.linux-foundation.org>,
        linux-tegra@...r.kernel.org, Dmitry Osipenko <digetx@...il.com>,
        kernel@...labora.com, Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v8 2/2] drm/gem: Don't map imported GEMs

On Wed, Aug 10, 2022 at 12:26 PM Dmitry Osipenko
<dmitry.osipenko@...labora.com> wrote:
>
> On 8/10/22 18:08, Rob Clark wrote:
> > On Wed, Aug 10, 2022 at 4:47 AM Daniel Vetter <daniel@...ll.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@....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@...ux.intel.com>
> >>>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.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.
>
> > 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.

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?

BR,
-R

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