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

[Dmitry Osipenko]

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.
>>
>>> 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.

> 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.

-- 
Best regards,
Dmitry