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

[Dmitry Osipenko]

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.

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

-- 
Best regards,
Dmitry