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

[Dmitry Osipenko]

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?

-- 
Best regards,
Dmitry