Virtualization - Jul 2022 - [Linaro-mm-sig] Re: [PATCH v6 02/22] drm/gem: Move mapping of imported dma-bufs to drm_gem_mmap_obj()

Am 30.06.22 um 01:06 schrieb Dmitry Osipenko:
> On 6/29/22 11:43, Thomas Hellstr?m (Intel) wrote:
>> On 6/29/22 10:22, Dmitry Osipenko wrote:
>>> On 6/29/22 09:40, Thomas Hellstr?m (Intel) wrote:
>>>> On 5/27/22 01:50, Dmitry Osipenko wrote:
>>>>> 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. For example, 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.
>>>>>
>>>>> To fix this bug, move mapping of imported dma-bufs to
>>>>> drm_gem_mmap_obj().
>>>>> Now mmaping of imported dma-bufs works properly for all DRM
drivers.
>>>> Same comment about Fixes: as in patch 1,
>>>>> Cc: stable at vger.kernel.org
>>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko at
collabora.com>
>>>>> ---
>>>>>  ?? drivers/gpu/drm/drm_gem.c????????????? | 3 +++
>>>>>  ?? drivers/gpu/drm/drm_gem_shmem_helper.c | 9 ---------
>>>>>  ?? drivers/gpu/drm/tegra/gem.c??????????? | 4 ++++
>>>>>  ?? 3 files changed, 7 insertions(+), 9 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/drm_gem.c
b/drivers/gpu/drm/drm_gem.c
>>>>> index 86d670c71286..7c0b025508e4 100644
>>>>> --- a/drivers/gpu/drm/drm_gem.c
>>>>> +++ b/drivers/gpu/drm/drm_gem.c
>>>>> @@ -1038,6 +1038,9 @@ int drm_gem_mmap_obj(struct
drm_gem_object *obj,
>>>>> unsigned long obj_size,
>>>>>  ?????? if (obj_size < vma->vm_end -
vma->vm_start)
>>>>>  ?????????? return -EINVAL;
>>>>>  ?? +??? if (obj->import_attach)
>>>>> +??????? return dma_buf_mmap(obj->dma_buf, vma, 0);
>>>> If we start enabling mmaping of imported dma-bufs on a majority
of
>>>> drivers in this way, how do we ensure that user-space is not
blindly
>>>> using the object mmap without calling the needed
DMA_BUF_IOCTL_SYNC
>>>> which is needed before and after cpu access of mmap'ed
dma-bufs?
>>>>
>>>> I was under the impression (admittedly without looking) that
the few
>>>> drivers that actually called into dma_buf_mmap() had some
private
>>>> user-mode driver code in place that ensured this happened.
>>> Since it's a userspace who does the mapping, then it should be
a
>>> responsibility of userspace to do all the necessary syncing.
>> Sure, but nothing prohibits user-space to ignore the syncing thinking
>> "It works anyway", testing those drivers where the syncing is
a NOP. And
>> when a driver that finally needs syncing is tested it's too late to
fix
>> all broken user-space.
>>
>>>  ? I'm not
>>> sure whether anyone in userspace really needs to map imported
dma-bufs
>>> in practice. Nevertheless, this use-case is broken and should be
fixed
>>> by either allowing to do the mapping or prohibiting it.
>>>
>> Then I'd vote for prohibiting it, at least for now. And for the
future
>> moving forward we could perhaps revisit the dma-buf need for syncing,
>> requiring those drivers that actually need it to implement emulated
>> coherent memory which can be done not too inefficiently (vmwgfx being
>> one example).
> Alright, I'll change it to prohibit the mapping. This indeed should be
a
> better option.
Oh, yes please. But I would expect that some people start screaming.

Over time I've got tons of TTM patches because people illegally tried to 
mmap() imported DMA-bufs in their driver.

Anyway this is probably the right thing to do and we can work on fixing 
the fallout later on.

Regards,
Christian.