Re: [PATCH v6 02/22] drm/gem: Move mapping of imported dma-bufs to drm_gem_mmap_obj()

[Dmitry Osipenko]

On 6/29/22 09:40, Thomas Hellström (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@xxxxxxxxxxxxxxx
>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
>> ---
>>   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. 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.

-- 
Best regards,
Dmitry