Safety of opening up /dev/dma_heap/* to physically present users (udev uaccess tag) ?

[Hans de Goede]

Hi Dmitry,

On 5/7/24 3:32 PM, Dmitry Baryshkov wrote:
> On Mon, May 06, 2024 at 01:49:17PM +0200, Hans de Goede wrote:
>> Hi dma-buf maintainers, et.al.,
>>
>> Various people have been working on making complex/MIPI cameras work OOTB
>> with mainline Linux kernels and an opensource userspace stack.
>>
>> The generic solution adds a software ISP (for Debayering and 3A) to
>> libcamera. Libcamera's API guarantees that buffers handed to applications
>> using it are dma-bufs so that these can be passed to e.g. a video encoder.
>>
>> In order to meet this API guarantee the libcamera software ISP allocates
>> dma-bufs from userspace through one of the /dev/dma_heap/* heaps. For
>> the Fedora COPR repo for the PoC of this:
>> https://hansdegoede.dreamwidth.org/28153.html
> 
> Is there any reason for allocating DMA buffers for libcamera through
> /dev/dma_heap/ rather than allocating them via corresponding media
> device and then giving them away to DRM / VPU / etc?
> 
> At least this should solve the permission usecase: if the app can open
> camera device, it can allocate a buffer.

This is with a software ISP, the input buffers with raw bayer data
come from a v4l2 device, but the output buffers with the processed
data are purely userspace managed in this case.

Regards,

Hans