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

Dmitry Baryshkov
2024-05-07T15:09:16Z

On Tue, May 07, 2024 at 04:34:24PM +0200, Hans de Goede wrote:
> 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.

Ah, I see. Then why do you require the DMA-ble buffer at all? If you are
providing data to VPU or DRM, then you should be able to get the buffer
from the data-consuming device. If the data is further processed by
a userspace app, then it should not require DMA memory at all.

My main concern is that dma-heaps is both too generic and too limiting
for the generic library implementation.

--
With best wishes
Dmitry