Re: [Linux-fbdev-devel] fbdev API problems

[Matt S. <mat...@ho...>]

On Thu, 2001-11-22 at 00:37, Sven wrote:
> On Thu, Nov 22, 2001 at 12:34:03AM -0800, Matt Sottek wrote:
> > >The framebuffer itself is pretty safe to export to userland.
> > 
> > I assume you mean to allow people to mmap the framebuffer? Yes this can
> > usually be safe, but only the screen data. Most chips use graphics
> > memory of some type for dma command buffers. Most chips cannot safely
> > give you full access to such memory areas. The sysmem_start and
> > sysmem_length seem to cover the whole graphics region, not just the
> > visible framebuffer. This is likely not safe.
> 
> DMA is not useable outside of the DRI anyway for now, or do you know of any
> userland app doing this ?
> 
> I guess you need to have root access to the fbdev or something such to be able
> to do dma, not sure though.

Yes only the DRI is doing dma. It doesn't matter who is doing it.

What I was pointing out is that you cannot just let users map the
"framebuffer" memory (meaning all of the graphics memory) because some
of that memory may contain active dma command buffers which could be
hijacked to do insecure things. Like, for instance, blit
something from system memory into the framebuffer such that you
can then read it, or write to random areas of memory.

> 
> Anyway, even X is not able to do DMA outside of the DRI setup. I would have
> liked it when writing Xv support for the permedia3 chip.
> 
So write a DRM :)

-Matt