RE: Introduce a new helper framework for buffer synchronization

[Inki Dae <inki.dae@samsung.com>]

> -----Original Message-----
> From: Maarten Lankhorst [mailto:maarten.lankhorst@canonical.com]
> Sent: Monday, May 13, 2013 5:01 PM
> To: Inki Dae
> Cc: Rob Clark; Daniel Vetter; DRI mailing list; linux-arm-
> kernel@lists.infradead.org; linux-media@vger.kernel.org; linux-fbdev;
> Kyungmin Park; myungjoo.ham; YoungJun Cho
> Subject: Re: Introduce a new helper framework for buffer synchronization
> 
> Op 09-05-13 09:33, Inki Dae schreef:
> > Hi all,
> >
> > This post introduces a new helper framework based on dma fence. And the
> > purpose of this post is to collect other opinions and advices before RFC
> > posting.
> >
> > First of all, this helper framework, called fence helper, is in progress
> > yet so might not have enough comments in codes and also might need to be
> > more cleaned up. Moreover, we might be missing some parts of the dma
> fence.
> > However, I'd like to say that all things mentioned below has been tested
> > with Linux platform and worked well.
> 
> > ....
> >
> > And tutorial for user process.
> >         just before cpu access
> >                 struct dma_buf_fence *df;
> >
> >                 df->type = DMA_BUF_ACCESS_READ or DMA_BUF_ACCESS_WRITE;
> >                 ioctl(fd, DMA_BUF_GET_FENCE, &df);
> >
> >         after memset or memcpy
> >                 ioctl(fd, DMA_BUF_PUT_FENCE, &df);
> NAK.
> 
> Userspace doesn't need to trigger fences. It can do a buffer idle wait,
> and postpone submitting new commands until after it's done using the
> buffer.

Hi Maarten,

It seems that you say user should wait for a buffer like KDS does: KDS uses
select() to postpone submitting new commands. But I think this way assumes
that every data flows a DMA device to a CPU. For example, a CPU should keep
polling for the completion of a buffer access by a DMA device. This means
that the this way isn't considered for data flow to opposite case; CPU to
DMA device.

> Kernel space doesn't need the root hole you created by giving a
> dereferencing a pointer passed from userspace.
> Your next exercise should be to write a security exploit from the api you
> created here. It's the only way to learn how to write safe code. Hint:
> df.ctx = mmap(..);
> 

Also I'm not clear to use our way yet and that is why I posted. As you
mentioned, it seems like that using mmap() is more safe. But there is one
issue it makes me confusing. For your hint, df.ctx = mmap(..), the issue is
that dmabuf mmap can be used to map a dmabuf with user space. And the dmabuf
means a physical memory region allocated by some allocator such as drm gem
or ion.

There might be my missing point so could you please give me more comments?

Thanks,
Inki Dae



> ~Maarten