linux-kernel - Re: Changing vma->vm_file in dma_buf

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Date:   Wed, 16 Sep 2020 16:14:19 +0200
From:   Christian König <christian.koenig@....com>
To:     Jason Gunthorpe <jgg@...pe.ca>, akpm@...ux-foundation.org,
        sumit.semwal@...aro.org, linux-media@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Changing vma->vm_file in dma_buf_mmap()

Am 16.09.20 um 16:07 schrieb Jason Gunthorpe:
> On Wed, Sep 16, 2020 at 11:53:59AM +0200, Daniel Vetter wrote:
>
>> But within the driver, we generally need thousands of these, and that
>> tends to bring fd exhaustion problems with it. That's why all the private
>> buffer objects which aren't shared with other process or other drivers are
>> handles only valid for a specific fd instance of the drm chardev (each
>> open gets their own namespace), and only for ioctls done on that chardev.
>> And for mmap we assign fake (but unique across all open fd on it) offsets
>> within the overall chardev. Hence all the pgoff mangling and re-mangling.
> Are they still unique struct files? Just without a fdno?

Yes, exactly.

>> Hence why we'd like to be able to forward aliasing mappings and adjust the
>> file and pgoff, while hopefully everything keeps working. I thought this
>> would work, but Christian noticed it doesn't really.
> It seems reasonable to me that the dma buf should be the owner of the
> VMA, otherwise like you say, there is a big mess attaching the custom
> vma ops and what not to the proper dma buf.
>
> I don't see anything obviously against this in mmap_region() - why did
> Chritian notice it doesn't really work?

To clarify I think this might work.

I just had the same "Is that legal?", "What about security?", etc.. 
questions you raised as well.

It seems like a source of trouble so I thought better ask somebody more 
familiar with that.

Christian.

>
> Jason