Changing vma->vm_file in dma_buf_mmap()

[Christian König]

[SNIP]
> But Jason pointed me to the right piece of code. See this comment in 
> in mmap_region():
>
>> 		/* ->mmap() can change vma->vm_file, but must guarantee that
>> * vma_link() below can deny write-access if VM_DENYWRITE is set
>> * and map writably if VM_SHARED is set. This usually means the
>> * new file must not have been exposed to user-space, yet.
>> */
>> 		vma <https://elixir.bootlin.com/linux/v5.9-rc5/C/ident/vma>->vm_file 
>> <https://elixir.bootlin.com/linux/v5.9-rc5/C/ident/vm_file>  =  get_file 
>> <https://elixir.bootlin.com/linux/v5.9-rc5/C/ident/get_file>(file 
>> <https://elixir.bootlin.com/linux/v5.9-rc5/C/ident/file>);
>> 		error  =  call_mmap 
>> <https://elixir.bootlin.com/linux/v5.9-rc5/C/ident/call_mmap>(file 
>> <https://elixir.bootlin.com/linux/v5.9-rc5/C/ident/file>,  vma <https://elixir.bootlin.com/linux/v5.9-rc5/C/ident/vma>);
>
> So changing vma->vm_file is allowed at least under certain circumstances.
>
> Only the "file must not have been exposed to user-space, yet" part 
> still needs double checking. Currently working on that.

Ok, I think we can guarantee for all DMA-bufs what is required here.

While searching the code I've found that at least vgem_prime_mmap() and 
i915_gem_dmabuf_mmap() are doing the same thing of modifying vma->vm_file.

So I'm leaning towards that this works as expected and we should just 
document this properly.

Daniel and Jason what do you think?

Christian.
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