[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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