Re: [PATCH 1/4] mm: introduce vma_set_file function v2

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Oct 09, 2020 at 09:39:00AM +0200, Daniel Vetter wrote:
> I just noticed this here in the patch because everyone else does not do
> this. But looking at the mmap_region() code in mmap.c we seem to indeed
> have this problem for the error path:
> 
> unmap_and_free_vma:
> 	vma->vm_file = NULL;
> 	fput(file);
> 
> Note that the success path does things correctly (a bit above):
> 
> 	file = vma->vm_file;
> out:
> 
> So it indeed looks like dma-buf is the only one that does this fully
> correctly. So maybe we should do a follow-up patch to change the
> mmap_region exit code to pick up whatever vma->vm_file was set instead,
> and fput that?

Given that this new vma_set_file() should be the only way to
manipulate vm_file from the mmap op, I think this reflects a bug in
mm/mmap.c.. Should be:

unmap_and_free_vma:
        fput(vma->vm_file);
        vma->vm_file = NULL;

Then everything works the way you'd expect without tricky error
handling

Jason