linux-kernel - Re: [PATCH 1/4] mm: introduce vma_set

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Date:   Fri, 9 Oct 2020 09:12:20 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     christian.koenig@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
        dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
        chris@...is-wilson.co.uk, airlied@...hat.com,
        akpm@...ux-foundation.org, sumit.semwal@...aro.org
Subject: Re: [PATCH 1/4] mm: introduce vma_set_file function v2

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