Re: [PATCH v4 8/8] vfio: Validate dma-buf revocation semantics

[Pranjal Shrivastava <praan@google.com>]

On Wed, Jan 21, 2026 at 09:47:12AM -0400, Jason Gunthorpe wrote:
> On Wed, Jan 21, 2026 at 02:59:16PM +0200, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@nvidia.com>
> > 
> > Use the new dma_buf_attach_revocable() helper to restrict attachments to
> > importers that support mapping invalidation.
> > 
> > Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> > ---
> >  drivers/vfio/pci/vfio_pci_dmabuf.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > index 5fceefc40e27..85056a5a3faf 100644
> > --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> > +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > @@ -31,6 +31,9 @@ static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
> >  	if (priv->revoked)
> >  		return -ENODEV;
> >  
> > +	if (!dma_buf_attach_revocable(attachment))
> > +		return -EOPNOTSUPP;
> > +
> >  	return 0;
> >  }
> 
> We need to push an urgent -rc fix to implement a pin function here
> that always fails. That was missed and it means things like rdma can
> import vfio when the intention was to block that. It would be bad for
> that uAPI mistake to reach a released kernel.
> 
> It's tricky that NULL pin ops means "I support pin" :|
> 

I've been wondering about this for a while now, I've been sitting on the
following:

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index a4d8f2ff94e4..962bce959366 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -1133,6 +1133,8 @@ int dma_buf_pin(struct dma_buf_attachment *attach)

        if (dmabuf->ops->pin)
                ret = dmabuf->ops->pin(attach);
+       else
+               ret = -EOPNOTSUPP;

        return ret;
 }

But didn't get a chance to dive in the history yet. I thought there's a
good reason we didn't have it? Would it break exisitng dmabuf users?

Praan