Re: [PATCH 2/2] virtio/virtio_ring: Fix the dma_max_mapping_size call

[Michael S. Tsirkin]

On Tue, Jul 23, 2019 at 05:38:30PM +0200, Christoph Hellwig wrote:
> On Mon, Jul 22, 2019 at 11:33:35AM -0400, Michael S. Tsirkin wrote:
> > On Mon, Jul 22, 2019 at 04:55:09PM +0200, Eric Auger wrote:
> > > Do not call dma_max_mapping_size for devices that have no DMA
> > > mask set, otherwise we can hit a NULL pointer dereference.
> > > 
> > > This occurs when a virtio-blk-pci device is protected with
> > > a virtual IOMMU.
> > > 
> > > Fixes: e6d6dd6c875e ("virtio: Introduce virtio_max_dma_size()")
> > > Signed-off-by: Eric Auger <eric.auger@xxxxxxxxxx>
> > > Suggested-by: Christoph Hellwig <hch@xxxxxx>
> > 
> > Christoph, I wonder why did you suggest this?
> > The connection between dma_mask and dma_max_mapping_size
> > is far from obvious.  The documentation doesn't exist.
> > Do we really have to teach all users about this hack?
> > Why not just make dma_max_mapping_size DTRT?
> 
> Because we should not call into dma API functions for devices that
> are not DMA capable.

I'd rather call is_device_dma_capable then, better than poking
at DMA internals.

-- 
MST