Re: [PATCH] vfio/pci: Don't export DMABUFs for unmappable BARs

[Matt Evans <mattev@meta.com>]

Hi Leon,

On 16/04/2026 09:11, Leon Romanovsky wrote:
> > 
> On Wed, Apr 15, 2026 at 11:16:23AM -0700, Matt Evans wrote:
>> Although vfio_pci_core_feature_dma_buf() validates that both requested
>> DMABUF ranges and the PCI resources being referenced are page-aligned,
>> there may be reasons other than alignment that cause a BAR to be
>> unmappable.
>>
>> Add a check for vdev->bar_mmap_supported[index], similar to the VFIO
>> mmap path.
>>
>> Fixes: 5d74781ebc86c ("vfio/pci: Add dma-buf export support for MMIO regions")
>> Signed-off-by: Matt Evans <mattev@meta.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 f87fd32e4a01..4ccaf3531e02 100644
>> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
>> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
>> @@ -249,6 +249,9 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
>>   	if (get_dma_buf.region_index >= VFIO_PCI_ROM_REGION_INDEX)
>>   		return -ENODEV;
>>   
>> +	if (!vdev->bar_mmap_supported[get_dma_buf.region_index])
>> +		return -EINVAL;
>> +
> 
> And it looks like AI has valid concern about this line too.
> https://urldefense.com/v3/__https://sashiko.dev/*/patchset/20260415181623.1021090-1-mattev@meta.com__;Iw!!Bt8RZUm9aw!5DxsN8cDUviPIZqEjG0pZ_VYYbl_RdmWucTGdTZ3ZzlVP_Ysb0n7ykr0eXwFXdpuqvZH2FK3$

Ah, Sashiko has a point, and I think its suggestion of checking lower 
down in the default .get_dmabuf_phys (vfio_pci_core_get_dmabuf_phys()) 
and preserving driver overrides is decent.  Will revisit.

To your other question:
 > I noticed this check in vfio_pci_core_mmap(). Isn't that sufficient?

The scenario in mind is doing a DMABUF-export for BARs that you haven't 
necessarily noticed can't be mmap()ed, and both paths should be checking.

Cheers,


Matt


> 
> Thanks
> 
>>   	dma_ranges = memdup_array_user(&arg->dma_ranges, get_dma_buf.nr_ranges,
>>   				       sizeof(*dma_ranges));
>>   	if (IS_ERR(dma_ranges))
>> -- 
>> 2.47.3
>>