Re: [PATCH] vfio: Reuse file f_inode as vfio device inode

[Yi Liu <yi.l.liu@intel.com>]

On 2024/6/28 23:28, Yan Zhao wrote:
> On Fri, Jun 28, 2024 at 05:48:11PM +0800, Yi Liu wrote:
>> On 2024/6/28 13:21, Yan Zhao wrote:
>>> On Thu, Jun 27, 2024 at 09:42:09AM -0300, Jason Gunthorpe wrote:
>>>> On Thu, Jun 27, 2024 at 05:51:01PM +0800, Yan Zhao wrote:
>>>>
>>>>>>>> This doesn't seem right.. There is only one device but multiple file
>>>>>>>> can be opened on that device.
>>>>> Maybe we can put this assignment to vfio_df_ioctl_bind_iommufd() after
>>>>> vfio_df_open() makes sure device->open_count is 1.
>>>>
>>>> Yeah, that seems better.
>>>>
>>>> Logically it would be best if all places set the inode once the
>>>> inode/FD has been made to be the one and only way to access it.
>>> For group path, I'm afraid there's no such a place ensuring only one active fd
>>> in kernel.
>>> I tried modifying QEMU to allow two openings and two assignments of the same
>>> device. It works and appears to guest that there were 2 devices, though this
>>> ultimately leads to device malfunctions in guest.
>>>
>>>>> BTW, in group path, what's the benefit of allowing multiple open of device?
>>>>
>>>> I don't know, the thing that opened the first FD can just dup it, no
>>>> idea why two different FDs would be useful. It is something we removed
>>>> in the cdev flow
>>>>
>>> Thanks. However, from the code, it reads like a drawback of the cdev flow :)
>>> I don't understand why the group path is secure though.
>>>
>>>           /*
>>>            * Only the group path allows the device to be opened multiple
>>>            * times.  The device cdev path doesn't have a secure way for it.
>>>            */
>>>           if (device->open_count != 0 && !df->group)
>>>                   return -EINVAL;
>>>
>>>
>>
>> The group path only allow single group open, so the device FDs retrieved
>> via the group is just within the opener of the group. This secure is built
>> on top of single open of group.
> What if the group is opened for only once but VFIO_GROUP_GET_DEVICE_FD
> ioctl is called for multiple times?

this should happen within the process context that has opened the group. it
should be safe, and that would be tracked by the open_count.

-- 
Regards,
Yi Liu