iommufd: Add iommufd_object_tombstone_user() helper

Add the iommufd_object_tombstone_user() helper, which allows the caller
to destroy an iommufd object created by userspace.

This is useful on some destroy paths when the kernel caller finds the
object should have been removed by userspace but is still alive. With
this helper, the caller destroys the object but leave the object ID
reserved (so called tombstone). The tombstone prevents repurposing the
object ID without awareness of the original user.

Since this happens for abnormal userspace behavior, for simplicity, the
tombstoned object ID would be permanently leaked until
iommufd_fops_release(). I.e. the original user gets an error when
calling ioctl(IOMMU_DESTROY) on that ID.

The first use case would be to ensure the iommufd_vdevice can't outlive
the associated iommufd_device.

Link: https://patch.msgid.link/r/[email protected]
Suggested-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Lu Baolu <[email protected]>
Reviewed-by: Nicolin Chen <[email protected]>
Reviewed-by: Kevin Tian <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Co-developed-by: "Aneesh Kumar K.V (Arm)" <[email protected]>
Signed-off-by: "Aneesh Kumar K.V (Arm)" <[email protected]>
Tested-by: Nicolin Chen <[email protected]>
Signed-off-by: Xu Yilun <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>

0 files changed, 0 insertions, 0 deletions