[PATCH v2] nvme: remove unprivileged passthrough support

[Keith Busch]

On Mon, Oct 23, 2023 at 07:44:56AM +0200, Christoph Hellwig wrote:
> Yes, you need someone with root access to change the device node
> persmissions.  But we allowed that under the assumption it is safe
> to do so, which it turns out it is not.

Okay, iiuc, while we have to opt-in to allow this hole, we need another
option for users to set to allow this usage because it's not safe.

Here are two options I have considered for unpriveledged access, please
let me know if you have others or thoughts.

  Restrict access for processes with CAP_SYS_RAWIO, which can be granted
  to non-root users. This cap is already used in scsi subsystem, too.

  A per nvme-generic namespace sysfs attribute that only root can toggle
  that would override any caps and just rely on access permissions.