QMP: exposing the host TAP interface name

["Tim Foerster" via qemu development <qemu-devel@nongnu.org>]

Hi,

I'd like to query the host TAP interface name of a network backend via QMP, especially when the TAP FD is passed to QEMU using SCM_RIGHTS
and then assigned to a backend through fd=.

In my current setup, retrieving this information externally requires CAP_SYS_PTRACE. This gives the management process far more access
than needed and conflicts with the Landlock sandboxing I want to apply, due to Landlock's ptrace restrictions [1].

QEMU already owns the FD, and tap_fd_get_ifname() can retrieve the name using TUNGETIFF on Linux. For a TAP passed through fd=, x-query-
network currently reports the following (simplified):

{
  "name": "hostnet0",
  "type": "tap",
  "info-str": "fd=42"
}

I'd like to obtain the actual interface name in a structured field, regardless of how the TAP was created or passed to QEMU. For example:

{
  "name": "hostnet0",
  "type": "tap",
  "info-str": "fd=42",
  "data": {
    "ifname": "vnet7"
  }
}

Would extending x-query-network be the preferred approach? If so, would you prefer a dedicated ifname field or a type-specific data object
selected by the existing type field?

Personally, I'd prefer an approach that is reasonably easy to backport downstream to older QEMU releases. Since x-query-network is
relatively new, extending it would also require bringing that command to releases which don't have it yet, potentially increasing the
backporting effort.

A type-specific data object could also accommodate additional TAP properties later, such as multiqueue mode or vnet_hdr. However, those
are outside my current scope, I only need the interface name.

Is there an existing alternative I've missed?

Thanks!

[1] https://docs.kernel.org/userspace-api/landlock.html#ptrace-restrictions