Re: [regression 5.10.y] Libvirt can no longer delete macvtap devices after backport of a6cec0bcd342 ("net: rtnetlink: add bulk delete support flag") to 5.10.y series (Debian 11)

Hi,

On Mon, Jan 05, 2026 at 01:30:59PM +0100, Thorsten Leemhuis wrote:

@stable team and/or @net maintainers: this imho needs a judgement call
from your side. See below for details.

On 1/2/26 21:18, Ben Hutchings wrote:

quoted

On Fri, 2025-12-19 at 10:19 +0100, Thorsten Leemhuis wrote:

quoted

On 12/18/25 20:50, Salvatore Bonaccorso wrote:

quoted

Is there soemthing missing?

Roland I think it would be helpful if you can test as well more recent
stable series versions to confirm if the issue is present there as
well or not, which might indicate a 5.10.y specific backporting
problem.

FWIW, it (as usual) would be very important to know if this happens with
mainline as well, as that determines if it's a general problem or a
backporting problem

[...]

The bug is this:

- libvirtd wrongly used to use NLM_F_CREATE (0x400) and NLM_F_EXCL
  (0x200) flags on an RTM_DELLINK operation.  These flags are only
  semantically valid for NEW-type operations.

- rtnetlink is rather lax about checking the flags on operations, so
  these unsupported flags had no effect.

- rtnetlink can now support NLM_F_BULK (0x200) on some DEL-type
  operations.  If the flag is used but is not valid for the specific
  operation then the operation now fails with EOPNOTSUPP.  Since
  NLM_F_EXCL == NLM_F_BULK and RTM_DELLINK does not support bulk
  operations, libvirtd now hits this error case.

I have not tested with mainline, but in principle the same issue should
occur with any other kernel version that has commit a6cec0bcd342 "net:
rtnetlink: add bulk delete support flag"

FWIW, merged for v5.19-rc1 and backported to v5.10.246 as 1550f3673972c5
End of October 2025 in parallel with 5b22f62724a0a0 ("net: rtnetlink:
fix module reference count leak issue in rtnetlink_rcv_msg") [v6.0-rc2],
which is a fix for the former.

quoted

together with an older version of libvirt.

This was fixed in libvirt commit 1334002340b, which appears to have gone
into version 7.1.0,

Could not find that commit when looking briefly, but that version was
released 2021-03-01.

For reference it is this one I think:
https://gitlab.com/libvirt/libvirt/-/commit/81334002340be6bd3a1a34c6584a85fe25cc049c

Regards,
Salvatore