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)

[Salvatore Bonaccorso <carnil@debian.org>]

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:
> > On Fri, 2025-12-19 at 10:19 +0100, Thorsten Leemhuis wrote:
> >> On 12/18/25 20:50, Salvatore Bonaccorso wrote:
> >>>
> >>> 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.
> 
> > 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