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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)

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@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.

> but Debian 11 "bullseye" has 7.0.0.
> 
> We can certainly fix the libvirt side of this in Debian, but this also
> sounds like a case where the kernel should work around known buggy user-
> space.  On the other hand, this has been upstream for over 3 years so
> maybe it doesn't make sense now.

Yeah, I tend to the latter as well (the @net maintainers can speak up if
the disagree). But we have one more middle-ground option here maybe the
@stable team could do: revert the backports of 1550f3673972c5 and
5b22f62724a0a0 from 5.10.y, unless they are strongly needed there.

> Please let me know whether I (or anyone) should try to implement a
> workaround for this in the kernel.

Ciao, Thorsten

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Libvirt can no longer delete macvtap devices after kernel update  (Debian 11) Roland Schwarzkopf <rschwarzkopf@mathematik.uni-marburg.de> - 2025-12-17 14:10 +0100
  Re: Libvirt can no longer delete macvtap devices after kernel update  (Debian 11) Salvatore Bonaccorso <carnil@debian.org> - 2025-12-17 20:30 +0100
    Re: Libvirt can no longer delete macvtap devices after kernel update  (Debian 11) Roland Schwarzkopf <rschwarzkopf@mathematik.uni-marburg.de> - 2025-12-18 15:10 +0100
      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) Thorsten Leemhuis <regressions@leemhuis.info> - 2025-12-19 11:10 +0100
        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) Ben Hutchings <benh@debian.org> - 2026-01-02 21:20 +0100
          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) Thorsten Leemhuis <regressions@leemhuis.info> - 2026-01-05 13:50 +0100
      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) Roland Schwarzkopf <rschwarzkopf@mathematik.uni-marburg.de> - 2025-12-19 18:20 +0100

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