Re: [PATCH] virsh: add transient interfaces check for iface-unbridge

[Laine Stump <laine@xxxxxxxxx>]

(BTW, I assume that you are running suse, and using netcf behind the iface-* commands. I didn't realize that the suse port of netcf was being officially used anywhere, as there hasn't been any suse-specific modification to upstream netcf since the suse port was originally added in 2011. If there are downstream patches, it would be great to have them upstream as well.)

On 12/17/2014 05:34 AM, Lin Ma wrote:

iface-unbridge(netcf interface backend) checks multiple interfaces
attaching based on static configuration.
If guests interfaces(says tun/tap devices) are attaching to the bridge,
iface-unbridge doesn't check them, It causes the bridge is removed
although it's in use by other guests.

Please refer to:
https://bugzilla.suse.com/show_bug.cgi?id=813117

In my opinion, if we're going to do this, then we should also modify virsh net-destroy to not allow shutting down a network if there are guests still using it - it's essentially the same thing. This isn't a simple bug that has a definite simple fix, it requires agreement on philosophy. For a very long time virsh has allowed removing networks (net-destroy) even when a guest is connected (just as it permits "virsh destroy" of a guest that hasn't cleanly shutdown its OS); I don't know if this was originally an omission, if it was done because there was originally no simple way to check (the reporting of number of connections in the network status xml is a fairly recent addition), or if it was done on purpose in order to not prevent something that someone may legitimately want to do, but virsh iface-unbridge was written to have the same laissez faire attitude about guest connections.

The existing check for multiple *configured* attached devices though, is there because I wanted iface-unbridge to be able to undo exactly what iface-bridge was doing - put a single ethernet interface behind a bridge. If there are multiple configured interfaces, that can't be done because there is no way to know which of those two interfaces should get the IP configuration from the bridge device (and if there are *0* configured interfaces, the IP config from the bridge would be lost).

On 12/17/2014 10:48 AM, Lin Ma wrote:

 

>>> Michal Privoznik <mprivozn@xxxxxxxxxx> 2014-12-17 下午 20:34 >>>
>On 17.12.2014 11:34, Lin Ma wrote:
>> iface-unbridge(netcf interface backend) checks multiple interfaces
>> attaching based on static configuration.
>> If guests interfaces(says tun/tap devices) are attaching to the bridge,
>> iface-unbridge doesn't check them, It causes the bridge is removed
>> although it's in use by other guests.
>>
>> Please refer to:
>> https://bugzilla.suse.com/show_bug.cgi?id=813117
>
> The bug is not publicly viewable. We tend to not put the BZ URL in the
> commit message in that case.

sorry about that, the URL will be removed, Thanks.

Even better - if there is no sensitive information in the BZ, perhaps you could make it public (or maybe the BZ can be sanitized to mark only the sensitive parts as private).

>> @@ -1103,6 +1103,21 @@ cmdInterfaceUnbridge(vshControl *ctl, const vshCmd *cmd)
>> goto cleanup;
>> }
>>
>> + /* verify whether there is any transient interface attached to bridge. */
>> + if (!(br_xml_transient_if = virInterfaceGetXMLDesc(br_handle, 0)))
>> + goto cleanup;
>> + if (!(xml_doc_transient_if = virXMLParseStringCtxt(br_xml_transient_if,
>> + _("(bridge interface definition)"),
>> + &ctxt_transient_if))) {
>> + vshError(ctl, _("Failed to parse configuration of %s"), br_name);
>> + goto cleanup;
>> + }
>> +
>> + if (virXPathNode("./bridge/interface[2]", ctxt_transient_if) != NULL) {
>> + vshError(ctl, "%s", _("The bridge is in use by transient interfaces"));
>> + goto cleanup;
>> + }
>> +
>
> This effectively copies the code just a few lines above (not to be seen
> in this context though). The only difference is, that your code gest
> LIVE bridge XML, while pre-exisitng code dumps INACTIVE XML. Can we turn
> already existing code into checking LIVE XML instead?

No. That is there for a different reason, and is checking something different (see above).

It feels more
> appropriate too. Live bridge can have only one interface plugged in (due
> to hot-unplugging) regardless of what INACTIVE XML says.

The interfaces listed in the config are the ethernet/bond/vlan interfaces that are configured to be attached to the bridge when it is created. Any interfaces beyond that are most likely tap devices connecting guests (although that isn't necessarily the case).

Actually, at the beginning of writing this patch, I tended to turn already existing code
into checking LIVE XML instead, But if I do so, it can't deal with this extreme circumstance:
says no any physical interface attached to the bridge, But one guest tap backend is
attaching to this bridge. The bridge will be removed in this case instead of reporting "No
interface attached to bridge".

Again, if there is no configured interface that is attached to the bridge, there is no place to put the bridge's IP config, so executing "iface-unbridge" would permanently lose that information, and must be avoided.

Now I thought about it, no body configures the bridge and guest interface like that,
that extreme circumstance is meaningless and useless scenario, So I agreed with your idea.

any else suggestions?

I think the reasons for those two checks are being misunderstood. When there is no configured interface attached to the bridge, removing the bridge will lose the IP config; where there are multiple configured interfaces attached to the bridge, the desired outcome is ambiguous. The live XML gives us different information.

My opinion is that this bug report should be closed as NOTABUG (or virsh net-destroy should also be changed to be prohibited when guests are connected, but it's possible that could break existing scripts - I remember that before "virsh net-update" existed, when somebody wanted to change a running network, they would make the changes to the network, net-destroy and net-start it, then go through all the running domains, detaching and re-attaching all interfaces using that network. It's very likely there are still scripts like that hanging around)

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