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network: drop use of dummy tap device in bridges 

A long time ago we introduced a dummy tap device (e.g. virbr0-nic) that
we attached to the bridge device created for virtual networks:

  commit 5754dbd56d
  Author: Laine Stump <laine@redhat.com>
  Date:   Wed Feb 9 03:28:12 2011 -0500

    Give each virtual network bridge its own fixed MAC address

This was a hack to workaround a Linux kernel bug where it would not
honour any attempt to set a MAC address on a bridge. Instead the
bridge would adopt the numerically lowest MAC address of all NICs
attached to the bridge. This lead to the MAC addrss of the bridge
changing over time as NICs were attached/detached.

The Linux bug was actually fixed 3 years before the libvirt
workaround was added in:

  commit 92c0574f11598c8036f81e27d2e8bdd6eed7d76d
  Author: Stephen Hemminger <shemminger@vyatta.com>
  Date:   Tue Jun 17 16:10:06 2008 -0700

    bridge: make bridge address settings sticky

    Normally, the bridge just chooses the smallest mac address as the
    bridge id and mac address of bridge device. But if the administrator
    has explictly set the interface address then don't change it.

    Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>

but libvirt needed to support RHEL-5 kernels at that time, so
none the less added the workaround.

We have long since dropped support for RHEL-5 vintage distros,
so there's no reason to keep the dummy tap device for the purpose
of setting the bridge MAC address.

Later the dummy TAP device was used for a second purpose related
to IPv6 DAD (Duplicate Address Detection) in:

  commit db488c7917
  Author: Benjamin Cama <benoar@dolka.fr>
  Date:   Wed Sep 26 21:02:20 2012 +0200

    network: fix dnsmasq/radvd binding to IPv6 on recent kernels

This was again dealing with a regression in the Linux kernel, where
if there were no devices attached to the bridge in the UP state,
IPv6 DAD would not be performed. The virbr0-nic was attached but
in the DOWN state, so the above libvirt fix tenporarily brought
the NIC online. The Linux commit causing the problem was in v2.6.38

  commit 1faa4356a3bd89ea11fb92752d897cff3a20ec0e
  Author: stephen hemminger <shemminger@vyatta.com>
  Date:   Mon Mar 7 08:34:06 2011 +0000

    bridge: control carrier based on ports online

A short while later Linux was tweaked so that DAD would still occur
if the bridge had no attached devices at all in 3.1:

  commit b64b73d7d0c480f75684519c6134e79d50c1b341
  Author: stephen hemminger <shemminger@vyatta.com>
  Date:   Mon Oct 3 18:14:45 2011 +0000

    bridge: leave carrier on for empty bridge

IOW, the only reason we need the DAD hack of bringing virbr0-nic
online is because virbr0-nic exists. Once it doesn't exist, then
we hit the "empty bridge" case which works in Linux.

We can rely on distros having Linux kernel >= 3.1, so both things
that the virbr0-nic are doing are redundant.

Fixes https://gitlab.com/libvirt/libvirt/-/issues/53
Reviewed-by: Laine Stump <laine@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
This commit is contained in:
Daniel P. Berrangé 2020-08-03 14:52:13 +01:00
parent 36f922ef8e
commit ee6c936fbb
1 changed files with 8 additions and 50 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

58
src/network/bridge_driver.c
View File

@ -2247,8 +2247,7 @@ networkAddAddrToBridge(virNetworkObjPtr obj,
static int
networkStartHandleMACTableManagerMode(virNetworkObjPtr obj,
const char *macTapIfName)
networkStartHandleMACTableManagerMode(virNetworkObjPtr obj)
{
virNetworkDefPtr def = virNetworkObjGetDef(obj);
const char *brname = def->bridge;
@ -2257,12 +2256,6 @@ networkStartHandleMACTableManagerMode(virNetworkObjPtr obj,
def->macTableManager == VIR_NETWORK_BRIDGE_MAC_TABLE_MANAGER_LIBVIRT) {
if (virNetDevBridgeSetVlanFiltering(brname, true) < 0)
return -1;
if (macTapIfName) {
if (virNetDevBridgePortSetLearning(brname, macTapIfName, false) < 0)
return -1;
if (virNetDevBridgePortSetUnicastFlood(brname, macTapIfName, false) < 0)
return -1;
}
}
return 0;
}
@ -2330,10 +2323,8 @@ networkStartNetworkVirtual(virNetworkDriverStatePtr driver,
virErrorPtr save_err = NULL;
virNetworkIPDefPtr ipdef;
virNetDevIPRoutePtr routedef;
g_autofree char *macTapIfName = NULL;
virMacMapPtr macmap;
g_autofree char *macMapFile = NULL;
int tapfd = -1;
bool dnsmasqStarted = false;
bool devOnline = false;
bool firewalRulesAdded = false;
@ -2360,29 +2351,6 @@ networkStartNetworkVirtual(virNetworkDriverStatePtr driver,
if (virNetDevBridgeCreate(def->bridge, &def->mac) < 0)
return -1;
if (def->mac_specified) {
/* To set a mac for the bridge, we need to define a dummy tap
* device, set its mac, then attach it to the bridge. As long
* as its mac address is lower than any other interface that
* gets attached, the bridge will always maintain this mac
* address.
*/
macTapIfName = networkBridgeDummyNicName(def->bridge);
if (!macTapIfName)
goto error;
/* Keep tun fd open and interface up to allow for IPv6 DAD to happen */
if (virNetDevTapCreateInBridgePort(def->bridge,
&macTapIfName, &def->mac,
NULL, NULL, &tapfd, 1, NULL, NULL,
VIR_TRISTATE_BOOL_NO,
NULL, def->mtu, NULL,
VIR_NETDEV_TAP_CREATE_USE_MAC_FOR_BRIDGE |
VIR_NETDEV_TAP_CREATE_IFUP |
VIR_NETDEV_TAP_CREATE_PERSIST) < 0) {
goto error;
}
}
if (!(macMapFile = virMacMapFileName(driver->dnsmasqStateDir,
def->bridge)) ||
!(macmap = virMacMapNew(macMapFile)))
@ -2426,7 +2394,7 @@ networkStartNetworkVirtual(virNetworkDriverStatePtr driver,
goto error;
}
if (networkStartHandleMACTableManagerMode(obj, macTapIfName) < 0)
if (networkStartHandleMACTableManagerMode(obj) < 0)
goto error;
/* Bring up the bridge interface */
@ -2482,15 +2450,6 @@ networkStartNetworkVirtual(virNetworkDriverStatePtr driver,
if (v6present && networkWaitDadFinish(obj) < 0)
goto error;
/* DAD has finished, dnsmasq is now bound to the
* bridge's IPv6 address, so we can set the dummy tun down.
*/
if (tapfd >= 0) {
if (virNetDevSetOnline(macTapIfName, false) < 0)
goto error;
VIR_FORCE_CLOSE(tapfd);
}
if (virNetDevBandwidthSet(def->bridge, def->bandwidth, true, true) < 0)
goto error;
@ -2514,16 +2473,11 @@ networkStartNetworkVirtual(virNetworkDriverStatePtr driver,
def->forward.type != VIR_NETWORK_FORWARD_OPEN)
networkRemoveFirewallRules(def);
if (macTapIfName) {
VIR_FORCE_CLOSE(tapfd);
ignore_value(virNetDevTapDelete(macTapIfName, NULL));
}
virNetworkObjUnrefMacMap(obj);
ignore_value(virNetDevBridgeDelete(def->bridge));
virErrorRestore(&save_err);
/* coverity[leaked_handle] - 'tapfd' is not leaked */
return -1;
}
@ -2555,9 +2509,13 @@ networkShutdownNetworkVirtual(virNetworkDriverStatePtr driver,
if (dnsmasqPid > 0)
kill(dnsmasqPid, SIGTERM);
/* We no longer create a dummy NIC, but if we've upgraded
* from old libvirt, we still need to delete any dummy NIC
* that might exist. Keep this logic around for a while...
*/
if (def->mac_specified) {
g_autofree char *macTapIfName = networkBridgeDummyNicName(def->bridge);
if (macTapIfName)
if (macTapIfName && virNetDevExists(macTapIfName))
ignore_value(virNetDevTapDelete(macTapIfName, NULL));
}
@ -2597,7 +2555,7 @@ networkStartNetworkBridge(virNetworkObjPtr obj)
if (virNetDevBandwidthSet(def->bridge, def->bandwidth, true, true) < 0)
goto error;
if (networkStartHandleMACTableManagerMode(obj, NULL) < 0)
if (networkStartHandleMACTableManagerMode(obj) < 0)
goto error;
return 0;