Re: [PATCH net-next v2] bridge: also trigger RTM_NEWLINK when interface is released from bridge

[David Ahern <dsahern@gmail.com>]

On 9/20/17 3:09 PM, David Miller wrote:
> From: Vincent Bernat <vincent@bernat.im>
> Date: Sat, 16 Sep 2017 16:18:33 +0200
> 
> David, I am CC:'ing you because you've done work in this area over the
> past year.  I'm applying this patch, it's been sitting since the 16th
> and likes entirely correct to me.  But if you have objections just let
> me know.
> 
>> Currently, when an interface is released from a bridge via
>> ioctl(), we get a RTM_DELLINK event through netlink:
>>
>> Deleted 2: dummy0: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 master bridge0 state UNKNOWN
>>     link/ether 6e:23:c2:54:3a:b3
>>
>> Userspace has to interpret that as a removal from the bridge, not as a
>> complete removal of the interface. When an bridged interface is
>> completely removed, we get two events:
>>
>> Deleted 2: dummy0: <BROADCAST,NOARP> mtu 1500 master bridge0 state DOWN
>>     link/ether 6e:23:c2:54:3a:b3
>> Deleted 2: dummy0: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN group default
>>     link/ether 6e:23:c2:54:3a:b3 brd ff:ff:ff:ff:ff:ff
>>
>> In constrast, when an interface is released from a bond, we get a
>> RTM_NEWLINK with only the new characteristics (no master):
>>
>> 3: dummy1: <BROADCAST,NOARP,SLAVE,UP,LOWER_UP> mtu 1500 qdisc noqueue master bond0 state UNKNOWN group default
>>     link/ether ae:dc:7a:8c:9a:3c brd ff:ff:ff:ff:ff:ff
>> 3: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default
>>     link/ether ae:dc:7a:8c:9a:3c brd ff:ff:ff:ff:ff:ff
>> 4: bond0: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
>>     link/ether ae:dc:7a:8c:9a:3c brd ff:ff:ff:ff:ff:ff
>> 3: dummy1: <BROADCAST,NOARP> mtu 1500 qdisc noqueue state DOWN group default
>>     link/ether ae:dc:7a:8c:9a:3c brd ff:ff:ff:ff:ff:ff
>> 3: dummy1: <BROADCAST,NOARP> mtu 1500 qdisc noqueue state DOWN group default
>>     link/ether ca:c8:7b:66:f8:25 brd ff:ff:ff:ff:ff:ff
>> 4: bond0: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
>>     link/ether ae:dc:7a:8c:9a:3c brd ff:ff:ff:ff:ff:ff
>>
>> Userland may be confused by the fact we say a link is deleted while
>> its characteristics are only modified. A first solution would have
>> been to turn the RTM_DELLINK event in del_nbp() into a RTM_NEWLINK
>> event. However, maybe some piece of userland is relying on this
>> RTM_DELLINK to detect when a bridged interface is released. Instead,
>> we also emit a RTM_NEWLINK event once the interface is
>> released (without master info).
>>
>> Deleted 2: dummy0: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 master bridge0 state UNKNOWN
>>     link/ether 8a:bb:e7:94:b1:f8
>> 2: dummy0: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default
>>     link/ether 8a:bb:e7:94:b1:f8 brd ff:ff:ff:ff:ff:ff
>>
>> This is done only when using ioctl(). When using Netlink, such an
>> event is already automatically emitted in do_setlink().

The DELLINK is for AF_BRIDGE family (ifi_family). Adding family to
print_linkinfo and running the monitor I get:


[LINK]family 0: 35: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc
noqueue master br0 state UNKNOWN group default
    link/ether d6:c3:73:86:3c:73 brd ff:ff:ff:ff:ff:ff

[LINK]family 7: 35: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500
master br0 state UNKNOWN
    link/ether d6:c3:73:86:3c:73

[LINK]Deleted family 7: 35: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu
1500 master br0 state UNKNOWN
    link/ether d6:c3:73:86:3c:73

[LINK]family 0: 35: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc
noqueue state UNKNOWN group default
    link/ether d6:c3:73:86:3c:73 brd ff:ff:ff:ff:ff:ff

And that seems correct. So I think the RTM_NEWLINK added by this patch
should not be needed.