netdev - Re: [PATCH net-next 1/1] veth: tweak creation of veth device

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Date:   Thu, 12 Oct 2017 12:49:37 -0400
From:   Roman Mashak <mrv@...atatu.com>
To:     David Miller <davem@...emloft.net>
Cc:     jhs@...atatu.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 1/1] veth: tweak creation of veth device

David Miller <davem@...emloft.net> writes:

>> When creating veth pair, at first rtnl_new_link() creates veth_dev, i.e.
>> one end of the veth pipe, but not registers it; then veth_newlink() gets
>> invoked, where peer dev is created _and_ registered, followed by veth_dev
>> registration, which may fail if peer information, that is VETH_INFO_PEER
>> attribute, has not been provided and the kernel will allocate unique veth
>> name.
>> 
>> So, we should ask the kernel to allocate unique name for veth_dev only
>> when peer info is not available.
>> 
>> Example:
>> 
>> % ip link dev veth0 type veth
>> RTNETLINK answers: File exists
>> 
>> After fix:
>> % ip link dev veth0 type veth
>> % ip link show dev veth0
>> 5: veth0@...h1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
>>     link/ether f6:ef:8b:96:f4:ec brd ff:ff:ff:ff:ff:ff
>> %
>> 
>> Signed-off-by: Roman Mashak <mrv@...atatu.com>
>
> I'm not so sure about this.
>
> If we specify an explicit tb[IFLA_NAME], we shouldn't completely ignore that
> request from the user just because they didn't give any peer information.
>
> I see what happens in this case, the peer gets 'veth0' and then since
> the user asked for 'veth0' for the non-peer it conflicts.

So, the only way is to require user space to _always_ pass in
VETH_INFO_PEER, which may break existing code (fixing iproute2 is easiest).

Otherwise ignore netlink messages lacking of VETH_INFO_PEER and return
error.

IMO, neither of these solutions seem reasonable.

Also, there are valid use cases where a user does not care about veth
name sitting in container, but assigns a name following certain
pattern to a host-side veth.

> Well, too bad.  The user must work to orchestrate things such that
> this doesn't happen.  That means either providing the IFLA_NAME for
> both the peer and the non-peer, or specifying neither.
>
> I'm not applying this, sorry.