netdev - Re: [PATCH] veth: Cleanly handle a missing peer

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Date:	Wed, 12 Sep 2007 09:25:45 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Pavel Emelyanov <xemul@...nvz.org>
Cc:	David Miller <davem@...emloft.net>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org,
	Stephen Hemminger <shemminger@...ux-foundation.org>
Subject: Re: [PATCH] veth: Cleanly handle a missing peer_tb argument on creation.

Pavel Emelyanov <xemul@...nvz.org> writes:

> Eric W. Biederman wrote:
>> Pavel Emelyanov <xemul@...nvz.org> writes:
>> 
>>>> +	}
>>>>  
>>>> -		tbp = peer_tb;
>>>> -	} else
>>>> -		tbp = tb;
>>> The intention of this part was to get the same parameters for
>>> peer as for the first device if no "peer" argument was specified
>>> for ip utility. Does it still work?
>> 
>> I know it is problematic because we try to assign the same name
>> to both network devices, if we assign a name to the primary
>> network device.  That can't work.
>
> This can - as you can see I reallocate the name lower.

Hmm. I just see:
	if (tbp[IFLA_IFNAME])
		nla_strlcpy(ifname, tbp[IFLA_IFNAME], IFNAMSIZ);

Then lower I see:
	if (tb[IFLA_IFNAME])
		nla_strlcpy(dev->name, tb[IFLA_IFNAME], IFNAMSIZ);

If (tb == tbp) then dev->name == ifname
Unless I'm completely misreading that code.

>> Beyond that I had some really weird crashes while testing this
>> piece of code, especially when I did not specify a peer parameter.
>
> Can you please give me the exact command that caused an oops.
> I try simple ip link add type veth and everything is just fine.

It might have been 64bit specific. 

What I have in my history is:
./ip/ip link add veth23 type veth

I forget exactly how it failed but as I recall it wasn't as
nice as an oops.  My memory may be a bit foggy though.

If I haven't provided a bit enough clue I guess I can go back
and remove the patch and try to reproduce the failure again.

>> So it was just easier to avoid the problem with this patch then
>> to completely root cause it.
>
> Let me handle this problem. AFAIR this was one of wishes from 
> Patrick that we make two equal devices in case peer is not given, 
> not just the default peer.

Ok.  I have if we can track down the weird cases I have no problem
if we handle this.  I think it still might be simpler if just
copy tb onto peer_tb instead of using tbp.

Eric
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