netdev - Re: changing dev->needed_headroom/needed

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Date:	Tue, 20 Aug 2013 11:00:18 +0100
From:	Florian Fainelli <f.fainelli@...il.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	netdev <netdev@...r.kernel.org>, linux-wireless@...r.kernel.org
Subject: Re: changing dev->needed_headroom/needed_tailroom?

2013/8/5 Johannes Berg <johannes@...solutions.net>:
> On Fri, 2013-08-02 at 06:11 -0700, Eric Dumazet wrote:
>> On Fri, 2013-08-02 at 10:55 +0200, Ben Hutchings wrote:
>>
>> > I don't think this is safe when the interface is running (even if
>> > carrier is off).  Some functions may read dev->needed_headroom twice and
>> > rely on getting the same value each time.
>>
>> It should be no problem. Remaining unsafe places should be fixed.
>
> Most interesting would be stack devs, which I hadn't even considered. In
> any case, since I can't completely _rely_ on it, it's an optimisation,
> the only bugs would be around the double-access and then running
> over/under the SKB or so?

As far as I could test this with an Ethernet driver which adjusted its
needed_headroom by 64 bytes whenever some hardware feature was
enabled/disabled, this expectedly broke bridge and vlans at least.
Bridge code does not use the slave ports needed_headroom values, and
VLAN devices get the parent device needed_headroom only when creating
the vlan device. The good thing is since the needed_headroom space you
need is most likely fixed for a given configuration type, you should
see a pretty "stable" corruption of your SKB head.

>
>> We already had this discussion in the past, and some patches were
>> issued. Check commit ae641949df01b85117845bec45328eab6d6fada1
>> ("net: Remove all uses of LL_ALLOCATED_SPACE")
>
> That would have addressed some of that, I guess.
>
>
> I'm asking because some of the crypto stuff we do has fairly large
> head/tailroom requirements and it seems I may need to add more. But if
> you don't have crypto, it would be much smaller, so I figured we could
> switch it.

We could probably do it via a pair of new NETDEV_* notify event to
signal new needed_headroom/tailroom values to stacked devices. Would
that be acceptable?
-- 
Florian
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