netdev - Re: Linux kernel patch: elide fib_validate_source() completely when possible

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Message-ID: <20140123030329.GI7269@order.stressinduktion.org>
Date:	Thu, 23 Jan 2014 04:03:29 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	David Miller <davem@...emloft.net>
Cc:	ja@....bg, gregory.hoggarth@...iedtelesis.co.nz,
	netdev@...r.kernel.org
Subject: Re: Linux kernel patch: elide fib_validate_source() completely when possible - bad side effect?

On Wed, Jan 22, 2014 at 06:53:05PM -0800, David Miller wrote:
> From: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Date: Thu, 23 Jan 2014 03:02:24 +0100
> 
> > On Wed, Jan 22, 2014 at 11:04:52PM +0200, Julian Anastasov wrote:
> >> 	I don't remember someone mentioning about such
> >> side-effect, I guess it is overlooked. IMHO, it is not a good
> >> reason to restore the old behavior. Lets see other opinions.
> > 
> > A routing lookup to check for broadcast is much too expansive, I agree. But
> > can't we just check skb->pkt_type == PACKET_BROADCAST?
> 
> We can't universally know what a broadcast is, because the prefix of
> every network is a local piece of information.
> 
> We might not have the route necessary to know the source address is
> a broadcast if the packet came through more than 1 hop already.

But in this case we only need those local pieces.

E.g. we register all local registered broadcast addresses in a structure
like inet_addr_lst so we only need to check if the packet would leave
this host with a broadcast hardware address. If the packet is forwarded
the router must do the same check as only it knows the local broadcast
addresses. I hope this is correct. ;)

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