netdev - Re: [PATCH/RFC rocker-net-next 1/6] net: flow: Cancel innermost nested attribute first

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Date:	Mon, 5 Jan 2015 22:01:13 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	David Miller <davem@...emloft.net>
Cc:	simon.horman@...ronome.com, john.fastabend@...il.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH/RFC rocker-net-next 1/6] net: flow: Cancel innermost
 nested attribute first

On 01/05/15 at 04:17pm, David Miller wrote:
> From: Simon Horman <simon.horman@...ronome.com>
> Date: Mon,  5 Jan 2015 15:50:05 +0900
> 
> > Cancel innermost nested attribute first on error when putting flow actions.
> > 
> > Signed-off-by: Simon Horman <simon.horman@...ronome.com>
> > 
> > ---
> > 
> > Its unclear to me if this makes any difference.
> > But it seems more logical to me.
> 
> Hmmm.  Be careful here.  nla_nest_cancel() is just rolling back
> the length of the SKB to right before the netlink attribute being
> given as the cancellation point.
> 
> So you really have to cancel attributes in exactly the reverse order
> in which they were added.  Otherwise we'll make a trim call with a
> negative adjustment that actually expands the SKB past an already
> cancelled attribute.

As I told John in the other the other thread: It is often clearer
and less error prone to only ever cancel the outer most nesting
level if we are undoing all of them anyway. Dave is absolutely right,
a wrong order will lead to bugs which are hard to track down.
Thinking about this I'll send a patch to WARN() in nlmsg_trim() on
mark < skb->data.
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