netdev - Re: [PATCH RFC v3 net 1/2] ipv6: Remove the net->ipv6.ip6_null

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Date:	Wed, 08 Oct 2014 15:08:38 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	kafai@...com
Cc:	netdev@...r.kernel.org, hannes@...essinduktion.org
Subject: Re: [PATCH RFC v3 net 1/2] ipv6: Remove the
 net->ipv6.ip6_null_entry check

From: Martin KaFai Lau <kafai@...com>
Date: Mon, 6 Oct 2014 17:05:14 -0700

> The above BACKTRACK have already caught the rt == net->ipv6.ip6_null_entry case
> 
> Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Signed-off-by: Martin KaFai Lau <kafai@...com>
 ...
> @@ -936,8 +936,7 @@ restart:
>  	if (rt->rt6i_nsiblings)
>  		rt = rt6_multipath_select(rt, fl6, oif, strict | reachable);
>  	BACKTRACK(net, &fl6->saddr);
> -	if (rt == net->ipv6.ip6_null_entry ||
> -	    rt->rt6i_flags & RTF_CACHE)
> +	if (rt->rt6i_flags & RTF_CACHE)
>  		goto out;
>  
>  	dst_hold(&rt->dst);

I think this is sort of going in the wrong direction.

The BACKTRACK() macro hides a lot of side effects inside of it's
implementation, and worst of all it hides a change of control flow
with it's "goto out;" and "goto restart;"

I'd rather see us clean this up in some way that someone auditing this
code won't be tricked into missing the control flow side effects, than
adding more dependencies upon BACKTRACK()'s implementation.

Thanks.
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