netdev - Re: [PATCH net-next 3/7] ipv6: improve ipv6_find

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Message-ID: <CAA=3Oqky-GUbj-ZDqYAtxpg-fjDGAdo04OHbKXp7uQRKMcR_sQ@mail.gmail.com>
Date:	Tue, 4 Dec 2012 12:47:31 -0800
From:	Ansis Atteka <aatteka@...ira.com>
To:	Jesse Gross <jesse@...ira.com>
Cc:	Pablo Neira Ayuso <pablo@...filter.org>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	dev@...nvswitch.org
Subject: Re: [PATCH net-next 3/7] ipv6: improve ipv6_find_hdr() to skip empty
 routing headers

On Tue, Dec 4, 2012 at 10:15 AM, Jesse Gross <jesse@...ira.com> wrote:
> On Mon, Dec 3, 2012 at 10:06 AM, Pablo Neira Ayuso <pablo@...filter.org> wrote:
>> On Mon, Dec 03, 2012 at 09:28:55AM -0800, Jesse Gross wrote:
>>> On Mon, Dec 3, 2012 at 6:04 AM, Pablo Neira Ayuso <pablo@...filter.org> wrote:
>>> > On Thu, Nov 29, 2012 at 10:35:45AM -0800, Jesse Gross wrote:
>>> >> @@ -159,9 +162,10 @@ int ipv6_find_hdr(const struct sk_buff *skb, unsigned int *offset,
>>> >>       }
>>> >>       len = skb->len - start;
>>> >>
>>> >> -     while (nexthdr != target) {
>>> >
>>> > If the offset is set as parameter via ipv6_find_hdr, we now are always
>>> > entering the loop even if we found the target header we're looking
>>> > for, before that didn't happen.
>>> >
>>> > Something seems wrong here to me.
>>>
>>> If the target header is a routing header then you might still need to
>>> continue searching because the first one that you see could be empty.
>>
>> OK, but if it's not a routing header what we're searching for (which
>> seems to be the case of netfilter/IPVS) we waste way more cycles on
>> copying the IPv6 header again and with way more things that are
>> completely useless.
>
> We could add a check to short circuit this but it seems like a
> premature optimization to me.
>
> Ansis, can you comment?

Pablo, I think that the only concern you have here is about
optimizations, right?

We could either:
1. add an "if" statement that terminates loop early; or
2. switch back to "while () {}" loop and change condition from
"nexthdr != target" to something like "nexthdr != target || (nexthdr
== NEXTHDR_ROUTING && flags && (*flags & IP6_FH_F_SKIP_RH))".

This function seemed like a good candidate to extend it for this. I
think that iptables could make use of it too (to figure out whether L4
checksums need to be updated in presence of routing headers and NAT).

Thanks,
Ansis
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