netdev - Re: Why do we prefer skb->priority to tc filters?

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Message-ID: <CAHA+R7N88DFPpfPeN5cqru7oODfew1ZUdT80R0fj1QkQsx-siw@mail.gmail.com>
Date:	Wed, 11 Mar 2015 15:12:19 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev <netdev@...r.kernel.org>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	David Miller <davem@...emloft.net>
Subject: Re: Why do we prefer skb->priority to tc filters?

On Wed, Mar 11, 2015 at 2:47 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2015-03-11 at 13:46 -0700, Cong Wang wrote:
>
>> That is just a permission check when val > 6, given the fact most
>> daemons have root permission, I doubt your argument makes a difference
>> for discussion. At least with userns having root permission is more common.
>
> Some setups use ip[6]tables rules to mangle skb->priority to select a
> HTB class.
>
> Google definitely uses this model, as netfilter code runs on multiple
> cpus, while HTB classifier runs under qdisc spinlock, so far.

I knew we can modify skb->priority in a few ways, for example skbedit.

That is not my concern, all what I am thinking is there is some
way in application layer to bypass our tc filters, which is not expected
to happen for me. Given our specific case, I want to propose to clear
skb->priority after moving out of a netns:

diff --git a/net/core/dev.c b/net/core/dev.c
index 962ee9d..2301f01 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1694,6 +1694,7 @@ int __dev_forward_skb(struct net_device *dev,
struct sk_buff *skb)
        }

        skb_scrub_packet(skb, true);
+       skb->priority = 0;
        skb->protocol = eth_type_trans(skb, dev);
        skb_postpull_rcsum(skb, eth_hdr(skb), ETH_HLEN);
--
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