netdev - Re: [PATCH v2 net-next 4/4] net/sched: act

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Date:   Tue, 27 Sep 2016 14:27:13 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Shmulik Ladkani <shmulik.ladkani@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        WANG Cong <xiyou.wangcong@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Florian Westphal <fw@...len.de>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next 4/4] net/sched: act_mirred: Implement
 ingress actions

On Tue, 2016-09-27 at 23:59 +0300, Shmulik Ladkani wrote:
> Up until now, 'action mirred' supported only egress actions (either
> TCA_EGRESS_REDIR or TCA_EGRESS_MIRROR).
> 
> This patch implements the corresponding ingress actions
> TCA_INGRESS_REDIR and TCA_INGRESS_MIRROR.


> -		if (m->tcfm_mac_header_xmit)
> +	/* If action's target direction differs than filter's direction,
> +	 * and devices expect a mac header on xmit, then mac push/pull is
> +	 * needed.
> +	 */
> +	if (at != tcf_mirred_act_direction(m->tcfm_eaction) &&

Note that m->tcfm_eaction is read here.

> +	    m->tcfm_mac_header_xmit) {
> +		if (at & AT_EGRESS) {
> +			/* caught at egress, act ingress: pull mac */
> +			mac_len = skb_network_header(skb) - skb_mac_header(skb);
> +			skb_pull_rcsum(skb2, mac_len);
> +		} else {
> +			/* caught at ingress, act egress: push mac */
>  			skb_push_rcsum(skb2, skb->mac_len);
> +		}
>  	}
>  
>  	/* mirror is always swallowed */
> -	if (m->tcfm_eaction != TCA_EGRESS_MIRROR)
> +	if (tcf_mirred_is_act_redirect(m->tcfm_eaction))
>  		skb2->tc_verd = SET_TC_FROM(skb2->tc_verd, at);
>  
>  	skb2->skb_iif = skb->dev->ifindex;
>  	skb2->dev = dev;
> -	err = dev_queue_xmit(skb2);

Note that m->tcfm_eaction is read another time here.

> +	if (tcf_mirred_act_direction(m->tcfm_eaction) & AT_EGRESS)
> +		err = dev_queue_xmit(skb2);
> +	else
> +		netif_receive_skb(skb2);
>  

Since this runs lockless, another cpu might change m->tcfm_eaction in
the middle, and you could call dev_queue_xmit(skb2) while the skb2 was
prepared for the opposite action.

I guess some drivers could crash, because they expect to find a MAC
header.

If not, a comment would be nice.

Thanks.