netdev - Re: [bpf-next, v4 1/5] flow_dissector: implements flow dissector BPF hook

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Date:   Tue, 18 Sep 2018 11:07:10 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Petar Penkov <peterpenkov96@...il.com>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, ast@...nel.org, daniel@...earbox.net,
        simon.horman@...ronome.com, ecree@...arflare.com,
        songliubraving@...com, tom@...bertland.com,
        Petar Penkov <ppenkov@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [bpf-next, v4 1/5] flow_dissector: implements flow dissector BPF
 hook



On 09/14/2018 07:46 AM, Petar Penkov wrote:
> From: Petar Penkov <ppenkov@...gle.com>
> 
> Adds a hook for programs of type BPF_PROG_TYPE_FLOW_DISSECTOR and
> attach type BPF_FLOW_DISSECTOR that is executed in the flow dissector
> path. The BPF program is per-network namespace

...

>  
> +	rcu_read_lock();
> +	attached = skb ? rcu_dereference(dev_net(skb->dev)->flow_dissector_prog)
> +		       : NULL;


Some skbs have a NULL skb->dev, so we are going to crash here.

(AF_UNIX generates skbs)

> +	if (attached) {
> +		/* Note that even though the const qualifier is discarded
> +		 * throughout the execution of the BPF program, all changes(the
> +		 * control block) are reverted after the BPF program returns.
> +		 * Therefore, __skb_flow_dissect does not alter the skb.
> +		 */