Subject: Re: [PATCH net-next v2 1/3] drop_monitor: Implement namespace filtering/reporting for software drops

From: Ido Schimmel <[email-masked]>
Date: Wed, 23 Nov 2022 17:16:43 +0200

On Wed, Nov 23, 2022 at 04:28:15PM +0200, Nikolay Borisov wrote:
>  static void trace_drop_common(struct sk_buff *skb, void *location)
>  {
>  	struct net_dm_alert_msg *msg;
> @@ -219,7 +233,11 @@ static void trace_drop_common(struct sk_buff *skb, void *location)
>  	int i;
>  	struct sk_buff *dskb;
>  	struct per_cpu_dm_data *data;
> -	unsigned long flags;
> +	unsigned long flags, ns_id = 0;
> +
> +	if (skb->dev && net_dm_ns &&
> +	    dev_net(skb->dev)->ns.inum != net_dm_ns)

I don't think this is going to work, unfortunately. 'skb->dev' is in a
union with 'dev_scratch' so 'skb->dev' does not necessarily point to a
valid netdev at all times. It can explode when dev_net() tries to
dereference it.

__skb_flow_dissect() is doing something similar, but I believe there the
code paths were audited to make sure it is safe.

Did you consider achieving this functionality with a BPF program
attached to skb::kfree_skb tracepoint? I believe BPF programs are run
with page faults disabled, so it should be safe to attempt this there.