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Date: Mon, 28 Nov 2022 09:54:52 +0200 From: Ido Schimmel <idosch@...sch.org> To: nb <nikolay.borisov@...tuozzo.com> Cc: nhorman@...driver.com, davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org, kernel@...nvz.org Subject: Re: [PATCH net-next v2 1/3] drop_monitor: Implement namespace filtering/reporting for software drops On Thu, Nov 24, 2022 at 01:41:38PM +0200, nb wrote: > > > On 23.11.22 г. 20:10 ч., Ido Schimmel wrote: > > On Wed, Nov 23, 2022 at 05:21:23PM +0200, nb wrote: > > > > > > > > > On 23.11.22 г. 17:16 ч., Ido Schimmel wrote: > > > > 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. > > > > > > How would that be different than the trace_drop_common which is called as > > > part of the trace_kfree_skb, as it's really passed as trace point probe via: > > > > Consider this call path: > > > > __udp_queue_rcv_skb() > > __udp_enqueue_schedule_skb() > > udp_set_dev_scratch() // skb->dev is not NULL, but not a pointer to a netdev either > > // error is returned > > kfree_skb_reason() // probe is called > > > > dev_net(skb->dev) in the probe will try to dereference skb->dev and > > crash. > > This can easily be rectified by using is_kernel() . The layout of 'struct udp_dev_scratch' is not fixed and it can be arranged to contain values that make it seem like a valid kernel address, but does not actually point to a 'struct net_device'. > > > > > On the other hand, a BPF program that is registered as another probe on > > the tracepoint will access the memory via bpf_probe_read_kernel(), which > > will try to safely read the memory and return an error if it can't. You > > can do that today without any kernel changes. > > I did a PoC for this and indeed it works, however I'd still like to pursue > this code provided there is upstream interest.
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