bpf: Add helper to retrieve socket in BPF

Hello Joe Stringer,

This is a semi-automatic email about new static checker warnings.

The patch 6acc9b432e67: "bpf: Add helper to retrieve socket in BPF" from Oct 2, 2018, leads to the following Smatch complaint:

    net/core/filter.c:4893 bpf_sk_lookup()
    error: we previously assumed 'skb->dev' could be null (see line 4885)

net/core/filter.c
4884
4885 if (skb->dev)

                    ^^^^^^^^

The patch adds a check

4886 caller_net = dev_net(skb->dev);
4887 else
4888 caller_net = sock_net(skb->sk);
4889 if (netns_id) {
4890 net = get_net_ns_by_id(caller_net, netns_id);
4891 if (unlikely(!net))
4892 goto out;
4893 sk = sk_lookup(net, tuple, skb, family, proto);

                                                   ^^^
  4894			put_net(net);
  4895		} else {
  4896                  net = caller_net;
  4897                  sk = sk_lookup(net, tuple, skb, family, proto);
                                                   ^^^

We dereference skb->dev inside the sk_lookup() function.

  4898          }
  4899  
  4900          if (sk)
  4901                  sk = sk_to_full_sk(sk);
  4902  out:
  4903          return (unsigned long) sk;
  4904  }

regards,
dan carpenter

Thanks for the report Dan, I'm working on a fix.