Subject: Re: [PATCH bpf-next 4/8] bpf, sockmap: Don't let child socket inherit psock or its ops on copy

From: Jakub Sitnicki <[email-masked]>
Date: Tue, 26 Nov 2019 16:54:33 +0100

On Mon, Nov 25, 2019 at 11:38 PM CET, Martin Lau wrote:
> On Sat, Nov 23, 2019 at 12:07:47PM +0100, Jakub Sitnicki wrote:
> [ ... ]
>
>> @@ -370,6 +378,11 @@ static inline void sk_psock_restore_proto(struct sock *sk,
>>  			sk->sk_prot = psock->sk_proto;
>>  		psock->sk_proto = NULL;
>>  	}
>> +
>> +	if (psock->icsk_af_ops) {
>> +		icsk->icsk_af_ops = psock->icsk_af_ops;
>> +		psock->icsk_af_ops = NULL;
>> +	}
>>  }
>
> [ ... ]
>
>> +static struct sock *tcp_bpf_syn_recv_sock(const struct sock *sk,
>> +					  struct sk_buff *skb,
>> +					  struct request_sock *req,
>> +					  struct dst_entry *dst,
>> +					  struct request_sock *req_unhash,
>> +					  bool *own_req)
>> +{
>> +	const struct inet_connection_sock_af_ops *ops;
>> +	void (*write_space)(struct sock *sk);
>> +	struct sk_psock *psock;
>> +	struct proto *proto;
>> +	struct sock *child;
>> +
>> +	rcu_read_lock();
>> +	psock = sk_psock(sk);
>> +	if (likely(psock)) {
>> +		proto = psock->sk_proto;
>> +		write_space = psock->saved_write_space;
>> +		ops = psock->icsk_af_ops;
> It is not immediately clear to me what ensure
> ops is not NULL here.
>
> It is likely I missed something.  A short comment would
> be very useful here.

I can see the readability problem. Looking at it now, perhaps it should
be rewritten, to the same effect, as:

static struct sock *tcp_bpf_syn_recv_sock(...)
{
	const struct inet_connection_sock_af_ops *ops = NULL;
        ...

        rcu_read_lock();
	psock = sk_psock(sk);
	if (likely(psock)) {
		proto = psock->sk_proto;
		write_space = psock->saved_write_space;
		ops = psock->icsk_af_ops;
	}
	rcu_read_unlock();

        if (!ops)
		ops = inet_csk(sk)->icsk_af_ops;
        child = ops->syn_recv_sock(sk, skb, req, dst, req_unhash, own_req);

If psock->icsk_af_ops were NULL, it would mean we haven't initialized it
properly. To double check what happens here:

In sock_map_link we do a setup dance where we first create the psock and
later initialize the socket callbacks (tcp_bpf_init).

static int sock_map_link(struct bpf_map *map, struct sk_psock_progs *progs,
			 struct sock *sk)
{
        ...
	if (psock) {
                ...
	} else {
		psock = sk_psock_init(sk, map->numa_node);
		if (!psock) {
			ret = -ENOMEM;
			goto out_progs;
		}
		sk_psock_is_new = true;
	}
        ...
        if (sk_psock_is_new) {
		ret = tcp_bpf_init(sk);
		if (ret < 0)
			goto out_drop;
	} else {
		tcp_bpf_reinit(sk);
	}

The "if (sk_psock_new)" branch triggers the call chain that leads to
saving & overriding socket callbacks.

tcp_bpf_init -> tcp_bpf_update_sk_prot -> sk_psock_update_proto

Among them, icsk_af_ops.

static inline void sk_psock_update_proto(...)
{
        ...
	psock->icsk_af_ops = icsk->icsk_af_ops;
	icsk->icsk_af_ops = af_ops;
}

Goes without saying that a comment is needed.

Thanks for the feedback,
Jakub