netdev - Re: [PATCH bpf-next 6/8] bpf: tcp: bpf iter batching and lock

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Date:   Tue, 29 Jun 2021 11:06:03 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Yonghong Song <yhs@...com>
CC:     <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>, <kernel-team@...com>,
        Neal Cardwell <ncardwell@...gle.com>, <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH bpf-next 6/8] bpf: tcp: bpf iter batching and lock_sock

On Tue, Jun 29, 2021 at 10:57:46AM -0700, Yonghong Song wrote:
> 
> 
> On 6/29/21 10:44 AM, Martin KaFai Lau wrote:
> > On Tue, Jun 29, 2021 at 10:27:17AM -0700, Yonghong Song wrote:
> > [ ... ]
> > 
> > > > +static int bpf_iter_tcp_realloc_batch(struct bpf_tcp_iter_state *iter,
> > > > +				      unsigned int new_batch_sz)
> > > > +{
> > > > +	struct sock **new_batch;
> > > > +
> > > > +	new_batch = kvmalloc(sizeof(*new_batch) * new_batch_sz, GFP_USER);
> > > 
> > > Since we return -ENOMEM below, should we have __GFP_NOWARN in kvmalloc
> > > flags?
> > will add in v2.
> > 
> > > 
> > > > +	if (!new_batch)
> > > > +		return -ENOMEM;
> > > > +
> > > > +	bpf_iter_tcp_put_batch(iter);
> > > > +	kvfree(iter->batch);
> > > > +	iter->batch = new_batch;
> > > > +	iter->max_sk = new_batch_sz;
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +
> > > [...]
> > > > +
> > > >    static int bpf_iter_tcp_seq_show(struct seq_file *seq, void *v)
> > > >    {
> > > >    	struct bpf_iter_meta meta;
> > > >    	struct bpf_prog *prog;
> > > >    	struct sock *sk = v;
> > > > +	bool slow;
> > > >    	uid_t uid;
> > > > +	int ret;
> > > >    	if (v == SEQ_START_TOKEN)
> > > >    		return 0;
> > > > +	if (sk_fullsock(sk))
> > > > +		slow = lock_sock_fast(sk);
> > > > +
> > > > +	if (unlikely(sk_unhashed(sk))) {
> > > > +		ret = SEQ_SKIP;
> > > > +		goto unlock;
> > > > +	}
> > > 
> > > I am not a tcp expert. Maybe a dummy question.
> > > Is it possible to do setsockopt() for listening socket?
> > > What will happen if the listening sock is unhashed after the
> > > above check?
> > It won't happen because the sk has been locked before doing the
> > unhashed check.
> 
> Ya, that is true. I guess I probably mean TCP_TIME_WAIT and
> TCP_NEW_SYN_RECV sockets. We cannot do setsockopt() for
> TCP_TIME_WAIT sockets since user space shouldn't be able
> to access the socket any more.
> 
> But how about TCP_NEW_SYN_RECV sockets?
_bpf_setsockopt() will return -EINVAL for non fullsock.