netdev - Re: [PATCH v6 bpf-next 03/11] tcp: Keep TCP_CLOSE sockets in the reuseport group.

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Date:   Thu, 20 May 2021 14:22:01 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Kuniyuki Iwashima <kuniyu@...zon.co.jp>
CC:     <andrii@...nel.org>, <ast@...nel.org>, <benh@...zon.com>,
        <bpf@...r.kernel.org>, <daniel@...earbox.net>,
        <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
        <kuni1840@...il.com>, <linux-kernel@...r.kernel.org>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH v6 bpf-next 03/11] tcp: Keep TCP_CLOSE sockets in the
 reuseport group.

On Thu, May 20, 2021 at 05:51:17PM +0900, Kuniyuki Iwashima wrote:
> From:   Martin KaFai Lau <kafai@...com>
> Date:   Wed, 19 May 2021 23:26:48 -0700
> > On Mon, May 17, 2021 at 09:22:50AM +0900, Kuniyuki Iwashima wrote:
> > 
> > > +static int reuseport_resurrect(struct sock *sk, struct sock_reuseport *old_reuse,
> > > +			       struct sock_reuseport *reuse, bool bind_inany)
> > > +{
> > > +	if (old_reuse == reuse) {
> > > +		/* If sk was in the same reuseport group, just pop sk out of
> > > +		 * the closed section and push sk into the listening section.
> > > +		 */
> > > +		__reuseport_detach_closed_sock(sk, old_reuse);
> > > +		__reuseport_add_sock(sk, old_reuse);
> > > +		return 0;
> > > +	}
> > > +
> > > +	if (!reuse) {
> > > +		/* In bind()/listen() path, we cannot carry over the eBPF prog
> > > +		 * for the shutdown()ed socket. In setsockopt() path, we should
> > > +		 * not change the eBPF prog of listening sockets by attaching a
> > > +		 * prog to the shutdown()ed socket. Thus, we will allocate a new
> > > +		 * reuseport group and detach sk from the old group.
> > > +		 */
> > For the reuseport_attach_prog() path, I think it needs to consider
> > the reuse->num_closed_socks != 0 case also and that should belong
> > to the resurrect case.  For example, when
> > sk_unhashed(sk) but sk->sk_reuseport == 0.
> 
> In the path, reuseport_resurrect() is called from reuseport_alloc() only
> if reuse->num_closed_socks != 0.
> 
> 
> > @@ -92,6 +117,14 @@ int reuseport_alloc(struct sock *sk, bool bind_inany)
> >  	reuse = rcu_dereference_protected(sk->sk_reuseport_cb,
> >  					  lockdep_is_held(&reuseport_lock));
> >  	if (reuse) {
> > +		if (reuse->num_closed_socks) {
> 
> But, should this be
> 
> 	if (sk->sk_state == TCP_CLOSE && reuse->num_closed_socks)
> 
> because we need not allocate a new group when we attach a bpf prog to
> listeners?
The reuseport_alloc() is fine as is.  No need to change.

I should have copied reuseport_attach_prog() in the last reply and
commented there instead.

I meant reuseport_attach_prog() needs a change.  In reuseport_attach_prog(),
iiuc, currently passing the "else if (!rcu_access_pointer(sk->sk_reuseport_cb))"
check implies the sk was (and still is) hashed with sk_reuseport enabled
because the current behavior would have set sk_reuseport_cb to NULL during
unhash but it is no longer true now.  For example, this will break:

1. shutdown(lsk); /* lsk was bound with sk_reuseport enabled */
2. setsockopt(lsk, ..., SO_REUSEPORT, &zero, ...); /* disable sk_reuseport */
3. setsockopt(lsk, ..., SO_ATTACH_REUSEPORT_EBPF, &prog_fd, ...);
   ^---- /* This will work now because sk_reuseport_cb is not NULL.
          * However, it shouldn't be allowed.
	  */

I am thinking something like this (uncompiled code):

int reuseport_attach_prog(struct sock *sk, struct bpf_prog *prog)
{
	struct sock_reuseport *reuse;
	struct bpf_prog *old_prog;

	if (sk_unhashed(sk)) {
		int err;

		if (!sk->sk_reuseport)
			return -EINVAL;

		err = reuseport_alloc(sk, false);
		if (err)
			return err;
	} else if (!rcu_access_pointer(sk->sk_reuseport_cb)) {
		/* The socket wasn't bound with SO_REUSEPORT */
		return -EINVAL;
	}

	/* ... */
}

WDYT?