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

[Kuniyuki Iwashima]

From:   Martin KaFai Lau <kafai@xxxxxx>
Date:   Thu, 20 May 2021 16:39:06 -0700
> On Fri, May 21, 2021 at 07:54:48AM +0900, Kuniyuki Iwashima wrote:
> > From:   Martin KaFai Lau <kafai@xxxxxx>
> > Date:   Thu, 20 May 2021 14:22:01 -0700
> > > On Thu, May 20, 2021 at 05:51:17PM +0900, Kuniyuki Iwashima wrote:
> > > > From:   Martin KaFai Lau <kafai@xxxxxx>
> > > > 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 missed sk_unhashed(sk) prevents calling reuseport_alloc()
> > if sk_state == TCP_LISTEN. I'll keep it as is.
> > 
> > 
> > > 
> > > 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.
> > > 	  */
> > 
> > Thank you for explanation, I understood the case.
> > 
> > Exactly, I've confirmed that the case succeeded in the setsockopt() and I
> > could change the active listeners' prog via a shutdowned socket.
> > 
> > 
> > > 
> > > 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?
> > 
> > I tested this change worked fine. I think this change should be added in
> > reuseport_detach_prog() also.
> > 
> > ---8<---
> > int reuseport_detach_prog(struct sock *sk)
> > {
> >         struct sock_reuseport *reuse;
> >         struct bpf_prog *old_prog;
> > 
> >         if (!rcu_access_pointer(sk->sk_reuseport_cb))
> > 		return sk->sk_reuseport ? -ENOENT : -EINVAL;
> > ---8<---
> Right, a quick thought is something like this for detach:
> 
> 	spin_lock_bh(&reuseport_lock);
> 	reuse = rcu_dereference_protected(sk->sk_reuseport_cb,
> 					  lockdep_is_held(&reuseport_lock));

Is this necessary because reuseport_grow() can detach sk?

        if (!reuse) {
                spin_unlock_bh(&reuseport_lock);
                return -ENOENT;
        }

Then we can remove rcu_access_pointer() check and move sk_reuseport check
here.


> 	if (sk_unhashed(sk) && reuse->num_closed_socks) {
> 		spin_unlock_bh(&reuseport_lock);
> 		return -ENOENT;
> 	}
> 
> Although checking with reuseport_sock_index() will also work,
> the above probably is simpler and faster?

Yes, if sk is unhashed and has sk_reuseport_cb, it stays in the closed
section of socks[] and num_closed_socks is larger than 0.


> 
> > 
> > 
> > Another option is to add the check in sock_setsockopt():
> > SO_ATTACH_REUSEPORT_[CE]BPF, SO_DETACH_REUSEPORT_BPF.
> > 
> > Which do you think is better ?
> I think it is better to have this sock_reuseport specific bits
> staying in sock_reuseport.c.

Exactly, I'll keep the change in sock_reuseport.c