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

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Date:   Fri, 21 May 2021 07:54:48 +0900
From:   Kuniyuki Iwashima <kuniyu@...zon.co.jp>
To:     <kafai@...com>
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>, <kuniyu@...zon.co.jp>,
        <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.

From:   Martin KaFai Lau <kafai@...com>
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@...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 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<---


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 ?