netdev - Re: [PATCH bpf-next 5/8] bpf: Allow selecting reuseport socket from a SOCKMAP

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Date:   Sun, 24 Nov 2019 20:17:12 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Jakub Sitnicki <jakub@...udflare.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        kernel-team@...udflare.com,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH bpf-next 5/8] bpf: Allow selecting reuseport socket from a
 SOCKMAP

Alexei Starovoitov wrote:
> On Sat, Nov 23, 2019 at 12:07:48PM +0100, Jakub Sitnicki wrote:
> > SOCKMAP now supports storing references to listening sockets. Nothing keeps
> > us from using it as an array of sockets to select from in SK_REUSEPORT
> > programs.
> > 
> > Whitelist the map type with the BPF helper for selecting socket. However,
> > impose a restriction that the selected socket needs to be a listening TCP
> > socket or a bound UDP socket (connected or not).
> > 
> > The only other map type that works with the BPF reuseport helper,
> > REUSEPORT_SOCKARRAY, has a corresponding check in its update operation
> > handler.
> > 
> > Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> > ---

[...]

> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 49ded4a7588a..e3fb77353248 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -8723,6 +8723,8 @@ BPF_CALL_4(sk_select_reuseport, struct sk_reuseport_kern *, reuse_kern,
> >  	selected_sk = map->ops->map_lookup_elem(map, key);
> >  	if (!selected_sk)
> >  		return -ENOENT;
> > +	if (!sock_flag(selected_sk, SOCK_RCU_FREE))
> > +		return -EINVAL;
> 
> hmm. I wonder whether this breaks existing users...

There is already this check in reuseport_array_update_check()

	/*
	 * sk must be hashed (i.e. listening in the TCP case or binded
	 * in the UDP case) and
	 * it must also be a SO_REUSEPORT sk (i.e. reuse cannot be NULL).
	 *
	 * Also, sk will be used in bpf helper that is protected by
	 * rcu_read_lock().
	 */
	if (!sock_flag(nsk, SOCK_RCU_FREE) || !sk_hashed(nsk) || !nsk_reuse)
		return -EINVAL;

So I believe it should not cause any problems with existing users. Perhaps
we could consolidate the checks a bit or move it into the update paths if we
wanted. I assume Jakub was just ensuring we don't get here with SOCK_RCU_FREE
set from any of the new paths now. I'll let him answer though.

> Martin,
> what do you think?

More eyes the better.

> Could you also take a look at other patches too?
> In particular patch 7?
> 

Agreed would be good to give 7/8 a look I'm not too familiar with the
selftests there.