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

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Date:   Mon, 25 Nov 2019 22:07:12 +0000
From:   Martin Lau <kafai@...com>
To:     Jakub Sitnicki <jakub@...udflare.com>
CC:     John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kernel-team@...udflare.com" <kernel-team@...udflare.com>
Subject: Re: [PATCH bpf-next 5/8] bpf: Allow selecting reuseport socket from a
 SOCKMAP

On Mon, Nov 25, 2019 at 11:40:41AM +0100, Jakub Sitnicki wrote:
> On Mon, Nov 25, 2019 at 05:17 AM CET, John Fastabend wrote:
> > 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;
If I read it correctly,
this is to avoid the following "if (!reuse)" to return -ENOENT,
and instead returns -EINVAL for non TCP_LISTEN tcp_sock.
It should at least only be done under the "if (!reuse)" then.

Checking SOCK_RCU_FREE to imply TCP_LISTEN is not ideal.
It is not immediately obvious.  Why not directly check
TCP_LISTEN?

Note that the SOCK_RCU_FREE check at the 'slow-path'
reuseport_array_update_check() is because reuseport_array does depend on
call_rcu(&sk->sk_rcu,...) to work, e.g. the reuseport_array
does not hold the sk_refcnt.

> >>
> >> 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.
> 
> That was exactly my thinking here.
> 
> REUSEPORT_SOCKARRAY can't be populated with sockets that don't have
> SOCK_RCU_FREE set. This makes the flag check in sk_select_reuseport BPF
> helper redundant for this map type.
> 
> SOCKMAP, OTOH, allows storing established TCP sockets, which don't have
> SOCK_RCU_FREE flag and shouldn't be used as reuseport targets. The newly
> added check protects us against it.
> 
> I have a couple tests in the last patch for it -
> test_sockmap_reuseport_select_{listening,connected}. Admittedly, UDP is
> not covered.
> 
> Not sure how we could go about moving the checks to the update path for
> SOCKMAP. At update time we don't know if the map will be used with a
> reuseport or a sk_{skb,msg} program.
or make these checks specific to the sockmap's lookup path.



digress a little from this patch,
will the upcoming patches/examples show the use case to have both
TCP_LISTEN and TCP_ESTABLISHED sk in the same sock_map?