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

[Martin Lau <kafai@fb.com>]

On Tue, Nov 26, 2019 at 03:30:57PM +0100, Jakub Sitnicki wrote:
> On Mon, Nov 25, 2019 at 11:07 PM CET, Martin Lau wrote:
> > 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@cloudflare.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.
> 
> Yes, exactly. For an established TCP socket in SOCKMAP we would get
> -ENOENT because sk_reuseport_cb is not set. Which is a bit confusing
> since the map entry exists.
> 
> Returning -EINVAL matches the REUSEPORT_SOCKARRAY update operation
> semantics for established TCP sockets.
> 
> But this is just about returning an informative error so you're
> completely right that this should be done under "if (!reuse)" branch to
> avoid the extra cost on the happy path.
> 
> > Checking SOCK_RCU_FREE to imply TCP_LISTEN is not ideal.
> > It is not immediately obvious.  Why not directly check
> > TCP_LISTEN?
> 
> I agree, it's not obvious. When I first saw this check in
> reuseport_array_update_check it got me puzzled too. I should have added
> an explanatory comment there.
> 
> Thing is we're not matching on just TCP_LISTEN. REUSEPORT_SOCKARRAY
> allows selecting a connected UDP socket as a target as well. It takes
> some effort to set up but it's possible even if obscure.
How about this instead:
if (!reuse)
 	/* reuseport_array only has sk that has non NULL sk_reuseport_cb.
	 * The only (!reuse) case here is, the sk has already been removed from
	 * reuseport_array, so treat it as -ENOENT.
	 *
	 * Other maps (e.g. sock_map) do not provide this guarantee and the sk may
	 * never be in the reuseport to begin with.
	 */
	return map->map_type == BPF_MAP_TYPE_REUSEPORT_SOCKARRAY ? -ENOENT : -EINVAL;

> 
> > 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.
> 
> Oh, so it's not only about socket state like I thought.
> 
> This raises the question - does REUSEPORT_SOCKARRAY allow storing
> connected UDP sockets by design or is it a happy accident? It doesn't
> seem particularly useful.
Not by design/accident on the REUSEPORT_SOCKARRAY side ;)

The intention of REUSEPORT_SOCKARRAY is to allow sk that can be added to
reuse->socks[].