On Wed, Mar 4, 2020 at 2:38 AM Leppanen, Jere (Nokia - FI/Espoo)
[off-list ref] wrote:

On Mon, 2 Mar 2020, Xin Long wrote:

quoted

As it says in rfc6458#section-9.2:

  The application uses the sctp_peeloff() call to branch off an
  association into a separate socket.  (Note that the semantics are
  somewhat changed from the traditional one-to-one style accept()
  call.)  Note also that the new socket is a one-to-one style socket.
  Thus, it will be confined to operations allowed for a one-to-one
  style socket.

Prior to this patch, sctp_peeloff() returned a one-to-many type socket,
on which some operations are not allowed, like shutdown, as Jere
reported.

This patch is to change it to return a one-to-one type socket instead.

Thanks for looking into this. I like the patch, and it fixes my simple
test case.

But with this patch, peeled-off sockets are created by copying from a
one-to-many socket to a one-to-one socket. Are you sure that that's
not going to cause any problems? Is it possible that there was a
reason why peeloff wasn't implemented this way in the first place?

I'm not sure, it's been there since very beginning, and I couldn't find
any changelog about it.

I guess it was trying to differentiate peeled-off socket from TCP style
sockets.

With this patch there's no way to create UDP_HIGH_BANDWIDTH style
sockets anymore, so the remaining references should probably be
cleaned up:

./net/sctp/socket.c:1886:       if (!sctp_style(sk, UDP_HIGH_BANDWIDTH) && msg->msg_name) {
./net/sctp/socket.c:8522:       if (sctp_style(sk, UDP_HIGH_BANDWIDTH))
./include/net/sctp/structs.h:144:       SCTP_SOCKET_UDP_HIGH_BANDWIDTH,

This patch disables those checks. The first one ignores a destination
address given to sendmsg() with a peeled-off socket - I don't know
why. The second one prevents listen() on a peeled-off socket.

My understanding is:
UDP_HIGH_BANDWIDTH is another kind of one-to-one socket, like TCP style.
it can get asoc by its socket when sending msg, doesn't need daddr.

Now I thinking to fix your issue in sctp_shutdown():

@@ -5163,7 +5163,7 @@ static void sctp_shutdown(struct sock *sk, int how)
        struct net *net = sock_net(sk);
        struct sctp_endpoint *ep;

-       if (!sctp_style(sk, TCP))
+       if (sctp_style(sk, UDP))
                return;

in this way, we actually think:
one-to-many socket: UDP style socket
one-to-one socket includes: UDP_HIGH_BANDWIDTH and TCP style sockets.