Subject: [BUG?] tcp regression in v4.7-r1: c14ac9451c34832554db33386a4393be8bba3a7b breaks pulseaudio over TCP

From: Sergei Trofimovich <[email-masked]>
Date: Sun, 10 Jul 2016 12:42:40 +0100

Hi netdev folk!

Commit c14ac9451c34832554db33386a4393be8bba3a7b
broke pulseaudio (PA) over TCP.

PA does unusual thing: it calls
    sendmsg(cmsg_type=SCM_CREDENTIALS)
on a TCP socket. It's not a new PA behaviour though.

Originally reported as PA bug (has more details)
    https://bugs.freedesktop.org/show_bug.cgi?id=96873

It looks like kernel used to ignore control messages
but now it does not:
    http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/diff/net/ipv4/tcp.c?id=c14ac9451c34832554db33386a4393be8bba3a7b

+	if (msg->msg_controllen) {
+		err = sock_cmsg_send(sk, msg, &sockc);
+		if (unlikely(err)) {
+			err = -EINVAL;
+			goto out_err;
+		}
+	}

This change breaks streaming of pulse clients.

Pulseaudio will be fixed at some point.

But kernel change does not look like intentional
breakage of old behaviour.

Perhaps kernel should have a grace period and only
warn about unsupported control messages for a socket?

Last working kernel: v4.6

Thanks!

-- 

  Sergei