sctp: implement SIOCINQ ioctl() (take 3)

This simple patch copies the current approach for SIOCINQ ioctl() from DCCP
into SCTP so that the userland code working with SCTP can use a similar
interface across different protocols to know how much space to allocate for
a buffer.
Signed-off-by: NDiego Elio Pettenò <flameeyes@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -3592,7 +3592,40 @@ SCTP_STATIC struct sock *sctp_accept(struct sock *sk, int flags, int *err)
 /* The SCTP ioctl handler. */
 SCTP_STATIC int sctp_ioctl(struct sock *sk, int cmd, unsigned long arg)
 {
-	return -ENOIOCTLCMD;
+	int rc = -ENOTCONN;
+
+	sctp_lock_sock(sk);
+
+	/*
+	 * SEQPACKET-style sockets in LISTENING state are valid, for
+	 * SCTP, so only discard TCP-style sockets in LISTENING state.
+	 */
+	if (sctp_style(sk, TCP) && sctp_sstate(sk, LISTENING))
+		goto out;
+
+	switch (cmd) {
+	case SIOCINQ: {
+		struct sk_buff *skb;
+		unsigned int amount = 0;
+
+		skb = skb_peek(&sk->sk_receive_queue);
+		if (skb != NULL) {
+			/*
+			 * We will only return the amount of this packet since
+			 * that is all that will be read.
+			 */
+			amount = skb->len;
+		}
+		rc = put_user(amount, (int __user *)arg);
+	}
+		break;
+	default:
+		rc = -ENOIOCTLCMD;
+		break;
+	}
+out:
+	sctp_release_sock(sk);
+	return rc;
 }
 
 /* This is the function which gets called during socket creation to