tipc: keep the skb in rcv queue until the whole data is read

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2160540
Tested: compile only

commit f4919ff
Author: Xin Long <lucien.xin@gmail.com>
Date:   Fri Jul 16 17:44:07 2021 -0400

    tipc: keep the skb in rcv queue until the whole data is read

    Currently, when userspace reads a datagram with a buffer that is
    smaller than this datagram, the data will be truncated and only
    part of it can be received by users. It doesn't seem right that
    users don't know the datagram size and have to use a huge buffer
    to read it to avoid the truncation.

    This patch to fix it by keeping the skb in rcv queue until the
    whole data is read by users. Only the last msg of the datagram
    will be marked with MSG_EOR, just as TCP/SCTP does.

    Note that this will work as above only when MSG_EOR is set in the
    flags parameter of recvmsg(), so that it won't break any old user
    applications.

    Signed-off-by: Xin Long <lucien.xin@gmail.com>
    Acked-by: Jon Maloy <jmaloy@redhat.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>

Signed-off-by: Xin Long <lxin@redhat.com>

Author: Xin Long

net/tipc/socket.c

@@ -1889,6 +1889,7 @@ static int tipc_recvmsg(struct socket *sock, struct msghdr *m,
 	bool connected = !tipc_sk_type_connectionless(sk);
 	struct tipc_sock *tsk = tipc_sk(sk);
 	int rc, err, hlen, dlen, copy;
+	struct tipc_skb_cb *skb_cb;
 	struct sk_buff_head xmitq;
 	struct tipc_msg *hdr;
 	struct sk_buff *skb;
@@ -1912,6 +1913,7 @@ static int tipc_recvmsg(struct socket *sock, struct msghdr *m,
 		if (unlikely(rc))
 			goto exit;
 		skb = skb_peek(&sk->sk_receive_queue);
+		skb_cb = TIPC_SKB_CB(skb);
 		hdr = buf_msg(skb);
 		dlen = msg_data_sz(hdr);
 		hlen = msg_hdr_sz(hdr);
@@ -1931,18 +1933,33 @@ static int tipc_recvmsg(struct socket *sock, struct msghdr *m,
 
 	/* Capture data if non-error msg, otherwise just set return value */
 	if (likely(!err)) {
-		copy = min_t(int, dlen, buflen);
-		if (unlikely(copy != dlen))
-			m->msg_flags |= MSG_TRUNC;
-		rc = skb_copy_datagram_msg(skb, hlen, m, copy);
+		int offset = skb_cb->bytes_read;
+
+		copy = min_t(int, dlen - offset, buflen);
+		rc = skb_copy_datagram_msg(skb, hlen + offset, m, copy);
+		if (unlikely(rc))
+			goto exit;
+		if (unlikely(offset + copy < dlen)) {
+			if (flags & MSG_EOR) {
+				if (!(flags & MSG_PEEK))
+					skb_cb->bytes_read = offset + copy;
+			} else {
+				m->msg_flags |= MSG_TRUNC;
+				skb_cb->bytes_read = 0;
+			}
+		} else {
+			if (flags & MSG_EOR)
+				m->msg_flags |= MSG_EOR;
+			skb_cb->bytes_read = 0;
+		}
 	} else {
 		copy = 0;
 		rc = 0;
-		if (err != TIPC_CONN_SHUTDOWN && connected && !m->msg_control)
+		if (err != TIPC_CONN_SHUTDOWN && connected && !m->msg_control) {
 			rc = -ECONNRESET;
+			goto exit;
+		}
 	}
-	if (unlikely(rc))
-		goto exit;
 
 	/* Mark message as group event if applicable */
 	if (unlikely(grp_evt)) {
@@ -1965,9 +1982,10 @@ static int tipc_recvmsg(struct socket *sock, struct msghdr *m,
 		tipc_node_distr_xmit(sock_net(sk), &xmitq);
 	}
 
-	tsk_advance_rx_queue(sk);
+	if (!skb_cb->bytes_read)
+		tsk_advance_rx_queue(sk);
 
-	if (likely(!connected))
+	if (likely(!connected) || skb_cb->bytes_read)
 		goto exit;
 
 	/* Send connection flow control advertisement when applicable */