net: af_unix: implement stream sendpage support

This patch implements sendpage support for AF_UNIX SOCK_STREAM
sockets. This is also required for a complete splice implementation.

The implementation is a bit tricky because we append to already existing
skbs and so have to hold unix_sk->readlock to protect the reading side
from either advancing UNIXCB.consumed or freeing the skb at the socket
receive tail.
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/unix/af_unix.c

@@ -518,6 +518,8 @@ static int unix_ioctl(struct socket *, unsigned int, unsigned long);
 static int unix_shutdown(struct socket *, int);
 static int unix_stream_sendmsg(struct socket *, struct msghdr *, size_t);
 static int unix_stream_recvmsg(struct socket *, struct msghdr *, size_t, int);
+static ssize_t unix_stream_sendpage(struct socket *, struct page *, int offset,
+				    size_t size, int flags);
 static int unix_dgram_sendmsg(struct socket *, struct msghdr *, size_t);
 static int unix_dgram_recvmsg(struct socket *, struct msghdr *, size_t, int);
 static int unix_dgram_connect(struct socket *, struct sockaddr *,
@@ -558,7 +560,7 @@ static const struct proto_ops unix_stream_ops = {
 	.sendmsg =	unix_stream_sendmsg,
 	.recvmsg =	unix_stream_recvmsg,
 	.mmap =		sock_no_mmap,
-	.sendpage =	sock_no_sendpage,
+	.sendpage =	unix_stream_sendpage,
 	.set_peek_off =	unix_set_peek_off,
 };
 
@@ -1720,6 +1722,101 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
 	return sent ? : err;
 }
 
+static ssize_t unix_stream_sendpage(struct socket *socket, struct page *page,
+				    int offset, size_t size, int flags)
+{
+	int err = 0;
+	bool send_sigpipe = true;
+	struct sock *other, *sk = socket->sk;
+	struct sk_buff *skb, *newskb = NULL, *tail = NULL;
+
+	if (flags & MSG_OOB)
+		return -EOPNOTSUPP;
+
+	other = unix_peer(sk);
+	if (!other || sk->sk_state != TCP_ESTABLISHED)
+		return -ENOTCONN;
+
+	if (false) {
+alloc_skb:
+		unix_state_unlock(other);
+		mutex_unlock(&unix_sk(other)->readlock);
+		newskb = sock_alloc_send_pskb(sk, 0, 0, flags & MSG_DONTWAIT,
+					      &err, 0);
+		if (!newskb)
+			return err;
+	}
+
+	/* we must acquire readlock as we modify already present
+	 * skbs in the sk_receive_queue and mess with skb->len
+	 */
+	err = mutex_lock_interruptible(&unix_sk(other)->readlock);
+	if (err) {
+		err = flags & MSG_DONTWAIT ? -EAGAIN : -ERESTARTSYS;
+		send_sigpipe = false;
+		goto err;
+	}
+
+	if (sk->sk_shutdown & SEND_SHUTDOWN) {
+		err = -EPIPE;
+		goto err_unlock;
+	}
+
+	unix_state_lock(other);
+
+	if (sock_flag(other, SOCK_DEAD) ||
+	    other->sk_shutdown & RCV_SHUTDOWN) {
+		err = -EPIPE;
+		goto err_state_unlock;
+	}
+
+	skb = skb_peek_tail(&other->sk_receive_queue);
+	if (tail && tail == skb) {
+		skb = newskb;
+	} else if (!skb) {
+		if (newskb)
+			skb = newskb;
+		else
+			goto alloc_skb;
+	} else if (newskb) {
+		/* this is fast path, we don't necessarily need to
+		 * call to kfree_skb even though with newskb == NULL
+		 * this - does no harm
+		 */
+		consume_skb(newskb);
+	}
+
+	if (skb_append_pagefrags(skb, page, offset, size)) {
+		tail = skb;
+		goto alloc_skb;
+	}
+
+	skb->len += size;
+	skb->data_len += size;
+	skb->truesize += size;
+	atomic_add(size, &sk->sk_wmem_alloc);
+
+	if (newskb)
+		__skb_queue_tail(&other->sk_receive_queue, newskb);
+
+	unix_state_unlock(other);
+	mutex_unlock(&unix_sk(other)->readlock);
+
+	other->sk_data_ready(other);
+
+	return size;
+
+err_state_unlock:
+	unix_state_unlock(other);
+err_unlock:
+	mutex_unlock(&unix_sk(other)->readlock);
+err:
+	kfree_skb(newskb);
+	if (send_sigpipe && !(flags & MSG_NOSIGNAL))
+		send_sig(SIGPIPE, current, 0);
+	return err;
+}
+
 static int unix_seqpacket_sendmsg(struct socket *sock, struct msghdr *msg,
 				  size_t len)
 {