can: isotp: support MSG_TRUNC flag when reading from socket

When providing the MSG_TRUNC flag via recvmsg() syscall the return value
provides the real length of the packet or datagram, even when it was longer
than the passed buffer.

Fixes: e057dd3fc20f ("can: add ISO 15765-2:2016 transport protocol")
Link: https://github.com/linux-can/can-utils/issues/347#issuecomment-1065932671
Link: https://lore.kernel.org/all/20220316164258.54155-3-socketcan@hartkopp.net
Suggested-by: Derek Will <derekrobertwill@gmail.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

diff --git a/net/can/isotp.c b/net/can/isotp.c
index 6b6c822..f6f8ba1 100644 (file)
--- a/net/can/isotp.c
+++ b/net/can/isotp.c
@@ -1047,29 +1047,28 @@ static int isotp_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
        struct sock *sk = sock->sk;
        struct sk_buff *skb;
        struct isotp_sock *so = isotp_sk(sk);
-       int err = 0;
-       int noblock;
+       int noblock = flags & MSG_DONTWAIT;
+       int ret = 0;
 
-       noblock = flags & MSG_DONTWAIT;
-       flags &= ~MSG_DONTWAIT;
+       if (flags & ~(MSG_DONTWAIT | MSG_TRUNC))
+               return -EINVAL;
 
        if (!so->bound)
                return -EADDRNOTAVAIL;
 
-       skb = skb_recv_datagram(sk, flags, noblock, &err);
+       flags &= ~MSG_DONTWAIT;
+       skb = skb_recv_datagram(sk, flags, noblock, &ret);
        if (!skb)
-               return err;
+               return ret;
 
        if (size < skb->len)
                msg->msg_flags |= MSG_TRUNC;
        else
                size = skb->len;
 
-       err = memcpy_to_msg(msg, skb->data, size);
-       if (err < 0) {
-               skb_free_datagram(sk, skb);
-               return err;
-       }
+       ret = memcpy_to_msg(msg, skb->data, size);
+       if (ret < 0)
+               goto out_err;
 
        sock_recv_timestamp(msg, sk, skb);
 
@@ -1079,9 +1078,13 @@ static int isotp_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
                memcpy(msg->msg_name, skb->cb, msg->msg_namelen);
        }
 
+       /* set length of return value */
+       ret = (flags & MSG_TRUNC) ? skb->len : size;
+
+out_err:
        skb_free_datagram(sk, skb);
 
-       return size;
+       return ret;
 }
 
 static int isotp_release(struct socket *sock)