Patch "can: isotp: support MSG_TRUNC flag when reading from socket" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    can: isotp: support MSG_TRUNC flag when reading from socket

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     can-isotp-support-msg_trunc-flag-when-reading-from-s.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 5fcebaa11da66d5959b0036bac7f4031a660e1cd
Author: Oliver Hartkopp <socketcan@xxxxxxxxxxxx>
Date:   Wed Mar 16 17:42:58 2022 +0100

    can: isotp: support MSG_TRUNC flag when reading from socket
    
    [ Upstream commit 42bf50a1795a1854d48717b7361dbdbce496b16b ]
    
    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@xxxxxxxxxxxx
    Suggested-by: Derek Will <derekrobertwill@xxxxxxxxx>
    Signed-off-by: Oliver Hartkopp <socketcan@xxxxxxxxxxxx>
    Signed-off-by: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/can/isotp.c b/net/can/isotp.c
index 9ea179a6aeb0..fabf8c66a38c 100644
--- a/net/can/isotp.c
+++ b/net/can/isotp.c
@@ -1046,29 +1046,28 @@ 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);
 
@@ -1078,9 +1077,13 @@ 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)