udp/recvmsg: Clear MSG_TRUNC flag when starting over for a new packet

Consider this scenario: When the size of the first received udp packet
is bigger than the receive buffer, MSG_TRUNC bit is set in msg->msg_flags.
However, if checksum error happens and this is a blocking socket, it will
goto try_again loop to receive the next packet.  But if the size of the
next udp packet is smaller than receive buffer, MSG_TRUNC flag should not
be set, but because MSG_TRUNC bit is not cleared in msg->msg_flags before
receive the next packet, MSG_TRUNC is still set, which is wrong.

Fix this problem by clearing MSG_TRUNC flag when starting over for a
new packet.

Signed-off-by: Xufeng Zhang <xufeng.zhang@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -1249,6 +1249,9 @@ csum_copy_err:
 
 	if (noblock)
 		return -EAGAIN;
+
+	/* starting over for a new packet */
+	msg->msg_flags &= ~MSG_TRUNC;
 	goto try_again;
 }
 

net/ipv6/udp.c

@@ -455,6 +455,9 @@ csum_copy_err:
 
 	if (noblock)
 		return -EAGAIN;
+
+	/* starting over for a new packet */
+	msg->msg_flags &= ~MSG_TRUNC;
 	goto try_again;
 }