In this test (and nearby ones), the receiver provides too little space to receive the file descriptors that are sent, so that the control message has to be truncated if it is to be received at all.  The warning may indicate that the cmsghdr struct received still claims that the message contains the full amount of data (so that socket.recvmsg() would overrun the buffer if it actually believed the cmsghdr).

With that in mind, you might want to check whether file descriptors that don't fit are still received (say the sender sends three file descriptors when the receiver only asks for two, and the receiver gets a control message saying it now has descriptors on numbers 3 and 4, but it also gets a file descriptor on number 5 which it isn't told about).  If they are, it's a kernel bug which could be used for denial of service by exhausting the receiving process' quota of file descriptors (and the warning would probably go away if it were fixed).