imsg is usually used over a privilege boundary. The sender is supposed
to send exactly len bytes of data. If it fails to do that the receiver
has to assume the sender is compromised and terminate itself.
E.g. (dhcpleased/engine.c):

case IMSG_CTL_LOG_VERBOSE:
	if (imsg_get_data(&imsg, &verbose,
	    sizeof(verbose)) == -1)
		fatalx("%s: invalid %s", __func__, i2s(type));

-- 
In my defence, I have been left unsupervised.