We were led astray (like many others before us) to believe that

msg_controllen should be CMSG_LEN() instead of CMSG_SPACE() because
the kernel fd passing code was erroring out when
     "cm->cmsg_len != control->m_len"
instead of
     "CMSG_ALIGN(cm->cmsg_len) != control->m_len".
On machines with 16-byte alignment, when one thinks about how the ALIGN
padding happens, it is clear that msg_controllen has to be CMSG_SPACE()
or the kernel cannot hope to bounds check the messages correctly.
For now, change the check to cm->cmsg_len > control->m_len to permit the
old ABI to continue working.  Later perhaps when all the old binaries
are gone we can stop permitting their use.
lots of discussion with kettenis

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