When sending a message over a SOCK_DGRAM that has control data `sosend_generic()` in `sys/kern/uipc_socket.c` checks `if ((atomic && resid > so->so_snd.sb_hiwat) ...` to return `ESMSGSIZE` but then next checks `if (space < resid + clen && (atomic || ...` and either returns `EWOULDBLOCK` or waits in `sbwait()` and then restarts.
`space` is calculated from `sbspace(&so->so_snd)` which is calculated to be the minimum of `sb_hiwat - sb_cc` and another value, and so strictly less or equal to `so->so_snd.sb_hiwat`.
This means then that if it is the case that `resid <= so->so_snd.sb_hiwat` but `resid + clen > so->so_snd.sb_hiwat` the code will always pass the first test and always fail the second and so will loop forever via the `goto restart` instruction.
I believe the it should be the case that `if ((atomic && resid + clen > so->so_snd.sb_hiwat) ...` the error `ESMSGSIZE` will be returned.
This may be related to 181092.