Re: PAX mprotect and JIT

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

> Date: Sat, 25 Feb 2017 22:35:27 +0100
> From: Joerg Sonnenberger <joerg%bec.de@localhost>
> 
> (1) Implement a new flag for mremap to allow duplicating a mapping
> (M_REMAPDUP). This patch is functional by itself.

Can we make this opt-in at mmap time -- that is, require you to have
specified MAP_REMAPDUP to mmap on a region in order to use it for
mremap on that region?  That way this introduces no new attack surface
for mappings that were not intended to be used with it in the first
place.