Non-root chroot

I'm wondering if there is any way to perform a simple chroot without 
having root privileges.  The goal is to test software builds with access 
to a limited set of dependencies, as poudriere does, but outside the 
FreeBSD ports system, and in some cases on hosts where the user has no 
root access.  This will prevent configure scripts with hard-coded search 
paths from finding things we don't want them to find.  Portability to 
other POSIX platforms would be desirable as well, but is not essential.

It's not clear to me why chroot() wasn't designed to support this use 
case.  There's lots of documentation stating that it's a security risk, 
but I don't see why it couldn't have been designed to be run by a 
regular user without escalating privileges inside the chroot.  I.e. if 
user "joe" does such a user-level chroot call, then all chrooted 
processes run as "joe", but with the path of the chroot dir prepended to 
every open() call (after $CWD is prepended to relative paths, of 
course), so that processes can only access files in the chroot dir. 
User "joe" would have the same privileges inside the chroot that he has 
on the host. One of the other security concerns mentioned is jail 
breaks, but if joe managed to escape the chroot, he'd only be hurting 
himself by borking the test build, so that's not a concern here.

It might be possible to port fakechroot 
(https://github.com/dex4er/fakechroot), proot 
(https://github.com/proot-me/proot), or something similar, but is there 
anything else on FreeBSD that can do this?

Thanks,

	Jason

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