On Tue, Jul 21, 2020 at 05:55:39PM +0100, Al Viro wrote:

How about fs/for_init.c and putting the damn helpers there?  With
calling conventions as close to syscalls as possible, and a fat
comment regarding their intended use being _ONLY_ the setup
in should-have-been-done-in-userland parts of init?

Where do you want the prototypes to go?  Also do you want devtmpfs
use the same helpers, which then't can't be marked __init (mount,
chdir, chroot), or separate copies?