The idea of an ID-shifting virtual filesystem that would remap user and
group IDs before passing requests through to an underlying real filesystem
has been around
for a few years but has never made it into the mainline. Implementations
have taken the form of shiftfs and shifting bind mounts. Now there is yet
another approach to the problem under consideration; this one involves
a theoretically simpler approach that makes
almost no changes to the kernel’s filesystem layer at all.

Source: LWN.net – [$] Filesystem UID mapping for User namespaces: yet another shiftfs