In response to a growing desire for ways to control groups of processes
from user space, the kernel has added a number of mechanisms that allow one
process to operate on another. One piece that is currently missing,
though, is the ability for a process to snatch a copy of an open file
descriptor from
another. That gap may soon be filled, though, if the pidfd_getfd()
system-call patch set from Sargun Dhillon is merged.

Source: LWN.net – [$] Grabbing file descriptors with pidfd_getfd()