Race-free process creation in the GNU C Library

That doesn't allow the receiver to verify anything, it's not just about knowing the pid, it's about knowing that it is still owned by the original process and not a recycled one. This is a real-world problem that has caused several CVEs, for example in polkit, and that so far has only been partially worked around by using unreliable heuristic like the start time in the target's proc/pid/status and other metadata, that can make it harder to exploit but not impossible