I think it's clear from the man page that the original idea was to be
able to pivot_root for individual processes. The reason it doesn't do
that, the reason it affects all processes, is to work around the
bootstrap problem, where processes that don't care what their root (or
current) directory is are still using the original root.
An extra parameter could be added to specify which behavior is desired,
probably defaulting to the current behavior. That would remove the need
to use a chroot bug.
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