This crate contains functions for obtaining file descriptors for files and directories under "/proc" on Linux.
This crate does a considerable amount of work to determine whether /proc is
mounted, with actual procfs, and without any additional mount points on top
of the paths we open.
Why all the effort to detect bind mount points? People are doing all kinds of things with Linux containers these days, with many different privilege schemes, and we want to avoid making any unnecessary assumptions. Rustix and its users will sometimes use procfs implicitly (when Linux gives them no better options), in ways that aren't obvious from their public APIs. These filesystem accesses might not be visible to someone auditing the main code of an application for places which may be influenced by the filesystem namespace. So with the checking here, they may fail, but they won't be able to succeed with bogus results.
As a caveat, QEMU intercepts selected paths in open to emulate their
contents, however this crate's extra checks bypass QEMU's interceptions, so
using this crate instead of just opening the paths directly can cause problems
when running under QEMU.
As a historical note, the functions used to be part of the rustix crate,
but were factored out into a separate crate to simplify the rustix crate, as
they can be implemented on top of rustix's public API.